5 -deoxy adenosyl-cobalamin

A related cleavage by alkaline cyanide can be viewed as a nucleophilic displacement of the deoxy-adenosyl anion by cyanide. The end product is dicyanocobalamin, in which the loosely bound nucleotide containing dimethyl benzimidazole is replaced by a second cyanide ion. Methyl and other simple alkyl cobalamins are stable to alkaline cyanide. A number of other cleavage reactions of alkyl cobalamins are known.392 393... 

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. 

Only cobalamin present on transcobalamin is available for the cells (for a review, see Quadros 2010), while the function of cobalamin bound to haptocorrin remains to be clarified (for an overview, see Morkbak et al. 2007). Transcobalamin carries both of the two co-enzymes, 5 -deoxy adenosyl-coba-lamin and methyl-cobalamin, and also other forms of the vitamin that can be converted into the coenzymes such as cyanocobalamin and hydroxo-cobalamin... 

A final prosthetic group that can be studied is the phosphate moiety of vitamin 6,2 (S-deoxy adenosyl cobalamin). This group is instrumental for the shifting of hydrogen atoms as part of the mechanism of several enzymes. 

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). 

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. 

Fig. 1. Structural formulae and S5mibols used. Left Cobalamins in their base-on form vitamin B12 (1, R = CN) coenz5mie B12 (2, R = 5 -deoxy-5 -adenosyl) methylcobalamin (3, R = CH3) cob(II)alamin (5, R = absent) aquocobalamin cation (7+, R = H2O) neopentyl-cobalamin (11, R = neopentyl) benzylcobalamin (12, R = benzyl). Right Cobinamides dicyanocobinamide (X = Y = CN) adenosylcob(III)inamide (8+, X = 5 -deoxy-5 -adenosyl, Y = H2O+), Co 8-methyl-cob(III)inamide (9+, X = CH3, Y = H2O+).

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