Proton transfer, ribonucleotide reductase

Ribonucleotide reductase differs from the other 5 -deoxyadenosyl-cobalamin requiring enzymes in a number of respects. Hydrogen is transferred from coenzyme to the C2-position of the ribose moiety without inversion of configuration. Also since lipoic acid functions in hydrogen transfer, exchange with solvent protons takes place. Furthermore, exchange between free and bound 5 -deoxyadenosylcobalamin occurs rapidly during catalysis. Evidence for a Co(I)-corrin as an intermediate for this reduction is presented in our section on electron spin resonance. 

Stubbe J, Nocera DG, Yee CS, Chang MCY. Radical initiation in the class I ribonucleotide reductase long-range proton-coupled electron transfer Chem Rev 2003 103 2167-201. 

Figure 7. Proposed proton-coupled electron transfer (PCET) pathway between the R2 and R1 subunits of the E. coli ribonucleotide reductase (RNR) complex. The conserved amino acids are shown schematically E. coli numbering).

Figure 25.11 Ribonucleotide reductase mechanism. (1) An electron is transferred from a cysteine residue on R1 to a tyrosine radical on R2. generating a highly reactive cysteine thiyl radical. (2) This radical abstracts a hydrogen atom from C-3 of the ribose unil. (3) The radica at C-3 releases OH from the C-2 carbon atom. Combined with a proton from a second cysteine residue, the OH is eliminated as water. (4) A hydride ion is transferred from a third cysteine residue with the concomitant formation of a disulfide bond. (5) The C-3 radical recaptures the originally abstracted hydrogen atom. (6) An electron is transferred from R2 to reduce the thiyl radical, which also accepts a proton. The deoxyribonucleotide is free to leave Rl. The disulfide formed in the active site must be reduced to begin another cycle.

Stubbe, J., Nocera, D. G., Yee, C. S., Chang, M. C. Y., Radical Initiation in the Class I Ribonucleotide Reductase Long range Proton coupled Electron Transfer , Chem. Rev. 2003, 103,2167 2201. 

Type A PCET reactions describe amino acid radical generation steps in many enzymes, since the electron and proton transfer from the same site as a hydrogen atom [188]. Similarly, substrate activation at C-H bonds typically occurs via a Type A configuration at oxidized cofactors such as those in lipoxygenase [47, 48] galactose oxidase [189-191] and ribonucleotide reductase (Y oxidation at the di-iron cofactor, vide infra) [192]. Here, the HATs are more akin to the transition metal mediated reactions of Section 17.3.1 since the final site of the electron and proton are on site differentiated at Ae (redox cofactor) and Ap (a ligand). 

ACP = acyl carrier protein ACPA D = ACPA desat-urase AlkB = octane 1-monooxygenase AOX = alternative oxidase DMQ hydroxylase = 5-demethoxyquinone hydroxylase EXAFS = extended X-ray absorption fine structure spectroscopy FMN = flavin mononucleotide FprA = flavoprotein A (flavo-diiron enzyme homologue) Hr = hemerythrin MCD = magnetic circular dichroism MME hydroxylase = Mg-protophorphyrin IX monomethyl ester hydroxylase MMO = methane monooxygenase MMOH = hydroxylase component of MMO NADH = reduced nicotinamide adenine dinucleotide PAPs = purple acid phosphatases PCET = proton-coupled electron transfer, PTOX = plastid terminal oxidase R2 = ribonucleotide reductase R2 subunit Rbr = rubrerythrin RFQ = rapid freeze-quench RNR = ribonucleotide reductase ROO = rubredoxin oxygen oxidoreductase XylM = xylene monooxygenase. 

In a PCET, the electron does not have to be coupled to a particular proton throughout the entire reaction.A prominent example is the class I ribonucleotide reductase from E. coli, which catalyses the reduction of nucleoside diphosphates (NDPs) to deoxynucleotide diphosphates (dNDPs), the building blocks of deoxyribonucleic acid (DNA). An electron is transferred from a... 

---