Cobalamins transfer reactions

Co within all compounds of the so-called cobalamin (or B12) family. The biological functions of cobalamin cofactors are defined by their axial substituents either a methyl or an adenosyl group. Both cofactors participate in biosynthesis the former in methyl transfer reactions while the latter is a free radical initiator, abstracting H atoms from substrates. Decades after their initial characterization, the fascination with the biological chemistry of cobalamins remains.1109... 

In mammals and in the majority of bacteria, cobalamin regulates DNA synthesis indirectly through its effect on a step in folate metabolism, catalyzing the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate via two methyl transfer reactions. This cytoplasmic reaction is catalyzed by methionine synthase (5-methyltetrahydrofolate-homocysteine methyl-transferase), which requires methyl cobalamin (MeCbl) (253), one of the two known coenzyme forms of the complex, as its cofactor. 5 -Deoxyadenosyl cobalamin (AdoCbl) (254), the other coenzyme form of cobalamin, occurs within mitochondria. This compound is a cofactor for the enzyme methylmalonyl-CoA mutase, which is responsible for the conversion of T-methylmalonyl CoA to succinyl CoA. This reaction is involved in the metabolism of odd chain fatty acids via propionic acid, as well as amino acids isoleucine, methionine, threonine, and valine. 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

Regulation of homocysteine metabolism appears to be especially important in the central nervous system, presumably because of the critical role of methyl transfer reactions in the production of neurotransmitters and other methylated products. It has been known for decades that mental retardation is a feature of the genetic diseases, such as CBS deficiency, that cause severe hyperhomocysteinemia and ho-mocystinuria. Impaired cognitive function is also seen in pernicious anemia, which causes hyperhomocysteinemia due to deficiency of cobalamin (see Chapter 28). Hyperhomocysteinemia also may be linked to depression, schizophrenia, multiple sclerosis, and Alzheimer s disease. The molecular mechanisms underlying these clinical associations have not yet been delineated. 

The interconversion of these forms of foiic acid takes place through various electron transfer reactions facilitated by specific enzyme systems and coenzymes, such as the reduced forms of fiavin-adenine dinucleotide (FADH2) and NADPH. The conversion between the N -, N -methylene form and -formyl forms is readily reversible, but the reduction of methylene to methyl and reduction of free THF to formyltetrahydrofolate is essentially irreversible. Conversion of N -methyltetrahydrofolate back to free THF. may require cobalamin. 

Vitamin B12 (cyanocobalamin) 3 is, in fact, not a natural product as the cyanide ligand to the cobalt ion is added during the isolation procedure. Coenzyme B12 (adenosylcobalamin) 4 and methylcobalamin 5 are the true final products of the biosynthetic pathway. Coenzyme 0,2 is the cofactor for a number of enzymic rearrangement reactions, such as that catalysed by methylmalonyl CoA mutase, and methylcobalamin is the cofactor for certain methyl transfer reactions, including the synthesis of methionine. A number of anaerobic bacteria produce related corrinoids in which the dimethylbenzimidazole moiety of the cobalamins (3 - 5) is replaced by other groups which may or may not act as ligands to the cobalt ion, such as adenine orp-cresol [12]. 

The primary reaction catalyzed by methionine synthase converts homocysteine (Hey) and methyltetrahydrofolate (CH3H4folate) to methionine and tetrahydrofolate (Figure 2). Occasional oxidation of the reactive cob(I)alamin intermediate produces an inactive cob(II)alamin enzyme, which is reactivated by a reductive methylation that uses S-adenosylmethionine (AdoMet) as the methyl donor and flavodoxin or a flavodoxin-like domain as an electron donor. Thus methionine synthase supports three distinct methyl transfer reactions each involving the cobalamin cofactor. 

The catalytically active corrinoids involved in methyl group transfer reactions of these methyl transferases have been characterized as protein-bound Co -corrins (such as cob(I)alamin) and methyl-Co -corrins (such as methyl-cobalamin) with the nucleotide base decoordinated (10,48,49). Recent studies have provided indirect evidence for the methylation of radicals by protein-bound methylcobamides (35), a possible second mechanism of biological methyl transfer... 

