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Hemocyanin (Hc) and phenoloxidase (PO) are members of the type 3 copper protein family. Although arthropod Hc and PO exhibit similar three-dimensional structures of the copper-containing active site, Hc functions as an oxygen transport protein, showing minimal or no phenoloxidase activity. Here, we present the crystal structure of the oxy form of Hc from Panulirus japonicus (PjHc) at 1.58 Å resolution. The structure of the di-copper active site of PjHc was found to be almost identical to that of PO. Although conserved amino acids and the water molecule crucial for the enzymatic activity were observed in PjHc at almost the same positions as those in PO, PjHc showed no enzymatic activity under our experimental conditions. One striking difference between PjHc and arthropod PO was the presence of a "blocker residue" near the binuclear copper site of PjHc. This blocker residue comprised a phenylalanine residue tightly stacked with an imidazole ring of a CuA coordinated histidine and hindered substrates from accessing the active site. Our results suggest that the blocker residue is also a determining factor of the catalytic activity of type 3 copper proteins., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Hemocyanin is a multimeric type-3 copper containing oxygen carrier protein that exhibits phenoloxidase-like activity and is found in selected species of arthropoda and mollusca. The phenoloxidase activity in the molluscan hemocyanins can be triggered by the proteolytic removal of the C-terminal β-rich sandwich domain of the protein or by the treatment with chemical agents like SDS, both of which enable active site access to the phenolic substrates. The mechanism by which SDS treatment enhances active site access to the substrates is however not well understood in molluscan hemocyanins. Here, using a combination of in silico molecular dynamics (MD) and docking studies on the crystal structure of Octopus dofleini hemocyanin (PDB code:1JS8), we demonstrate that the C-terminal β-domain of the protein plays a crucial role in regulating active site access to bulky phenolic substrates. Furthermore, MD simulation of hemocyanin in SDS revealed displacement of β-domain, enhanced active site access and a resulting increase in binding affinity for substrates. These observations were further validated by enzyme kinetics experiments., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Phenoloxidases play vital roles in invertebrate innate immune reactions, wound closure and sclerotization processes in arthropods. In chelicerates, where phenoloxidases are lacking, phenoloxidase-activity can be induced in the oxygen carrier hemocyanin in vitro by proteolytic cleavage, incubation with the artificial inducer SDS, or lipids. The role of protein-protein interaction has up to now received little attention. This is remarkable, as lipoproteins - complexes of proteins and lipids - are present at high concentrations in arthropod hemolymph. We characterized the three lipoproteins present in tarantula hemolymph, two high-density lipoproteins and one very high-density lipoprotein, and show that the two high-density lipoproteins have distinct structures: the more abundant high-density lipoprotein is an ellipsoid particle with axes of ~22.5 nm and ~16.8 nm, respectively. The second high-density lipoprotein, present only in trace amount, is a large discoidal lipoprotein with a diameter of ~38.4 nm and an on-edge thickness of ~7.1 nm. We further demonstrate that the interaction between lipoproteins and hemocyanin induces phenoloxidase activity in hemocyanin, and propose that this activation is due to protein-protein interaction rather than protein-lipid interaction, as neither lipid micelles nor lipid monomers were found to be activating. Activation was strongest in the presence of high-density lipoproteins; very high-density lipoproteins were found to be non-activating. This is the first time that the ability of lipoproteins to induce phenoloxidase activity of hemocyanin has been demonstrated, thus adding novel aspects to the function of lipoproteins apart from their known role in nutrient supply., (Copyright © 2015 Elsevier B.V. All rights reserved.)

