4g38

Mutational analysis of sulfite reductase hemoprotein reveals the mechanism for coordinated electron and proton transfer

spinqualitylabelsall models 4g38, resolution 1.56Å Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image   Print3D Structural highlights 4g38 is a 1 chain structure with sequence from Escherichia coli k-12. Full crystallographic information is available from OCA. For a guided tour on the structure components use FirstGlance. Ligands: K, PO4, SF4, SRM Related: 1aop, 4g39 Gene: b2763, cysI, JW2733 (Escherichia coli K-12) Activity: Sulfite reductase (NADPH), with EC number 1.8.1.2 Resources: FirstGlance, OCA, RCSB, PDBsum Function [CYSI_ECOLI] Component of the sulfite reductase complex that catalyzes the 6-electron reduction of sulfite to sulfide. This is one of several activities required for the biosynthesis of L-cysteine from sulfate.[HAMAP-Rule:MF_01540] Publication Abstract from PubMed Sulfite reductase catalyzes the six-electron reduction of sulfite to sulfide. The active site, found in the hemoprotein subunit (SiRHP), sits on the distal face of a negatively charged porphyrinoid called siroheme whose central iron atom is coupled to a proximal Fe(4)S(4) cluster. Four positively charged amino acids are positioned around the active site cavity. Together, these two arginines (R83 and R153) and two lysines (K215 and K217) mitigate the negative charge on the siroheme macrocycle. They also serve as a cage around the distally bound anion that tightens when substrate binds and an active site loop clamps down. Structures of native SiRHP point to these amino acids as being important, but their specific roles are ill-defined. Here, we have altered those four active site amino acids and one amino acid on the flexible loop (N149) to probe their roles in SiRHP activity. None of these positively charged residues is required for electron transfer, but only R83S and N149W variants can produce a fully reduced product. By measuring the electrons used per unit of reduced sulfur released, we show that K215, R153, and K217 are responsible for intermediate and late proton transfers, whereas N149 and R153 play a role in the structure of the flexible loop that controls anion binding and release. R83 is primarily responsible for siroheme binding. Together, the activities and structures of these variants reveal specific roles for each in anion binding and in coupled proton transfer that facilitates electron transfer. Mutational Analysis of Sulfite Reductase Hemoprotein Reveals the Mechanism for Coordinated Electron and Proton Transfer.,Smith KW, Stroupe ME Biochemistry. 2012 Nov 29. PMID:23153334 From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.

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