Folic acid (pteroylglutamic acid) and related compounds are present at high concentration in liver, but spinach, broccoli, peanuts, and fresh fruit are also good dietary sources. The RDA is 300 pg. Folates are important for the synthesis of tetrahydrofolate which is important with cobalamin for a series of 1-carbon transfer reactions leading to DNA synthesis, failure of which leads to megaloblastic anemia. 

The synthesis of methionine, the formation of methane, and the formation of acetate furnish examples of the capacity of Co-corrinoid compounds to act as carriers in methyl transfer reactions. At the present time it is difficult to judge whether this chemical capability of the Co-corrinoid compounds has been utilized more widely in nature for other transmethylation reactions. It has recently been reported that CHs-cobalamin can serve as a methyl donor in the formation of methylated bases in transfer RNA. Although a significant rate of nonenzymic reaction occurred, enzymes from both rat liver and the bacterium Propionibacterium shermanii increased die rate of methylation (Walerych et al., 1966). When extracts of a methane-forming bacterium were incubated with CHa-Bi2 and mer-... 

Mechanistic aspects of the action of folate-requiring enzymes involve one-carbon unit transfer at the oxidation level of formaldehyde, formate and methyl (78ACR314, 8OMI2I6OO) and are exemplified in pyrimidine and purine biosynthesis. A more complex mechanism has to be suggested for the methyl transfer from 5-methyl-THF (322) to homocysteine, since this transmethylation reaction is cobalamine-dependent to form methionine in E. coli. 

FIGURE 24.21 A mechanism for the methylmalonyl-CoA mntase reaction. In the first step, Co is rednced to Co dne to homolytic cleavage of the Co —C bond in cobalamin. Hydrogen atom transfer from methylmalonyl-CoA yields a methylmalonyl-CoA radical that can undergo rearrangement to form a snccinyl-CoA radical. Transfer of an H atom regenerates the coenzyme and yields snccinyl-CoA. 

Enzymatic methylation of homocysteine (HSCHjCHjCHNHjCOOH) by methylcobalamin to give methionine (CH3SCH2CH2CHNH2COOH) was discovered in 1962 by Woods and co-workers, who also noticed the occurrence of a much slower, nonenzymatic reaction giving the same products. Methylcobinamide showed the same activity as the cobalamin in both the enzymatic and nonenzymatic reactions (72, 7/). It was subsequently discovered that HS, MeS , PhS , and w-BuS will dealkylate a variety of methyl complexes [DMG, DMG-BF2, DPG, G, salen, (DO)(DOH)pn, cobalamin] and even ethyl-Co(DMG)2 complexes to give the thioethers, and it was suggested that the reaction involved transfer of the carbonium ion to the attacking thiolate 161, 164), e.g.,... 

Methionine synthase deficiency (cobalamin-E disease) produces homocystinuria without methylmalonic aciduria. This enzyme mediates the transfer of a methyl group from methyltetrahydrofolate to homocysteine to yield methionine (Fig. 40-4 reaction 4). A cobalamin group bound to the enzyme is converted to methylcobalamin prior to formation of methionine. 

It is the role of jV5-methyl THF which is key to understanding the involvement of cobalamin in megaloblastic anaemia. The metabolic requirement for N-methyl THF is to maintain a supply of the amino acid methionine, the precursor of S-adenosyl methionine (SAM), which is required for a number of methylation reactions. The transfer of the methyl group from jV5-methyl THF to homocysteine is cobalamin-dependent, so in B12 deficiency states, the production of SAM is reduced. Furthermore, the reaction which brings about the formation of Ns-methyl THF from N5,N10-methylene THF is irreversible and controlled by feedback inhibition by SAM. Thus, if B12 is unavailable, SAM concentration falls and Ah -methyl THF accumulates and THF cannot be re-formed. The accumulation of AT-methyl THF is sometimes referred to as the methyl trap because a functional deficiency of folate is created. 

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