The phenomenon of hyperpigmentation (melanosis) in shellfish has long been attributed to phenoloxidase enzymes. Over the last number of years, the oxygen carrier hemocyanin, has demonstrated several immune- and physiological functionalities, most notably, inducible phenoloxidase activity. In this study, hemocyanin purified from the hemolymph of Nephrops norvegicus displays diphenoloxidase activity in the presence of a number of elicitors and retains structural and functional integrity throughout the process of freeze-thawing (at -25 °C). Conversely, cellular phenoloxidase activity (present in cell-lysates), demonstrates >98% reduction in activity after freeze-thawing. We present evidence that hemocyanin may act as a causative agent of hyperpigmentation in N. norvegicus. The inhibition of hemocyanin-derived phenoloxidase activity is discussed, and for the first time, the biophysical interactions of shellfish hemocyanin with known phenoloxidase inhibitors are presented., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Background: Tyrosinases, tyrosinase-related proteins, catechol oxidases and hemocyanins comprise the type-3 copper protein family and are involved in a variety of biological processes, including pigment formation, innate immunity and oxygen transport. Although this family is present in the three domains of life, its origin and early evolution are not well understood. Previous analyses of type-3 copper proteins largely have focussed on specific animal and plant phyla., Results: Here, we combine genomic, phylogenetic and structural analyses to show that the original type-3 copper protein possessed a signal peptide and may have been secreted (we designate proteins of this type the α subclass). This ancestral type-3 copper protein gene underwent two duplication events, the first prior to the divergence of the unikont eukaryotic lineages and the second before the diversification of animals. The former duplication gave rise to a cytosolic form (β) and the latter to a membrane-bound form (γ). Structural comparisons reveal that the active site of α and γ forms are covered by aliphatic amino acids, and the β form has a highly conserved aromatic residue in this position. The subsequent evolution of this gene family in modern lineages of multicellular eukaryotes is typified by the loss of one or more of these three subclasses and the lineage-specific expansion of one or both of the remaining subclasses., Conclusions: The diversity of type-3 copper proteins in animals and other eukaryotes is consistent with two ancient gene duplication events leading to α, β and γ subclasses, followed by the differential loss and expansion of one or more of these subclasses in specific kingdoms and phyla. This has led to many lineage-specific type-3 copper protein repertoires and in some cases the independent evolution of functionally-classified tyrosinases and hemocyanins. For example, the oxygen-carrying hemocyanins in arthropods evolved from a β-subclass tyrosinase, whilst hemocyanins in molluscs and urochordates evolved independently from an α-subclass tyrosinase. Minor conformational changes at the active site of α, β and γ forms can produce type-3 copper proteins with capacities to either carry oxygen (hemocyanins), oxidize diphenols (catechol oxidase) or o-hydroxylate monophenols (tyrosinases) and appear to underlie some functional convergences.

Inhibitors of phenoloxidase are used routinely to characterise the structural and functional properties of phenoloxidases. Hemocyanin-derived phenoloxidase activity is also sensitive to standard phenoloxidase inhibitors. In this study, we characterise the effects of a number of phenoloxidase inhibitors on hemocyanin-derived phenoloxidase activity from the chelicerate, Limulus polyphemus. Both inhibition type and K(i) values were similar to those observed for hemocyanin-derived phenoloxidase from another chelicerate, Eurypelma californicum. In addition, substrate inhibition was observed at concentrations above 2mM dopamine. The conformation in which two of the inhibitors, namely tropolone and kojic acid, would bind near the Cu(II) centre of hemocyanin is proposed., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Phenoloxidases (POs) and haemocyanins constitute a family of copper-containing proteins widely distributed among invertebrates. Both of them are able, under appropriate conditions, to convert polyphenols to quinones and induce cytotoxicity through the production of reactive oxygen species, a fundamental event in many immune responses. In ascidians, PO activity has been described and studied in both solitary and colonial species and the enzyme is involved in inflammatory and cytotoxic reactions against foreign cells or molecules, and in the formation of the cytotoxic foci which characterise the nonfusion reaction of botryllids. Expressed genes for two putative POs (CiPO1 and CiPO2) have been recently identified in C. intestinalis. In the present study, we determined the cDNA sequences of two haemocyanin-like proteins from two colonial ascidians: Botryllus schlosseri from the Mediterranean Sea and Polyandrocarpa misakiensis from Japan. Multiple sequence alignments evidenced the similarity between the above sequences and crustacean proPOs whereas the analysis of the three-dimensional structure reveals high similarity with arthropod haemocyanins which share common precursors with arthropod proPOs. Botryllus HLP grouped in the same cluster with Ciona POs, whereas Polyandrocarpa HLP clustered with arthropod haemocyanins; all of them share the full conservation of the six histidines at the two copper-binding sites as well as of other motifs, also found in arthropod haemocyanin subunits, involved in the regulation of enzyme activity. In situ hybridisation indicated that the genes are transcribed inside morula cells, a characteristic haemocyte type in ascidians where PO activity is located, at the beginning of their differentiation. These results represent a first attempt to identify candidate molecules responsible of the PO activity in compound ascidians., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

o-Diphenol oxidase activities (o-diPO) of chemically modified functional unit RvH1-a of molluscan hemocyanin Rapana venosa were studied using L-Dopa and dopamine as substrates. With L-Dopa as substrate the native FU RvH1-a did not show any o-diPO activity. Therefore the native FU RvH1-a was converted to enzymatic active form, after treatment with SDS, trypsin, urea and different values of pH when its o-diPO activity was studied. The highest artificial induction of o-diPO activity was observed after incubation of FU with 3.0mM SDS, and RvH1-a shows both, dopamine (K(M)=6.53mM, k(cat)/K(M)=1.29) and L-Dopa (K(M)=2.0mM, k(cat)/K(M)=2.1) activity due to a more open active site of the enzyme and better access of the substrates. It was determined that the K(M) value of SDS-activated RvH1-a against dopamine is higher compared to those of hemocyanins from Helix vulgaris, Helix pomatia and native tyrosinase from Ipomoea batatas but much lower than that from Illex argentinus (ST94) tyrosinase and arthropodan hemocyanin from Carcinus aestuarii. The Km value of SDS-activated RvH1-a against L-Dopa is higher than those of hemocyanins from H. vulgaris and Cancer magister, but lower than that of the tyrosinase from Streptomyces albus., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Phenoloxidase enzymes and the associated pro-phenoloxidase activation cascade play an essential role in the immune response of arthropods. Phenoloxidase activity can be elicited in the oxygen carrier, hemocyanin, by the addition of the artificial inducer, SDS. There is some evidence to support hemocyanin acting as a phenoloxidase in vivo; however, the identity of natural activators remains unclear. This study explores the role of the phospholipid, phosphatidylserine, as a possible natural activator of hemocyanin-derived phenoloxidase activity. Characterisation of the structural changes associated with activation of hemocyanin-derived phenoloxidase suggests that phosphatidylserine induces similar conformational changes to those caused by the artificial inducer, SDS. We propose that anionic phospholipids, in particular phosphatidylserine, may act as natural activators of hemocyanin-derived phenoloxidase., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

