Enzymes coenzyme B12

Alternative mechanisms for the OH transfer process in enzyme-coenzyme B12-catalysed dehydration of 1,2-dihydroxyethane, to give acetaldehyde and water, have been explored using ah initio MO calculations.75 Transfer within an (HOCH— CH2OH) radical was ruled out because the activation energy is too high, and no intermediate bridge structure could be found to facilitate conversion of 1,2-dihydroxyethyl cation (if it could be formed from the radical) to 2,2-dihydroxyethyl cation. The radical cation (HOCH—CH20H2)+ transformed rapidly to a stable... 

Vitamin B12 (cyanocobalamin) [1] was first isolated more than 50 years ago [2] when it was found to be associated with the prevention of pernicious anaemia in humans [3]. Later, Barker discovered the principal, naturally-occurring form of vitamin B12 to be 5 -deoxyadenosylcobalamin [4], This compound has since become known as coenzyme B12, reflecting the fact that it is required by a number of enzymes in order for them to properly perform their biological function. Each enzyme-coenzyme B12 partnership has been found to facilitate a rearrangement in which a substrate hydrogen atom and functional group (X) on adjacent carbon atoms apparently change places [5,6] ... 

Naturally, the biosynthesis of cobalamins themselves require delivery of Co ions at a particular point in the reaction scheme. Cobaltochelatase catalyzes the ATP-dependent insertion of Co11 into the corrin ring during the biosynthesis of coenzyme B12 in Pseudomonas denitrifleans. Cobaltochelatase is a heterodimeric enzyme (140 KDA and 450 KDA subunits each inactive in isolation), and the two components have been isolated and purified to homogeneity.1119 The reaction product is divalent cobyrinic acid, demonstrating that hydrogenobyrinic acid and its diamide (255) are precursors of AdoCbl. 

Vitamin Bj2 is converted to co-enzyme B12 by extracts from microorganisms supplemented with ATP Coenzyme B12 is associated with many biochemical reactions-... 

Oxidation of unsaturated fatty acids requires two additional enzymes enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase. Odd-number fatty acids are oxidized by the /3-oxidation pathway to yield acetyl-CoA and a molecule of propionyl-CoA This is carboxylated to methylmalonyl-CoA, which is isomerized to succinyl-CoA in a reaction catalyzed by methylmalonyl-CoA mutase, an enzyme requiring coenzyme B12. 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

The biochemistry of coenzyme B12 generally revolves around either mutase enzyme activity, involving functional group migration, notably by stereospecific 1,2-shifts (Scheme 2.8), or methylation by methionine synthetase. The general mechanism for the mutase activity is a radical-based one and has been established by EPR spectroscopy to be of the general form shown in Scheme 2.9. 

Table 1 Rearrangement Reactions Catalyzed by Coenzyme B12 (AdoCbl)-Dependent Enzymes...

Answer One of the enzymes necessary for the conversion of propionate to oxaloacetate is methylmalonyl-CoA mutase (see Fig. 17-11). This enzyme requires as an essential cofactor the cobalt-containing coenzyme B12, which is synthesized from vitamin B12. A cobalt deficiency in animals would result in coenzyme B12 deficiency. 

Answer The catabolism of the carbon skeletons of valine, isoleucine, and methionine is impaired because of the absence of a functional methylmalonyl-CoA mutase. This enzyme requires coenzyme B12 as a cofactor, and a deficiency of this vitamin leads to elevated methylmalonic acid levels (methylmalonic acidemia). The symptoms and effects of this deficiency are severe (see Table 18-2 and Box 18-2). 

A troublesome feature of this mechanistic interpretation is the absence of direct supporting evidence that the Co-C bond in coenzyme B12 (whose dissociation energy has not yet been determined) is sufficiently weak that facile homolysis under the mild conditions of the enzymic reactions is a plausible process. In fact, alkylcobalamins, including coenzyme B12, exhibit considerable thermal stability and typically do not decompose at measurable rates, in the absence of light or reagents such as 02, until fairly elevated temperatures ( 200°C for methylcobalamin) (4). Among the possible interpretations of this behavior are ... 

Marsh EN (1999) Coenzyme B12 (cobalamin)-dependent enzymes. Essays in Biochemistry 34, 139-54. 

The metabolically important functions of the Bn-derivatives are directly concerned either with enzymatically controlled organometalhc reactions involving protein-bound adenosylcobamides (such as coenzyme B12, (3)), or methyl-Co -corrinoids (such as methylcobalamin, (4)), or with enzyme-controlled redox reactions. Studies on the underlying biologically relevant organometalhc chemistry of the Bi2-coenzymes in homogeneous (protic) solution, as well as the characterization of the enzymatic processes themselves have attracted considerable interest. ... 

The known coenzyme Bi2-dependent enzymes all perform chemical transformations in enzymatic radical reactions that are difficult to achieve by typical organic reactions. Homolytic cleavage of the Co bond of the protein-bound coenzyme B12 (3) to a 5 -deoxy-5 -adenosyl radical (9) and cob(n)alamin (5) is the entry to reversible H-abstraction reactions involving the 5 -position of the radical (9). Indeed, homolysis of the Co bond is the thermally most easily achieved transformation of coenzyme B12 (3) in neutral aqueous solution (with a homolytic (Co-C)-BDE of about 30 kcal mol ). However, to be relevant for the observed rates of catalysis by the coenzyme B12-dependent enzymes, the homolysis of the Co-C bond of the protein-bound coenzyme (3) needs to be accelerated by a factor of about 10 , in the presence of a substrate. Coenzyme B12 might then be considered, first of aU, to be a structurally sophisticated, reversible source for an alkyl radical, whose Co bond is labihzed in the protein-bound state (Figure 8), and the first major task of the... 

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