Bi2-dependent Isomerases

The Co—carbon bond in AdoCbl is stable in water, but is inherently labile, with a bond dissociation energy of around 30—35 kcal mol This instability is exploited by the AdoCbl-dependent isomerases to effect radical-based rearrangements which, as pointed out above, are initiated by homolytic cleavage of the Co-carbon bond. In the absence of substrate, the homolysis products are not observed, yet in their presence the homolytic cleavage 

FIGURE 15.6 Structure of P430 at the MCR active site and mechanism of methane formation. The structure was derived from PDB code IHBN. The bound CoM was omitted from the structure to focus on the tetrapyrrole. (From Ragsdale, 2009. Copyright 2009 with permission from Elsevier.) 

FIGURE 15.7 Ball and stick representation of adenosylcobalamin. (From Reed, 2004. Copyright 2004 with permission from Elsevier.) 

FIGURE 15.9 General reaction mechanism for AdoCbl-dependent isomerases. (From Banerjee Ragsdale, 2003. Reprinted with 

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The currently accepted chemical mechanism of action of EAL, shown in Figure 17 for the reaction of ( 5)-2-aminopropanol, follows the pattern of other coenzyme Bi2-dependent isomerases. The chemical mechanism begins by homolytic scission of the Co—C5 bond in adenosylcobalamin with the substrate bound at its site translocation of the ribosyl moiety of the 5 -deoxyadenosyl radical by torsion about the ALribosyl linkage to place the 5 -methylene radical in contact with Cl of the substrate, in analogy with DDH ° abstraction of C 1-hydrogen from the substrate by the 5 -deoxyadenosyl radical to form 5 -deoxyadenosine and the... 

LAM is not specific for (5)-/3-lysine, although it is the best of the known substrates, and it accepts ( S )-lysine and (6 )-lysine as well. The reaction is depicted in Equation (13), which emphasizes the relationship with other coenzyme Bi2-dependent isomerases as well as lack of specificity for the regio- and stereoisomers of lysine. 

The mechanism of 1,2-hydrogen migration is similar to other coenzyme Bi2-dependent isomerases. ... 

The first coenzyme Bi2-dependent isomerase to be crystallized and structurally analyzed by X-ray crystallography is MCM. Several crystal structures of MCM are available, and one is illustrated in Figure 23. Like GM and 5,6-LAM, the larger subunit is a TIM barrel. Also like GM, MCM, and methionine synthase, adenosylcobalamin is bound in a base-ofF mode. 

The reaction of RTPR is fundamentally different from other coenzyme B 12-dependent reactions, in that it is a redox process proceeding with net reduction of the substrate, whereas the others are isomerases or isomerase/lyases. Moreover, the reduction of C2 (0H) to C2 (H) proceeds with incorporation of solvent hydrogen, unlike the other coenzyme B12 reactions. RTPR shares in common with other coenzyme Bi2-dependent reactions the role of adenosylcobalamin, which initiates the radical mechanism. It might also share with DDH the mechanism by which C2 (OH) is eliminated. 

The metabolic role of many minerals and vitamins is as prosthetic groups or coenzymes in different enzyme systems. Consequently, mineral and vitamin deficiencies can cause a breakdown of the processing system and precipitate metabolic disease. For example, methylmalonyl-CoA isomerase (see p. 203) is an important vitamin Bi2-dependent enzyme in the gluconeogenic pathway. A deficiency of vitamin B12 (or cobalt) may reduce enzyme activity, decrease the efficiency of glucose synthesis and predispose the animal to ketosis. Similarly, ceruloplasmin is a copper-dependent enzyme responsible for releasing iron from cells into blood plasma. A copper deficiency may reduce ceruloplasmin activity, decrease the efficiency of iron utilisation for haemoglobin synthesis and predispose the animal to anaemia. 

Isomerases that are dependent on coenzyme B12 constitute the largest subfamily of Bi2 enzymes and are components of a number of fermentative pathways in microbes [10, 11]. A single member of this group of enzymes, methylmalonyl-CoA mutase, is found in both bacteria and in mammals where it is a mitochondrial enzyme involved in the catabolism of odd-chain fatty acids, branched chain amino... 

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