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7-Hydroxymethyl chlorophyll a reductase (HCAR) catalyzes the second half-reaction in chlorophyll b to chlorophyll a conversion. HCAR is required for the degradation of light-harvesting complexes and is necessary for efficient photosynthesis by balancing the chlorophyll a/b ratio. Reduction of the hydroxymethyl group uses redox cofactors [4Fe-4S] cluster and FAD to transfer electrons and is difficult because of the strong carbonoxygen bond. Here, we report the crystal structure of Arabidopsis HCAR at 2.7-Å resolution and reveal that two [4Fe-4S] clusters and one FAD within a very short distance form a consecutive electron pathway to the substrate pocket. In vitro kinetic analysis confirms the ferredoxin-dependent electron transport chain, thus supporting a proton-activated electron transfer mechanism. HCAR resembles a partial reconstruction of an archaeal F420-reducing [NiFe] hydrogenase, which suggests a common mode of efficient proton-coupled electron transfer through conserved cofactor arrangements. Furthermore, the trimeric form ofHCARprovides a biological clue of its interaction with light-harvesting complex II. [ABSTRACT FROM AUTHOR]

Using Cu 2 O nanowires as sacrificial templates, SnO 2 nanotubes have been synthesized via wet chemical process. The morphology and the fine crystalline structure of the produced SnO 2 nanotubes have been characterized. None of any oxidizing agents or acid was added into the reaction system, which makes the mechanism much different from the known ones based on acidic or oxidative/redox etching of metal oxide templates. Accordingly one discusses the growth mechanism and attributes it to the hydrolysis of Sn 4+ , to form SnO 2 , and the coordination of CuCl with Cl − , to form soluble [CuClx] 1−x , eliminating Cu 2 O nanowires. [ABSTRACT FROM AUTHOR]

Members of glycosyltransferase family 75 (GT75) not only reversibly catalyze the autoglycosylation of a conserved arginine residue with specific NDP-sugars but also exhibit NDPpyranose mutase activity that reversibly converts specific NDP-pyranose to NDP-furanose. The latter activity provides valuable NDP-furanosyl donors for glycosyltransferases and requires a divalent cation as a cofactor instead of FAD used by UDP-D-galactopyranose mutase. However, details of the mechanism for NDP-pyranose mutase activity are not clear. Here we report the first crystal structures of GT75 family NDPpyranose mutases. The novel structures of GT75 member MtdL in complex with Mn2+ and GDP, GDP-D-glucopyranose, GDPL-fucopyranose, GDP-L-fucofuranose, respectively, combined with site-directed mutagenesis studies, reveal key residues involved in Mn2+ coordination, substrate binding, and catalytic reactions. We also provide a possible catalytic mechanism for this unique type of NDP-pyranose mutase. Taken together, our results highlight key elements of an enzyme family important for furanose biosynthesis. [ABSTRACT FROM AUTHOR]