The intrinsic and inducible phenoloxidase (PO) activity of Rapana thomasiana hemocyanin (RtH) and its substructures were studied. With catechol as substrate, a weak o-diPO activity was measured for the didecameric RtH and its subunits. Some activation of the o-diPO activity of RtH was achieved by limited treatment with subtilisin and by incubation of RtH with 2.9 mM sodium dodecyl sulphate (SDS), suggesting an enhanced substrate access to the active sites. The highest artificial induction of o-diPO activity in RtH, however, was obtained by lyophilization of the protein. This is ascribed to conformational changes during the lyophilization process of the didecameric RtH molecules, affecting the accessibility of the active sites. These conformational changes must be very small, since Fourier-transform infrared and circular dichroism spectroscopies did not reveal any changes in secondary structure of lyophilized RtH. The difference in accessibility of the copper containing active site for substrates between catechol oxidase and functional unit RtH2-e was demonstrated by molecular modeling and surface area accessibility calculations. The low level of intrinsic PO activity in the investigated hemocyanin is related to the inaccessibility of the binuclear copper active sites to the substrates.

Phenoloxidases (POs) occur in all organisms and are involved in skin and hair coloring in mammals, and initiating melanization in wound healing. Mutation or overexpression of PO can cause albinism or melanoma, respectively. SDS can convert inactive PO and the oxygen carrier hemocyanin (Hc) into enzymatically active PO. Here we present single-particle cryo-EM maps at subnanometer resolution and pseudoatomic models of the 24-oligomeric Hc from scorpion Pandinus imperator in resting and SDS-activated states. Our structural analyses led to a plausible mechanism of Hc enzyme PO activation: upon SDS activation, the intrinsically flexible Hc domain I twists away from domains II and III in each subunit, exposing the entrance to the active site; this movement is stabilized by enhanced interhexamer and interdodecamer interactions, particularly in the central linker subunits. This mechanism could be applicable to other type 3 copper proteins, as the active site is highly conserved.

The coupled binuclear "type 3" Cu sites are found in hemocyanin (Hc), tyrosinase (Tyr), and the multicopper oxidases (MCOs), such as laccase (Lc), and play vital roles in O(2) respiration. Although all type 3 Cu sites share the same ground state features, those of Hc/Tyr have very different ligand-binding properties relative to those of the MCOs. In particular, the type 3 Cu site in the MCOs (Lc(T3)) is a part of the trinuclear Cu cluster, and if the third (i.e., type 2) Cu is removed, the Lc(T3) site does not react with O(2). Density functional theory calculations indicate that O(2) binding in Hc is approximately 9 kcal mol(-1) more favorable than for Lc(T3). The difference is mostly found in the total energy difference of the deoxy states (approximately 7 kcal mol(-1)), where the stabilization of deoxy Lc(T3) derives from its long equilibrium Cu-Cu distance of approximately 5.5-6.5 A, relative to approximately 4.2 A in deoxy Hc/Tyr. The O(2) binding in Hc is driven by the electrostatic destabilization of the deoxy Hc site, in which the two Cu(I) centers are kept close together by the protein for facile 2-electron reduction of O(2). Alternatively, the lack of O(2) reactivity in Lc(T3) reflects the flexibility of the active site, capable of minimizing the electrostatic repulsion of the 2 Cu(I)s. Thus, the O(2) reactivity of the MCOs is intrinsic to the trinuclear Cu cluster, leading to different O(2) intermediates as required by its function of irreversible reduction of O(2) to H(2)O.

