C coupling electron transfers and substrate

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C. Coupling Electron Transfers and Substrate Activation Electron transfers are key steps in many enzymatic reactions involving the oxi- dation or reduction of a bound substrate. Relevant examples include cytochrome c oxidase (Oz ~ 2H z O) and nitrogenase (N z ~ 2NH 3 ). To reinforce the claim that electron-transfer steps are of widespread importance, several other redox systems, representative of diverse metabolic processes, will be mentioned here. Xanthine oxidase (275 kDa; <Xz dimer) catalyzes the two-electron ox- idation 37-39 of xanthine to uric acid (Equation 6.7). xanthine + H 2 0 o ~ H HN N ~ I }-OH o N N H uric acid (6.7)
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330 o Fe- Fe: 14A o Fe- Fe: 16A o Fe- Fe: 14A o Fe - (Mg)2 : 21 A o Mg - Mg : 7A in dimer o Mg - Mg : 13A from (BChl)2 to BChl o BChl - BPh centers: 11 A o Fe - Q center: 7A • • • • • • • • • • • • • • Figure 6.15 Structure of the Rps. viridis photosynthetic reaction-center cofactors. The black dots delineate the outward (0)- and inward (I)-facing portions of the membrane. Adapted from Reference 36. Q A Fe ~ Q s + BPh BPh + BChl " (BChl)2 Q-pool e- and cyt be, complex Figure 6.16 Electron flow in the bacterial photosynthetic reaction center.
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[2Fe-2S1 11 o~ 14 ± 4A [2Fe-2S], 331 XH + Hp o 11 ± 3A MaCa X-OH + 2W Figure 6.17 Representation of the cofactors in one subunit of xanthine oxidase. This enzyme, which plays a prominent role in the biodegradation of purines, is the target of drugs administered to patients suffering from gout (joint inflam- mation, due to precipitation of sodium urate). Figure 6.17 displays the cofactors in a subunit: a Mo-pterin, termed MoCo; two [2Fe-2S] centers; and one FAD. The binuclear iron-sulfur sites serve to shuttle electrons between the reduced substrate (XH) and O 2 , The first step in the biosynthesis of DNA involves the reduction of ribonu- cleotides (Equation 6.8) catalyzed by ribonucleotide reductase. 4o The E. coli enzyme is an (X2{32 tetramer composed of a B1 protein (160 kDa) and a B2 protein (78 kDa). The BI protein (a dimer) contains redox-active dithiol groups, binding sites for ribonucleotide substrates, and regulatory binding sites for nu- cleotide diphosphates. Protein B2, also a dimer, possesses a phenolate radical (Tyr-122) that is stabilized by an antiferromagnetically coupled binuclear iron center (Figure 6.18). This radical is essential for enzyme activity, and is ~ 10 A from the protein-B lIprotein-B2 interface. Hence it cannot directly participate in an H-atom abstraction from the substrate (bound to protein B 1). Instead, the x- ray structure of the B2 protein 41 suggests that a long-range electron transfer from the Tyr radical to a residue (perhaps Trp-48) on the B 1 protein is operative during enzyme turnover. ®:v-a-v"" OH OH _ __ --. ®:V-O-CQH2 0 base ribonucleotide reductase OH H (6.8)
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332 -0-0 0--( Glu-238 TY'::84_<~O~OYO N 1 oyoqN GI"~204 HiS- 118 -C""-, (YHiS-241 / Glu-115 N N I I H H Figure 6.18 Schematic of the binuclear iron center and Tyr-122 radical in the B2 protein of E. coli ribonucleotide reductase.
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