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The amino acid analogue selenomethionine (SeMet) is shown to be efficiently incorporated into recombinant proteins expressed in Escherichia coli grown in a simple minimal medium without the addition of synthetic amino acids. Furthermore, satisfactory SeMet incorporation is obtained with a methionine-prototrophic strain transformed with commonly used vector systems. As examples, purified tryparedoxin 1 from Crithidia fasciculata, alkylhydroperoxide reductase (AhpC) from Mycobacterium marinum and the 16-kDa antigen from M. tuberculosis are shown to be efficiently labelled with SeMet, using the culture conditions and the host/vector systems described here. Enzymatic analysis reveals no differences between native and SeMet-labelled tryparedoxin 1 enzyme. Both proteins yield crystals under similar conditions. The culture conditions and host vector systems described greatly facilitate selenium-labelling of proteins for 3-D structure determination.

Thiol-dependent hydroperoxide metabolism in parasites is reviewed in respect to potential therapeutic strategies. The hydroperoxide metabolism of Crithidia fasciculata has been characterized to comprise a cascade of three enzymes, trypanothione reductase, tryparedoxin, and tryparedoxin peroxidase, plus two supportive enzymes to synthesize the redox mediator trypanothione from glutathione and spermidine. The essentiality of the system in respect to parasite vitality and virulence has been verified by genetic approaches. The system appears to be common to all genera of the Kinetoplastida. The terminal peroxidase of the system belongs to the protein family of peroxiredoxins which is also represented in Entamoeba and a variety of metazoan parasites. Plasmodial hydroperoxide metabolism displays similarities to the mammalian system in comprising glutathione biosynthesis, glutathione reductase, and at least one glutathione peroxidase homolog having the active site selenocysteine replaced by cysteine. Nothing precise is known about the antioxidant defence systems of Giardia, Toxoplasma, and Trichomonas species. Also, the role of ovothiols and mycothiols reportedly present in several parasites remains to be established. Scrutinizing known enzymes of parasitic antioxidant defence for suitability as drug targets leaves only those of the trypanosomatid system as directly or indirectly validated. By generally accepted criteria of target selection and feasibility considerations tryparedoxin and tryparedoxin peroxidase can at present be rated as the most appealing target structures for the development of antiparasitic drugs.

The thermophile NADH oxidase from Thermus thermophilus, cloned and expressed in Escherichia coli, has been purified to homogeneity and crystallized. Three different crystal forms were found to be suitable for X-ray diffraction analysis. Crystals of the tetragonal form, grown in the presence of 25% polyethylene glycol 4000 and 0.25 M-NaCl at pH 6.6, were chosen for further analysis. These crystals belong to the space group P4(1)(3)2(1)2 with refined lattice constants of a = 94.8 A and c = 49.0 A, indicating a cell content of one monomer per asymmetric unit of the crystal. The crystals diffract to a resolution of 2.2 A.

Fibroblast growth factors (FGFs) constitute a family of at least 20 structurally related heparin-binding polypeptides active in regulating cell growth, survival, differentiation and migration. FGF9, originally discovered as a glia-activating factor, shares 30% sequence identity with other FGFs and has a unique spectrum of target-cell specificity. FGF9 crystallized in the tetragonal space group I4(1), with unit-cell parameters a = b = 151.9, c = 117.2 A. The structure of the glycosylated protein has been refined to an R value of 21.0% with R(free) = 24.8%) at 2.6 A resolution. The four molecules in the asymmetric unit are arranged in two non-crystallographic dimers, with the dimer interface composed partly of residues from N- and C-terminal extensions from the FGF core structure. Most of the receptor-binding residues identified in FGF1- and FGF2-receptor complexes are buried in the dimer interface, with the beta8-beta9 loop stabilized in a particular conformation by an intramolecular hydrogen-bonding network. The potential heparin-binding sites are in a pattern distinct from FGF1 and FGF2. The carbohydrate moiety attached at Asn79 has no structural influence.

The crystal structures of the flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) containing isoforms of NADH oxidase from Thermus thermophilus have been determined by isomorphous and molecular replacement and refined to 2.3 A and 1.6 A resolution with R-values of 18.5% and 18.6% respectively. The structure of the homodimeric enzyme consists of a central 4-stranded antiparallel beta-sheet covered by helices, a more flexible domain formed by two helices, and a C-terminal excursion connecting the subunits. The active sites are located in a deep cleft between the subunits. The binding site of the flavin cofactor lacks the common nucleotide binding fold and is different from the FMN binding site found in flavodoxins.

This chapter explores the development of enzyme sensors based on the enzyme structure. The mechanism of enzyme action, including substrate binding, product formation, electron transfer to oxygen or mediators, can be elucidated and the information used to analyses the possibilities of direct electron transfer between the active site and electrodes. This can be extended to interactions of the enzyme with electrode surfaces, polymer mediators, different solvents, or other enzymes. It is found that immobilization of the enzyme on the surface of the sensor could be optimized and its function and interaction with the sensor improved. The stability and performance of the enzyme could also be improved and the criteria for the selection or design of new and faster mediators defined. Other criteria or tools are needed to enable designed biosensors to become reality. New software capable of analyzing and visualizing the enormous volume of information available from the enzyme structure has to be developed. Data manipulation alone is not always sufficient.