Hemocyanin and phenoloxidase belong to the type-3 copper protein family, sharing a similar active center whereas performing different roles. In this study, we demonstrated that purified hemocyanin (450 kDa) from the spiny lobster Panulirus argus shows phenoloxidase activity in vitro after treatment with trypsin, chymotrypsin and SDS (0.1% optimal concentration), but it is not activated by sodium perchlorate or isopropanol. The optimal pHs of the SDS-activated hemocyanin were 5.5 and 7.0. Hemocyanin from spiny lobster behaves as a catecholoxidase. Kinetic characterization using dopamine, L-DOPA and catechol shows that dopamine is the most specific substrate. Catechol and dopamine produced substrate inhibition above 16 and 2 mM respectively. Mechanism-based inhibition was also evidenced for the three substrates, being less significant for L-DOPA. SDS-activated phenoloxidase activity is produced by the hexameric hemocyanin. Zymographic analysis demonstrated that incubation of native hemocyanin with trypsin and chymotrypsin, produced bands of 170 and 190 kDa respectively, with intense phenoloxidase activity. Three polypeptide chains of 77, 80 and 89 kDa of hemocyanin monomers were identified by SDS-PAGE. Monomers did not show phenoloxidase activity induced by SDS or partial proteolysis.

The enzymatic activity of phenoloxidase is assayed routinely in the presence of SDS. Similar assay conditions elicit phenoloxidase activity in another type 3 copper protein, namely hemocyanin, which normally functions as an oxygen carrier. The nature of the conformational changes induced in type 3 copper proteins by the denaturant SDS is unknown. This comparative study demonstrates that arthropod hemocyanins can be converted from being an oxygen carrier to a form which exhibits phenoloxidase activity by incubation with SDS, with accompanying changes in secondary and tertiary structure. Structural characterisation, using various biophysical methods, suggests that the micellar form of SDS is required to induce optimal conformational transitions in the protein which may result in opening a channel to the di-copper centre allowing bulky phenolic substrates access to the catalytic site.

This review presents the common features and differences of the type 3 copper proteins with respect to their structure and function. In spite of these differences a common mechanism of activation and catalysis seems to have been preserved throughout evolution. In all cases the inactive proenzymes such as tyrosinase and catecholoxidase are activated by removal of an amino acid blocking the entrance channel to the active site. No other modification at the active site seems to be necessary to enable catalytic activity. Hemocyanins, the oxygen carriers in many invertebrates, also behave as silent inactive enzymes and can be activated in the same way. The molecular basis of the catalytic process is presented based on recent crystal structures of tyrosinase and hemocyanin. Minor conformational differences at the active site seem to decide about whether the active site is only able to oxidize diphenols as in catecholoxidase or if it is also able to o-hydroxylate monophenols as in tyrosinase.

Copper-cluster sites in biology exhibit unique spectroscopic features reflecting exchange coupling between oxidized Cu's and e (-) delocalization in mixed valent sites. These novel electronic structures play critical roles in O 2 binding and activation for electrophilic aromatic attack and H-atom abstraction, the 4e (-)/4H (+) reduction of O 2 to H 2O, and in the 2e (-)/2H (+) reduction of N 2O. These electronic structure/reactivity correlations are summarized below.

The molecular mechanism of the monooxygenase (phenolase) activity of type 3 copper proteins has been examined in detail both in the model systems and in the enzymatic systems. The reaction of a side-on peroxo dicopper(II) model compound ( A) and neutral phenols proceeds via a proton-coupled electron-transfer (PCET) mechanism to generate phenoxyl radical species, which collapse each other to give the corresponding C-C coupling dimer products. In this reaction, a bis(mu-oxo)dicopper(III) complex ( B) generated by O-O bond homolysis of A is suggested to be a real active species. On the other hand, the reaction of lithium phenolates (deprotonated form of phenols) with the same side-on peroxo dicopper(II) complex proceeds via an electrophilic aromatic substitution mechanism to give the oxygenated products (catechols). The mechanistic difference between these two systems has been discussed on the basis of the Marcus theory of electron transfer and Hammett analysis. Mechanistic details of the monooxygenase activity of tyrosinase have also been examined using a simplified enzymatic reaction system to demonstrate that the enzymatic reaction mechanism is virtually the same as that of the model reaction, that is, an electrophilic aromatic substitution mechanism. In addition, the monooxygenase activity of the oxygen carrier protein hemocyanin has been explored for the first time by employing urea as an additive in the reaction system. In this case as well, the ortho-hydroxylation of phenols to catechols has been demonstrated to involve the same ionic mechanism.

The phenoloxidase (PO) activity of the hemocyanins (Hcs) from two molluscan species, the gastropod Helix pomatia (Hp) and the cephalopod Sepia officinalis (So), was studied. With catechol as substrate the Hcs showed a weak o-diPO activity, which was moderately enhanced on limited proteolysis with subtilisin. The sites in the Hc molecules mainly responsible for this activity were identified. The highest intrinsic o-diPO activity and also by far the highest level of induction were found in the functional units (FUs) Hp f and So g, isolated from Hp beta-Hc and So Hc (subunit 2), respectively. The results thus support the earlier conclusion, made on the basis of sequence homology between molluscan Hcs, that Hp f and So g are functional and structural analogues. The subtilisin treatment of Hp f also induced monoPO activity, considered to be at the origin of browning of the sample.

