Adenosylcobalamin-dependent enzymes reactions

Golding, B. T., and Rao, D. N. R., 1986, Adenosylcobalamin-dependent enzymic reactions. In Enzyme Mechanisms (M. L. Page and A. Williams, eds.), London Royal Society of Chemistry, pp. 404n428. 

This step is the culmination of a reaction sequence in which propionyl-CoA, a toxic metabolite derived from the degradation of fats, is removed from circulation. Carboxylation of propionyl-CoA gives (5)-methylmalonyl-CoA, which is epimerized to (/ )-methylmalonyl-CoA. Conversion of the (7 )-isomer to succinyl-CoA allows further metabolism via the Krebs cycle [74]. Methylmalonyl-CoA mutase is also the only adenosylcobalamin-dependent enzyme known to participate in human metabolism, and as such has received significant study [30, 37, 38]. 

Homolytic scission of the Co—C5 bond in adenosylcobalamin is brought about thermally at high temperatures (>80 °C), by photolysis, or by the actions of adenosylcobalamin-dependent enzymes. The fate of the 5 -deoxyadenosyl radical depends on the reaction conditions. In an enzymatic site, the radical most often abstracts a hydrogen atom from a substrate to initiate a radical rearrangement process in the substrate. In the absence of a substrate or/and enzyme, the 5 -deoxyadenosyl radical faces alternative fates. In anoxic conditions, photolysis of adenosylcobalamin to the transient 5 -deoxyadenosyl radical ends in cyclization to 5, 8-cyclo-adenosine, as shown in Figure 5, in what must be a multistep process." In the presence of oxygen gas, the 5 -deoxyadenosyl radical reacts with oxygen to form adenosine-5 -aldehyde. ... 

Two classes of dioxygen-independent enzymatic reaction are now recognized to proceed through radical-based mechanisms, those catalysed by S-adenosylmethionine/iron-sulphur enzymes, and the adenosylcobalamine-dependent enzymes, as recently reviewed by Frey (2001). 

In mammals, there are only three vitamin B12 -dependent enzymes methionine synthetase, methylmalonyl CoA mutase, and leucine aminomutase. The enzymes use different coenzymes methionine synthetase uses methylcobal-amin, and cobalt undergoes oxidation during the reaction methylmalonyl CoA mutase and leucine aminomutase use adenosylcobalamin and catalyze the formation of a 5 -deoxyadenosyl radical as the catalytic intermediate. 

Class II RTPR has in common with other coenzyme Bi2-dependent enzymes the use of the 5 -deoxyade-nosyl radical from adenosylcobalamin as a radical initiator. The elimination of water from the 3 -radical intermediate might in part be analogous to the elimination of water in the reaction of DDH. The variant function of RTPR can be viewed as foreshadowing the wider functions of the 5 -deoxyadenosyl radical in other radical reactions, in particular the radical SAM enzymes. ... 

Numerous analogs of adenosylcobalamin have been tested for their ability to replace or to inhibit the action of the coenzyme in the adenosyl-cobalamin-dependent ribonucleotide reductase reaction the enzyme from L. leichmannii has been used in most of these studies. Kinetic studies have been used in most investigations of analog-enzyme interactions and thus the interpretation of data regarding the affinity of analogs for the reductase is subject to the limitations imposed on kinetic studies of a complex reaction. 

The means by which enzymes facilitate the homolytic cleavage of the Co-C5 bond has been addressed in detail in studies of MCM. This process can be kinetically monitored by the spectral change from Co(III) in coenzyme B12 to that of cob(II)alamin. This spectral change depends on the addition of the substrate to the complex of MCM and adenosylcobalamin. The kinetic barrier to bond cleavage is lowered by 17 kcalmol . " " Moreover, the rate of this change displays a kinetic isotope effect of >20 (Fh/F ) when the deuterated substrate is employed. " It was concluded that Co-C5 bond cleavage and hydrogen abstraction from the substrate are kinetically coupled. This effect has been reported for other coenzyme Bj2-dependent reactions as well. 

Bi2 dependent ethanolamine ammonialyase catalysed reaction. Binding of adenosylcobalamin to the protein causes a slow conformational change and the reactions of 2-amino-alcohols proceed by formation of an enzyme-substrate complex followed by radical and cobalt(ii) formation and finally disappearance of the cobait(ii). Rate differences with 2-aminoethanol and L-2-aminopropanol arise mainly in this latter step. 

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