Cobalamin, adenosyl coenzyme

Figure 1 Schematic representation of the molecular structure, numbering of atoms, and designations of pyrrole rings of cobalamins. R = Me is methyl B12 R = Ado is adenosyl-cobalamin (coenzyme B12) X = CN is cyanocobalamin (vitamin B12). Five-membered rings are labeled A-D, and the amide side-chains are labeled a-g.

Although numerous enzymatic reactions requiring vitamin B12 have been described, and 10 reactions for adenosylcobalamin alone have been identified, only three pathways in man have so far been recognized, one of which has only recently been identified (PI). Two of these require the vitamin in the adenosyl form and the other in the methyl form. These cobalamin coenzymes are formed by a complex reaction sequence which results in the formation of a covalent carbon-cobalt bond between the cobalt nucleus of the vitamin and the methyl or 5 -deoxy-5 -adenosyl ligand, with resulting coenzyme specificity. Adenosylcobalamin is required in the conversion of methylmalonate to succinate (Fig. 2), while methylcobalamin is required by a B12-dependent methionine synthetase that enables the methyl group to be transferred from 5-methyltetrahydrofolate to homocysteine to form methionine (Fig. 3). 

S-Methylmalonyl-CoA mutase (EC 5.4.99.2) is a deoxyadenoxyladen-osylcobalamin-dependent enzyme of mitochondria required to catalyze the conversion of methylmalonyl-CoA to succinyl-CoA. A decrease in the activity of methylmalonyl-CoA mutase leads to the urinary excretion of large amounts of methylmalonic acid (C22). The biochemical lesion may be at the mutase level due to an abnormality of apoenzyme protein or an inability to elaborate the required coenzyme form of vitamin B12> i.e., adenosyl-cobalamin. In rare cases the abnormality may be due to an inability to convert the d form of methylmalonyl-CoA mutase to the l form as a result of a defective racemase (EC 5.1.99.1) (Kll). In patients, the nature of the abnormality can be determined by tissue culture studies (D13) and by clinical trial, since patients with a defect in adenosylcobalamin production will show clinical improvement when treated with very large doses of vitamin B12 (Mil). 

Cobalt accepts a methyl group from methyl-tetrahydrofolate, forming methyl Co +-cobalamin. Transfer of the methyl group onto homocysteine results in the formation of Co+-cobalamin, which can accept a methyl group from methyl-tetrahydrofolate to reform methyl Co +-cobalamin. However, except under strictly anaerobic conditions, demethylated Co+-cobalamin is susceptible to oxidation to Co +-cobalamin, which is catalyticaUy inactive. Reactivation of the enzyme requires reductive methylation, with S-adenosyl methionine as the methyl donor, and a flavoprotein linked to NADPH. For this reductive reactivation to occur, the dimethylbenzimidazole group of the coenzyme must be displaced from the cobalt atom by a histidine residue in the enzyme (Ludwig and Matthews, 1997). 

Ethanolamine ammonia lyase. EAL converts ethanol-amine to acetaldehyde, with loss of ammonia. EAL depends upon adenosylcobamides, such as coenzyme B12 (3), but a range of other adenosylcobamides are also accepted as cofactors, while cobalamins with /3-ligands other than the 5 -deoxy-5 -adenosyl group (of AdoCbl) are inhibitors. Active EAL is multimeric and has an apparent molecular mass of about 560 600kDa. Similar to the mechanism of DD, a radical mechanism is proposed for the isomerization of the vicinal amino-alcohol substrates (ethanolamine, (/f)- and (5)-aminopropanol) by EAL, starting with the abstraction of an H atom from the C-1 position of the substrates. 

The coenzyme form of pantothenic acid is coenzyme A and is represented as CoASH. The thiol group acts as a carrier of acyl group. It is an important coenzyme involved in fatty acid oxidation, pyruvate oxidation and is also biosynthesis of terpenes. The epsilon amino group of lysine in carboxylase enzymes combines with the carboxyl carrier protein (BCCP or biocytin) and serve as an intermediate carrier of C02. Acetyl CoA pyruvate and propionyl carboxylayse require the participation of BCCP. The coenzyme form of folic acid is tetrahydro folic acid. It is associated with one carbon metabolism. The oxidised and reduced forms of lipoic acid function as coenzyme in pyruvate and a-ketoglutarate dehydrogenase complexes. The 5-deoxy adenosyl and methyl cobalamins function as coenzyme forms of vitamin B12. Methyl cobalamin is involved in the conversion of homocysteine to methionine. 

Ribonucleotide reductases with an absolute requirement for adenosyl-cobalamin (Fig. 1) as a coenzyme have been demonstrated only in microorganisms (7). These reductases may be classified into two groups based on the nature of the nucleotide substrate utilized ribonucleoside triphosphate reductases, which act upon the ribonucleoside triphosphates... 

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. 

Adenosyl-cobalamine catalyzes hydrogen shifts as a special isomerisation reaction. With exception of reduction of ribonucleotides the H-shift occurs intramolecularly. Methyl-cobalamine and tetrahydrofolic add are the coenzymes in methylating homocysteine to methionine. 

The cooperation between the coenzyme and the residues within the active sites dictates the catalytic reaction and the substrate specificity, leading to the well-known versatility of PLP chemistry, unparalleled by any other coenzyme. Recently, the catalytic versatility of the PLP-dependent enzymes has been found to be expanded and modulated by the copresence of other coenzymes, as heme, S-adenosyl methionine (SAM), and cobalamine. 

Two mechanistic aspects of the cobalamin-dependent methyltransferases distinguish them from adenosyl-cobalamin-dependent enzymes. No methylcobalamin synthetases are known. Methylcobalamin is produced at enzymatic sites as an intermediate in the primary overall reaction. Second, methylcobalamin does not function as a coenzyme in the sense that it assists in catalysis to date, it is instead a catalytic intermediate in its enzymatic reactions. 

It is not known if all archaea contain adenylated corrinoids or not, however, because most archaea contain a corrinoid adenosyl transferase (CobA) orthologue it is presumed that the corrin biosynthetic intermediates are adenylated. The archaeal ATP Co(I)rrinoid adenosyltransferase from M. mazei strain Gol has been cloned and characterized. Unlike the bacterial enzyme, the M. mazei CobA was found to prefer cobalamin as a substrate over cobinamide, have increased selectivity for ATP over other nucleotides, and was able to utilize 2-deoxynucleotides (dCTP) as well as ribonucleotides. " As it is the methylated form of cobalamin, not the adenylated form, that is involved in methanogenesis, a possible role of the adenylated form of the coenzyme has not been determined. 

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