Tyrosinase (monophenol, L-DOPA: oxygen oxidoreductase, EC 1.14.18.1), a kind of copper-containing phenoloxidase, arouses great interests of scientists for its important role in periostracum formation. A cDNA clone encoding a putative tyrosinase, termed OT47 because of its estimated molecular mass of 47kDa, was isolated from the pearl oyster, Pinctada fucata. This novel tyrosinase shares similarity with the cephalopod tyrosinases and other type 3 copper proteins within two conserved copper-binding sites. RT-PCR analysis showed that OT47 mRNA was expressed only in the mantle edge. Further in situ hybridization analysis and tyrosinase activity staining revealed that OT47 was expressed at the outer epithelial cells of the middle fold, different from early histological results in Mercenaria mercenaria, suggesting a different model of periostracum secretion in P. fucata. Taken together, these results suggest that OT47 is most likely involved in periostracum formation. The identification and characterization of oyster tyrosinase also help to further understand the structural and functional properties of molluscan tyrosinase.

Tyrosinases are nearly ubiquitously distributed in all domains of life. They are essential for pigmentation and are important factors in wound healing and primary immune response. Their active site is characterized by a pair of antiferromagnetically coupled copper ions, CuA and CuB, which are coordinated by six histidine residues. Such a "type 3 copper centre" is the common feature of tyrosinases, catecholoxidases and haemocycanins. It is also one of several other copper types found in the multi-copper oxidases (ascorbate oxidase, laccase). The copper pair of tyrosinases binds one molecule of atmospheric oxygen to catalyse two different kinds of enzymatic reactions: (1) the ortho-hydroxylation of monophenols (cresolase activity) and (2) the oxidation of o-diphenols to o-diquinones (catecholase activity). The best-known function is the formation of melanins from L-tyrosine via L-dihydroxyphenylalanine (L-dopa). The complicated hydroxylation mechanism at the active centre is still not completely understood, because nothing is known about their tertiary structure. One main reason for this deficit is that hitherto tyrosinases from eukaryotic sources could not be isolated in sufficient quantities and purities for detailed structural studies. This is not the case for prokaryotic tyrosinases from different Streptomyces species, having been intensively characterized genetically and spectroscopically for decades. The Streptomyces tyrosinases are non-modified monomeric proteins with a low molecular mass of ca. 30kDa. They are secreted to the surrounding medium, where they are involved in extracellular melanin production. In the species Streptomyces, the tyrosinase gene is part of the melC operon. Next to the tyrosinase gene (melC2), this operon contains an additional ORF called melC1, which is essential for the correct expression of the enzyme. This review summarizes the present knowledge of bacterial tyrosinases, which are promising models in order to get more insights in structure, enzymatic reactions and functions of "type 3 copper" proteins in general.

The respiratory protein hemocyanin is present in molluscans and in some species of arthropods, and its dioxygen binding site strongly resembles that of the monophenol-hydroxylating and catechol-quinonising enzyme tyrosinase. Moreover, some hemocyanins show a certain extent of tyrosinase activity, so a common ancestry between the two proteins has been suggested. However, in the case purified hemocyanin of Scyllarides latus any attempts to evoke tyrosinase activity failed. A distinct tyrosinase has been purified to homogeneity from the hemolymph, and kinetically characterised. The purified tyrosinase showed both monophenolase and diphenolase enzyme activity and therefore it could be well defined as a true tyrosinase. This finding suggests that in the case of the studied crustacean the evolutionary functional divergence between dioxygen transport and oxidation of phenolics has already reached its completeness.

Hemocyanin and phenoloxidase are both copper-binding proteins involved in the immune system for a wide range of animal species. In crayfish, these proteins were purified and characterized from plasma and hemocytes, respectively. Recently, we have reported that the processing of one of the hemocyanin subunits occurs by a proteolytic cleavage under acidic conditions which results in the release of an antibacterial peptide designated as astacidin 1 from the C-terminus [J. Biol. Chem. 278 (2003) 7927]. In the present paper, we show that cleavage of crayfish hemocyanin subunit 2 at the N-terminal part results in that the processed hemocyanin exhibits phenoloxidase activity. The calculated mass of the cloned hemocyanin 2 is 78,372Da, which corresponds to the size obtained after SDS-PAGE under reducing conditions of the purified hemocyanin and pI is estimated to be 5.70. The complete hemocyanin 2 sequence shows 74% and 44% similarity with hemocyanin 1 and prophenoloxidase of crayfish, respectively. Crayfish hemocyanin exhibited phenoloxidase activity in presence of trypsin, but no activity could be detected if treated with sodium dodecyl sulfate. These results show that hemocyanin of crayfish is involved in several immune responses such as an oxygen carrier protein, as a precursor for an antibacterial peptide, and a molecule with phenoloxidase function.

