5 -Deoxyadenosyl radical

Figure 13.18 S-adenosyl methionine (SAM), a source of 5 -deoxyadenosyl radicals. SAM binds to the subsite iron (in blue) of the reduced [4Fe-4S] cluster via its a-aminocarboxylate group. The 5 -deoxyadenosine radical is formed by electron transfer which occurs either (a) by outer-sphere mechanism or (b) by p-sulfide alkylation followed by homolytic cleavage of the 5 -S-CH2Ado bond. In both cases, methionine is released. (From Fontecave et al., 2004. Copyright 2004, with permission from Elsevier.)...

It is believed that the reaction starts with homolytic cleavage of the cobalt-carbon bond (at a cost of perhaps 100 kJ mol-1)8 to yield a Co(ll) atom and a 5 -decxy-adenosyl radical. This radical then abstracts a hydrogen atom (in Eq. 19.35 from the methyl group). Migration of the —GO)SR group takes place, followed by return of the hydrogen atom from 5 -deoxyadenosine to the substrate. This regenerates the 5 -deoxyadenosyl radical, which can recombine with the Co( I) atom to form the coenzyme. 

A second group of ribonucleotide reductases (Class II), found in many bacteria, depend upon the cobalt-containing vitamin B12 coenzyme which is discussed in Section B. These enzymes are monomeric or homodimeric proteins of about the size of the larger a subunits of the Class I enzymes. The radical generating center is the 5 -deoxyadenosyl coenzyme.350 364 365... 

Another important experiment398 showed that lsO from [2-lsO]propanediol was transferred into the 1 position without exchange with solvent. Furthermore, lsO from (S)-[l-lsO]propanediol was retained in the product while that from the (R) isomer was not. Thus, it appears that the enzyme stereospecifically dehydrates the final intermediate. From these and other experiments, it was concluded that initially a 5 -deoxyadenosyl radical is formed via Eq. 16-35. This radical then abstracts the hydrogen atom marked by a shaded box in Eq. 16-36 to form a substrate radical and 5 -deoxy-adenosine. One proposal, illustrated in Eq. 16-36, is that the substrate radical immediately recombines with... 

According to this mechanism the Co(II) of the B12r formed in Eq. 16-35 has no active role in the isomerization and does not form an organocobalt intermediate as in Eq. 16-36. Its only role is to be available to recombine with the 5 -deoxyadenosyl radical at the end of the reaction sequence. Support for this interpretation has been obtained from study of model reactions and of organic radicals generated in other ways.400... 

It is thought that the activator becomes reduced by the excess electron of the Fe/ S cluster and then cleaves to produce a 5 -deoxyadenosyl radical, which in turn produces the glycyl radical [52], So the evidence of the redox-catalytic nature of the Fe/S cluster is not very strong, as there is no indication of direct coordination of the activator or its products to the cluster. 

There may be common themes in the role of protein-coenzyme contacts in these B -dependent enzymatic processes. In particular, these contacts could alter the relative stability of the Co(III)—R, Co(II), and Co(I) states to enhance reactivity. For coenzyme B 12-dependent enzymes, the deoxyadenosyl radical generates a substrate-derived radical, either directly or via a radical chain mechanism through the intermediacy of a protein-side-chain-based radical, such as S of cysteine or O of tyrosine. This protein-bound substrate-derived radical then undergoes rearrangement, possibly assisted by protein contacts. Thus, cofactor-protein contacts are probably very important in the activation of the Co—C bond, in altering the Co redox potentials, and in assisting in the rearrangements. 

The rearrangement proceeds through a stepwise process (Figure 3) initiated by the key homolysis step (i). Of the two radicals formed, cob(II)alamin (B12r) and 5 -deoxyadenosyl (Ado ), only Bi2r is relatively long-lived. It is believed that, relatively soon after it is formed, Ado abstracts the H from the substrate (SH). The EPR studies [10,58 (see Ref. 12 therein),69] reveal that the radical, S, remains close to the Co(II) center. 

Scheme 1.6.9. Generation of the allyl analog 35 of the 5 -deoxyadenosyl radical 31.

A widely accepted mechanism of coenzyme Bir dependent rearrangements encompasses, as the initial step, the homolytic cleavage of the carbon-cobalt bond to generate the 5 -deoxyadenosyl radical. The thermodynamic and kinetic aspects of this and related processes involving the homolysis of transition metal-carbon bonds are discussed. 

V. GENERATION OF RADICAL INTERMEDIATES A. The Difficulty with Cleaving the S-5 -Deoxyadenosyl Bond... 

There are no direct chemical models for cleaving a sulfiir-carbon bond using iron-sulfur clusters however, several suggestions have been advanced to explain how a S -deoxyadenosyl S -radical might be generated from SAM. One of the earliest proposals implicated an... 

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