Bi2-Dependent Ribonucleotide Reductase

About 10 coenzyme B -dependent enzymes are now known (reviewed in References 13,14, and 76 see Table 1) four carbon skeleton mutases (methylmalonyl-CoA mutase (MMCM), glutamate mutase (GM), methylene glu-tarate mutase (MGM), isobutyryl-CoA mutase (ICM) ), diol dehydratase (DD), glycerol dehydratase, ethanol-amine anunonia lyase (EAL), two amino mutases, and Bi2-dependent ribonucleotide reductase. The coenzyme Bi2-dependent enzymes are unevenly distributed in the living world, and MMCM is the only enzyme that is also indispensable in human metabolism. ... 

Brown, K. L., Li, J. (1998) Activation parameters for the carbon-cobalt bond homolysis of coenzyme B12 induced by the Bi2-dependent ribonucleotide reductase from Lactobacillus leichmannii, J. Am. Chem. Soc. 120, 9466-9474. 

Finke has established a chemical precedent for the proposed mechanism for thiyl radical formation in the Bi2-dependent ribonucleotide reductase. Thermolysis of AdoCbl with excess /3-mercaptoethanol under anaerobic conditions yielded 90% Co—C homolysis and 10% heterolysis, as determined by product characterization. The homolysis products were 5 -deoxyadenosine, cob(II)alamin, and the disulfide 2,2 -dithiodiethanol. Kinetic studies established a zero-order dependence on thiol at high [RSH], consistent with rate-limiting Co—C homolysis and formation of a discrete Ado- that subsequently abstracts an H atom from the thiol. Consequently, the... 

With the exception of the cobamide-dependent ribonucleotide reductase, the 5 -deoxyadenosyl radical is indicated to be the direct activator of the substrate. In ribonucleotide reductases, a protein-derived thiyl radical takes this role (see the following text). Accordingly, two vmusual ftinctions would be given to the apoen-zyme in the course of the coenzyme Bi2-catalyzed enzymatic reactions first, it would have to contribute to the activation of the bound coenzyme toward the (reversible) formation of radicals, then it would have to help harness the reactivity of the radical intermediates generated (64). 

The present chapter reviews applications in biocatalysis of the ONIOM method. The focus is on studies performed in our research group, in most cases using the two-layer ONIOM(QM MM) approach as implemented in Gaussian [23], The studied systems include methane monooxygenase (MMO), ribonucleotide reductase (RNR) [24, 25], isopenicillin N synthase (IPNS) [26], mammalian Glutathione peroxidase (GPx) [27,28], Bi2-dependent methylmalonyl-CoA mutase [29] and PLP-dependent P-lyase [30], These systems will be described in more detail in the following sections. ONIOM applications to enzymatic systems performed by other research groups will be only briefly described. 

Much less is known about the final step of the pathway, specifically the Bi2-dependent conversion of o( into queuosine. In his review of queuosine biosynthesis, Iwata-Reuyl draws a parallel between the reduction of o( and the reaction performed by ribonucleotide reductases and proposes a mechanism involving a thiyl radical and redox-active disulfide (Figure 36). 

The homolytic cleavage of the Co - C bond of the protein-boimd organo-metallic cofactor AdoCbl (2) is the initial step of the coenzyme Bi2-catalyzed enzymatic reactions. Halpern quoted that adenosyl cobamides can be considered as reversibly functioning sources for organic radicals [119]. A neutral aqueous solution of 2 is remarkably stable with a half-Ufe of 10 s (in the dark at room temperature), but decomposes, mainly with the homolysis of the Co-C bond, at higher temperatures [119,123]. The coenzyme B12-catalyzed enzyme reactions occur with maximal rates of approximately 100 s [173,239]. Rapid formation of Co(ll)corrins occurs only with addition of substrate to a solution of holoenzyme (or of apoenzymes and 2), as demonstrated in most of the known coenzyme Bi2-dependent enzymes, e.g., in methyl-malonyl-CoA mutase [121], glutamate mutase [202] and ribonucleotide reductase [239]. 

The metal dependence of ribonucleotide reductase is cobalt as vitamin Bi2 (prokaryotes) and iron (eukaryotes), with some evidence for manganese as cofactor [66]. The reader is referred to Ref. [66] for a recent discussion on the mechanism of these interesting enzymes. In essence, the reaction can be described ... 

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