Tyrosinase is a copper monooxygenase containing a coupled dinuclear copper active site (type-3 copper), which catalyzes oxygenation of phenols (phenolase activity) as well as dehydrogenation of catechols (catecholase activity) using O(2) as the oxidant. In this study, catalase activity (conversion of H(2)O(2) to (1/2)O(2) and H(2)O) and peroxygenase activity (H(2)O(2)-dependent oxygenation of substrates) of mushroom tyrosinase have been examined kinetically by using amperometric O(2) and H(2)O(2) sensors. The catalase activity has been examined by monitoring the initial rate of O(2) production from H(2)O(2) in the presence of a catalytic amount of tyrosinase in 0.1 M phosphate buffer (pH 7.0) at 25 degrees C under initially anaerobic conditions. It has been found that the catalase activity of mushroom tyrosinase is three-order of magnitude greater than that of mollusk hemocyanin. The higher catalase activity of tyrosinase could be attributed to easier accessibility of H(2)O(2) to the dinuclear copper site of tyrosinase. Mushroom tyrosinase has also been demonstrated for the first time to catalyze oxygenation reaction of phenols with H(2)O(2) (peroxygenase activity). The reaction has been investigated kinetically by monitoring the H(2)O(2) consumption rate in 0.5 M borate buffer (pH 7.0) under aerobic conditions. Similarity of the substituent effects of a series of p-substituted phenols in the peroxygenase reaction with H(2)O(2) to those in the phenolase reaction with O(2) as well as the absence of kinetic deuterium isotope effect with a perdeuterated substrate (p-Cl-C(6)D(4)OH vs p-Cl-C(6)H(4)OH) clearly demonstrated that the oxygenation mechanisms of phenols in both systems are the same, that is, the electrophilic aromatic substitution reaction by a (micro-eta(2):eta(2)-peroxo)dicopper(II) intermediate of oxy-tyrosinase.

Binuclear Cu proteins play vital roles in O(2) binding and activation in biology and can be classified into coupled and noncoupled binuclear sites based on the magnetic interaction between the two Cu centers. Coupled binuclear Cu proteins include hemocyanin, tyrosinase, and catechol oxidase. These proteins have two Cu centers strongly magnetically coupled through direct bridging ligands that provide a mechanism for the 2-electron reduction of O(2) to a mu-eta(2):eta(2) side-on peroxide bridged Cu(II)(2)(O(2)(2-)) species. This side-on bridged peroxo-Cu(II)(2) species is activated for electrophilic attack on the phenolic ring of substrates. Noncoupled binuclear Cu proteins include peptidylglycine alpha-hydroxylating monooxygenase and dopamine beta-monooxygenase. These proteins have binuclear Cu active sites that are distant, that exhibit no exchange interaction, and that activate O(2) at a single Cu center to generate a reactive Cu(II)/O(2) species for H-atom abstraction from the C-H bond of substrates. O(2) intermediates in the coupled binuclear Cu enzymes can be trapped and studied spectroscopically. Possible intermediates in noncoupled binuclear Cu proteins can be defined through correlation to mononuclear Cu(II)/O(2) model complexes. The different intermediates in these two classes of binuclear Cu proteins exhibit different reactivities that correlate with their different electronic structures and exchange coupling interactions between the binuclear Cu centers. These studies provide insight into the role of exchange coupling between the Cu centers in their reaction mechanisms.

The structural basis that define the physiological functions of binuclear copper enzymes is discussed in the frame of the data generated by a broad spectroscopic approach, spanning from paramagnetic NMR and pulsed EPR to x-ray absorption spectroscopies. The structural features discussed for the different oxidation and ligation states accessible to a binuclear copper sites are the coordination geometry for the first and second shell, the metal-metal distance and the role of the bridging exogenous ligand(s). A structural model will be presented to rationalize both the differentiation in function within the protein families and the reaction mechanism of those proteins that are enzymatically active.

The dinuclear copper enzyme tyrosinase (Ty) from genetically engineered Streptomyces antibioticus has been investigated in its paramagnetic half-met form [Cu(I)-Cu(II)]. The cw EPR, pulsed EPR, and hyperfine sublevel correlation spectroscopy (HYSCORE) experiments on the half-met-Ty and on its complexes with three different types of competitive inhibitor are reported. The first type includes p-nitrophenol, a very poor substrate for the monooxygenase activity of Ty. The second type comprises hydroxyquinones, such as kojic acid and l-mimosine, and the third type of inhibitor is represented by toluic acid. The electronic and structural differences of the half-met-Ty form induced at the cupric site by the different inhibitors have been determined. Probes of structural effects are the hyperfine coupling constants of the non coordinating Ndelta histidyl nitrogens. By using the available crystal structures of hemocyanin as a template in combination with the spectroscopic results, a structural model for the active site of half-met-Ty is obtained and a model for the binding modes of both mono- and diphenols could be proposed.

