A-Glycyl radical

Benzylsuccinate synthase has an amino acid sequence homologous to that of pyruvate formate lyase (p. 799 -801), contains a glycyl radical, and is activated in a manner similar to activation of pyruvate formate lyase. Propose a mechanism for formation of benzylsuccinate. See Krieger et al.383... 

Logan, D. T., Andersson, J., Sj berg, B.-M., and Nordlund, P., 1999, A glycyl radical site in the crystal structure of a class III ribonucleotide reductase. Science 283 1499nl504. 

Recently, a glycerol dehydratase was discovered in the anaerobic bacterium Clostridium butyricum, whose active site contains a glycyl radical formed by the action of the 5 -deoxyadenosyl radical on a specific glycine residue of the protein (20). The 5 -deoxyadenosyl radical is generated not from coenzyme Bi2, but by one-electron reduction of the stmcturally much simpler molecule 5-adenosylmethionine (SAM), named poor man s B12 by Barker. Hence, this glycerol dehydratase performs the same reaction, probably with a similar pathway, to the coenzyme B 12-dependent glycerol dehydratase. 

Activity depends on the presence of a glycyl radical on the large component 163... 

Despite the advances, little is known about the mechanism of ribonucleotide reduction by class III enzymes. However, it is very likely that the reaction proceeds by a radical mechanism, similar to that of class I and II enzymes (Scheme 10-2) [22, 53], As a matter of fact, the class III enzyme is also a radical enzyme with a glycyl radical absolutely required for activity. Furthermore, mechanism-based inhibitors of class I and II RNRs, such as nucleotide analogs carrying azido, chloro or fluoro groups at the 2 position, are also excellent inhibitors of class III enzymes [54]. 

In class II RRs, which are homodimeric enzymes, the radical species is generated by adenosyl cobalamine. They can fimction either aerobically or anaerobically and they are not found in higher organisms [78]. Anaerobic class III RRs have a a2 2 quaternary structure, they contain an FeS cluster and a glycyl radical and belong to the radical enzyme family described below. No structure is available for class III RRs [79]. 

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