Ribonucleotide reductase during catalysis

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

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. 

From these data it seems feasible that a Co(II)-species is generated during catalysis, and that homolysis of the Co—C-bond is a prerequisite for enzyme catalysis in ribonucleotide reductase. However, the kinetics of appearance of the Co(II)-signal indicates that the rate of formation of Co(II) is much slower than either the rate of ribonucleotide reduction... 

Lactobacillus leichmanii ribonucleotide reductase has a molecular weight of 76 000, with a single polypeptide chain of about 690 amino acids. The large size of the apoenzyme probably reflects the need for it to have sites to interact with the coenzyme, a dithiol, a substrate and allosteric effectors. A transient radical species was observed during catalysis. 

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