N-terminal amino acid sequences for the two hemocyanin subunits from the deep-sea crustacean Bathynomus giganteus have been determined by Edman degradation, providing the first sequence information for a hemocyanin from an isopod. In addition, purified hemocyanin from B. giganteus exhibited phenoloxidase activity in the presence of sodium dodecyl sulfate. Although a natural activator has not yet been identified, a preliminary study of the enzyme indicated a K(m) of 5mM for dopamine and an initial rate of 0.1 micromol per min per mg protein, values consistent with a significant role for this enzyme in the innate immune system of B. giganteus. Moreover, after separation of hemolymph by alkaline polyacrylamide gel electrophoresis, the only detectable phenoloxidase activity coincided with the two hemocyanin subunits. The hemocyanin of this primitive crustacean may fulfill dual functions, both as oxygen carrier and as the phenoloxidase crucial for host defense.

The functional conversion of hemocyanin (Hc), an oxygen transporter, into an enzyme was investigated in crustaceans. Hc is converted into a phenoloxidase-like enzyme by hemocyte components, which is triggered by beta-1,3-glucan. This activation is severely hampered with leupeptin and E-64 treatment, indicating that the serine/cysteine proteases in the hemocytes are involved in the activation. In a SDS-PAGE-analysis, no change was observed between normal and activated Hc under reduced conditions. However, under non-reduced condition of normal Hc, several minor bands were observed at oligomeric position of Hc subunit, which disappeared upon activation. These results indicate that a split of the reductive bond, such as the disulfide bond between subunits, is essential for Hc activation. This is the first report to show the enzymatic conversion of Hc and the presence of the covalent bond in the Hc subunit of crustaceans.

The enzymes tyrosinase, catecholoxidase and hemocyanin all share similar active sites, although their physiological functions differ. Hemocyanins serve as oxygen carrier proteins, and tyrosinases and catecholoxidases (commonly referred to as phenoloxidases in arthropods) catalyze the hydroxylation of monophenols or the oxidation of o-diphenols to o-quinones, or both. Tyrosinases are activated in vivo by limited proteolytic cleavage, which might open up substrate access to the catalytic site. It has recently been demonstrated that if hemocyanins are subjected to similar proteolytic treatments (in vitro) they also exhibit at least catecholoxidase reactivity. On the basis of their molecular structures, hemocyanins are used as model systems to understand the substrate-active-site interaction between catecholoxidases and tyrosinases.

Two closely related copper proteins, phenoloxidase and haemocyanin, are known to be involved in different physiological functions such as the primary immune response and oxygen transport. Although the proteins differ structurally, they have the same active site by which dioxygen is bound. Recent results reveal that haemocyanin also exhibits phenoloxidase activity. A scenario is proposed for the evolutionary relationships among copper oxygen-binding proteins (COPs).

An enzyme generally catalyzes one well defined reaction with high specificity and efficiency. We report here in contrast that the copper protein hemocyanin of the tarantula Eurypelma californicum exhibits two different functions. These occur at the same active site. While hemocyanin usually is an oxygen carrier, its function can be transformed totally to monophenoloxidase and o-diphenoloxidase activity after limited proteolysis with trypsin or chymotrypsin. N-acetyldopamine (NADA) is more effectively oxidized than L-dopa or dopamine. This irreversible functional switch of tarantula hemocyanin function is limited to the two subunits b and c of its seven subunit types. A conserved phenylalanine in the hemocyanin molecule acts as a placeholder for other substrates that are phenylalanine derivatives. The proteolytic cleavage removes an N-terminal fragment, including the critical phenylalanine residue, which opens an entrance for substrates. Therefore no new arrangement of the active site, with its two copper atoms and the mu - eta2:eta2 bound O2 molecule, is necessary to develop the catalytic function.

We have measured the x-ray absorption near edge structure (XANES) spectra of the enzyme tyrosinase from the mushroom Agaricus bisporus in solution in the oxy and deoxy forms. The spectra, obtained under the same conditions as the analogous forms of mollusc hemocyanin (Hc), show that the oxidation state of copper changes from Cu(II) (oxy form) to Cu(I) (deoxy form), and the copper active site(s) of A. bisporus tyrosinase in solution undergoes the same main conformational changes as Hc. We have applied the multiple scattering theory to simulate the XANES spectra of various alternative geometries of the copper site, accounting for the residual differences between Hc and tyrosinase. While oxy-Hc is reasonably fitted only by the pseudo-square-pyramidal geometry reported by its crystallographic data, oxytyrosinase can be fitted, starting from the Hc coordinates, either by distortions toward a pseudo-tetrahedral geometry, with inequivalent copper sites, or by an apically distorted square-pyramidal geometry (with an elongation of the apical distance of no more than 0.2 A).

Dipteran arylphorin receptors, insect hexamerins, cheliceratan and crustacean hemocyanins, and crustacean and insect tyrosinases display significant sequence similarities. We have undertaken a systematic comparison of primary and secondary structures of these proteins. On the basis of multiple sequence alignments the phylogeny of these proteins was investigated. Hexamerin subunits, hemocyanin subunits, and tyrosinases share extensive similarities throughout the entire amino acid sequence. Our studies suggest the origin of arthropod hemocyanins from ancient tyrosinase-like proteins. Insect hexamerins likely evolved from hemocyanins of ancient crustaceans, supporting the proposed sister-group position of these subphyla. Arylphorin receptors, responsible for incorporation of hexamerins into the larval fat body of diptera, are related to hexamerins, hemocyanins, and tyrosinase. The receptor sequences display extensive similarities to the first and third domains of hemocyanins and hexamerins. In the middle region only limited amino acid conservation was observed. Elements important for hexamer formation are deleted in the receptors. Phylogenetic analysis indicated that dipteran arylphorin receptors diverged from ancient hexamerins, probably early in insect evolution.

Pro-phenol oxidase [pro-PO; zymogen of phenol oxidase (monophenol, L-dopa:oxygen oxidoreductase, EC 1.14.18.1)] is present in the hemolymph plasma of the silkworm Bombyx mori. Pro-PO is a heterodimeric protein synthesized by hemocytes. A specific serine proteinase activates both subunits through a limited proteolysis. The amino acid sequences of both subunits were deduced from their respective cDNAs; amino acid sequence homology between the subunits was 51%. The deduced amino acid sequences revealed domains highly homologous to the copper-binding site sequences (copper-binding sites A and B) of arthropod hemocyanins. The overall sequence homology between silkworm pro-PO and arthropod hemocyanins ranged from 29 to 39%. Phenol oxidases from prokaryotes, fungi, and vertebrates have sequences homologous to only the copper-binding site B of arthropod hemocyanins. Thus, silkworm pro-PO DNA described here appears distinctive and more closely related to arthropod hemocyanins. The pro-PO-activating serine proteinase was shown to hydrolyze peptide bonds at the carboxyl side of arginine in the sequence-Asn-49-Arg-50-Phe-51-Gly-52- of both subunits. Amino groups of N termini of both subunits were indicated to be N-acetylated. The cDNAs of both pro-PO subunits lacked signal peptide sequences. This result supports our contention that mature pro-PO accumulates in the cytoplasm of hemocytes and is released by cell rupture, as for arthropod hemocyanins.

Clones encoding pro-phenol oxidase [pro-PO; zymogen of phenol oxidase (monophenol, L-dopa:oxygen oxidoreductase, EC 1.14.18.1)] A1 were isolated from a lambda gt10 library that originated from Drosophila melanogaster strain Oregon-R male adults. The 2294 bp of the cDNA included a 13-bp 5'-noncoding region, a 2070-bp encoding open reading frame of 690 amino acids, and a 211-bp 3'-noncoding region. A hydrophobic NH2-terminal sequence for a signal peptide is absent in the protein. Furthermore, there are six potential N-glycosylation sites in the sequence, but no amino sugar was detected in the purified protein by amino acid analysis, indicating the lack of an N-linked sugar chain. The potential copper-binding sites, amino acids 200-248 and 359-414, are highly homologous to the corresponding sites of hemocyanin of the tarantula Eurypelma californicum, the horseshoe crab Limulus polyphemus, and the spiny lobster Panulirus interruptus. On the basis of the phylogenetic tree constructed by the neighbor-joining method, vertebrate tyrosinases and molluscan hemocyanins constitute one family, whereas pro-POs and arthropod hemocyanins group with another family. It seems, therefore, likely that pro-PO originates from a common ancestor with arthropod hemocyanins, independently to the vertebrate and microbial tyrosinases.

Mutations of the tyrosinase gene associated with a partial or complete loss of enzymatic activity are responsible for tyrosinase related oculocutaneous albinism (OCA1). A large number of mutations have been identified and their analysis has provided insight into the biology of tyrosinase and the pathogenesis of these different mutations. Missense mutations produce their effect on the activity of an enzyme by altering an amino acid at a specific site. The location of these mutations in the peptide can be used to indicate potential domains important for enzymatic activity. Missense mutations of the tyrosinase polypeptide cluster in four regions, suggesting that these are important functional domains. Two of the potential domains involve the copper binding sites while the others are likely involved in substrate binding. More critical analysis of the copper binding domain of tyrosinase can be gained by analyzing the structure of hemocyanin, a copper-binding protein with a high degree of homology to tyrosinase in the copper binding region. This analysis indicates a single catalytic site in tyrosinase for all enzymatic activities.

Extended Huckel theory calculations have been carried out on a model of the oxyhemocyanin active site that includes six imidazoles, the two copper cations, and a dioxygen molecule. The results obtained for the very likely mu-eta 2:eta 2 arrangement of the dioxygen molecule show that the most favorable orientation of O2 is such that the two long Cu-N coordination bonds are perpendicular to the plane formed by the two metal atoms and O2. This arrangement leads to pentacoordinated coppers with a distorted square pyramidal geometry. The molecular electrostatic potential maps of the complexes exhibit a potential well located close to the peroxo anion midbond. The dependence of the energy and of the molecular electrostatic maps on the precise orientation and location of the imidazole rings has been investigated. These results, which show the important role played by the third remote imidazole ligand, are discussed in relation with the first step of tyrosinase-mediated phenol oxidation.