Methyl malonyl-CoA

In Rhodococcus ruber and Nocardia corallina the polymers composed of 3-hydroxybutyryl and 3-hydroxyvaleryl residues are synthesized from sugars by methyl-malonyl-CoA. Succinyl-CoA is decarboxylated via methyl-malonyl-CoA to propionyl-CoA as the precursor of 3-hydroxyvaleryl-CoA [40]. 

Biochemical findings are variable. The blood cobala-min and folate levels often are normal. Patients often have homocysteinemia with hypomethioninemia, the latter finding discriminating this group from homocystinuria secondary to cystathionine- P-synthase deficiency. Urinary excretion of methylmalonic acid may be high, reflecting the fact that vitamin B12 serves as a cofactor for the methyl-malonyl-CoA (coenzyme A) mutase reaction. 

The way biotin participates in carbon dioxide fixation was established in the early 1960s. In 1961 Kaziro and Ochoa using propionyl CoA carboxylase provided evidence for 14C02 binding in an enzyme-biotin complex. With excess propionyl CoA the 14C label moved into a stable position in methyl malonyl CoA. In the same year Lynen found biotin itself could act as a C02 acceptor in a fixation reaction catalyzed by B-methylcrotonyl CoA carboxylase. The labile C02 adduct was stabilized by esterification with diazomethane and the dimethyl ester shown to be identical with the chemically synthesized molecule. X-ray analysis of the bis-p-bromanilide confirmed the carbon dioxide had been incorporated into the N opposite to the point of attachment of the side chain. Proteolytic digestion and the isolation of biocytin established the biotin was bound to the e-NH2 of lysine. 

Free radical reactions Once thought to be rare, the homolytic cleavage of covalent bonds to generate free radicals has now been found in a range of biochemical processes. Some examples are the reactions of methyl-malonyl-CoA mutase (see Box 17-2), ribonucleotide reductase (see Fig. 22-41), and DNA photolyase (see Fig. 25-25). 

It may seem surprising that a coenzyme is needed for these carboxylation reactions. However, unless the cleavage of ATP were coupled to the reactions, the equilibria would lie far in the direction of decarboxylation. For example, the measured apparent equilibrium constant K for conversion of propionyl-CoA to S methyl-malonyl-CoA at pH 8.1 and 28°C51 is given by Eq. 14-4. 

An active, cobalt-containing, oxaloacetate transcarboxylase (methyl-malonyl-CoA pyruvate carboxyltransferase) has been isolated from Propionobacterium shermanii grown with 60Co2+ (137). The metal content corresponds to two equivalents of 60Co(II) per mole of enzyme. 

B12, cyanocobalamin Methylation of homocysteine to methionine, conversion of methyl malonyl CoA to succinyl CoA... 

SF Haydock, JF Aparicio, I Molnar, T Schwecke, LE Khaw, A Konig, AFA Marsden, IS Galloway, J Staunton, PF Leadlay. Divergent sequence motifs correlated with the substrate specificity of (methyl)malonyl-CoA acyl carrier protein trans-acylase domains in modular polyketide synthases. FEBS Lett 374 246-248, 1995. 

Figure 28-7. The metabolism of branched-chain amino acids and odd-chain fatty acids via propionyl-CoA. Propionyl-CoA is converted to D-methylmalonyl-CoA by propionyl-CoA carboxylase. D,L-Methylmalonyl-CoA racemase catalyzes the conversion of D-methylmalonyl-CoA to L-methylmalonyl-CoA. L-methyl malonyl-CoA mutase, an adenosyicobalamin-requiring enzyme, converts L-methylmalonyl-CoA to succinyl-CoA.TCA cycle is citric acid cycle or Kreb s cycle.

As a result of the reduced activity of the mutase in vitamin B12 deficiency, there is an accumulation of methyhnalonyl CoA, some of which is hydrolyzed to yield methylmalonic acid, which is excreted in the urine. As discussed in Section 10.10.3, this can be exploited as a means of assessing vitamin B12 nutritional status. There may also be some general metabolic acidosis, which has been attributed to depletion of CoA because of the accumulation of methyl-malonyl CoA. However, vitamin B12 deficiency seems to result in increased synthesis of CoA to maintain normal pools of metabolically useable coenzyme. Unlike coenzyme A and acetyl CoA, neither methylmalonyl CoA nor propionyl CoA (which also accumulates in vitamin B12 deficiency) inhibits pantothenate kinase (Section 12.2.1). Thus, as CoA is sequestered in these metabolic intermediates, there is relief of feedback inhibition of its de novo synthesis. At the same time, CoA may be spared by the formation of short-chain fatty acyl carnitine derivatives (Section 14.1.1), which are excreted in increased amounts in vitamin B12 deficiency. In vitamin Bi2-deficient rats, the urinary excretion of acyl carnitine increases from 10 to 11 nmol per day to 120nmolper day (Brass etal., 1990). 

Mancia, F., and Evans, P. R., 1998, Conformational changes on substrate binding to methyl-malonyl CoA mutase and new insights into the free radical mechanism. Structure 6 7119720. 

GC-MS has been responsible for the identification of a variety of unusual lipids associated with diseased conditions. Hydroxyocta-decadienoic esters of cholesterol for example, have been isolated from aortal atheroma placques [277] and branched chain and odd numbered fatty acids identified in the glycerolipids of brain, spinal cord and sciatic nerve [278] from a patient with methylmalonic aciduria. The latter compounds are thought to arise by the replacement of malonyl CoA with methyl malonyl CoA, and acetyl CoA with propionyl CoA at certain stages of fatty acid synthesis. In these and other examples, the lipids need to be hydrolysed to permit the identification of the constituent fatty acids. As the class of lipids is usually known from the separation procedure used, the nature of the fatty acids may allow the characterisation of the complete molecule. However, volatilisation of the intact lipid into the mass spectrometer when possible would be preferable, particularly when it is present in a mixture and separation of the components is first made by GC. 

The 5-deoxyadenosyl cobalamin and methyl cobalamin function as coenzyme forms and are required for the action of several enzymes. Methyl malonyl CoA mutase uses 5-deoxyadenosyl cobalamin as coenzyme. Methyl cobalamin functions as a carrier of methyl group to homocysteine and convert it to methionine... 

The related cobalt cofactor vitamin B12 (8), the coenzyme of methyl-malonyl CoA mutase and homocysteine transmethylase, contains a... 

Propionyl CoA is the product from the catabolism of valine, isoleucine, methionine, and odd-numbered fatty acids. The carboxylation reaction, found in the mitochondria, produces methyl malonyl CoA. The latter undergoes a cobalamin (vitamin Bj2)-catalyzed rearrangement, forming succinyl CoA, which is metabolized further in the Krebs cycle. 

The pathway from propionyl CoA to succinyl CoA is especially interesting because it entails a rearrangement that requires vitamin (also known as cobalamin). Propionyl CoA is carboxylated at the expense ot the hydrolysis of a molecule of ATP to yield the D isomer of methyl-malonyl CoA (Figure 22.12). This carboxylation reaction is catalyzed by... 

Propionyl-CoA and L-methylmalonyl-CoA are intermediates in the conversion of these amino acids to succinyl-CoA. Methyl-malonyl-CoA mutase is a vitamin B 12-requiring enzyme. Note that threonine is also degraded via the propionyl-CoA/suc-cinyl-CoA pathway (see Figure 15.4). 

One form of methionine synthase common in bacteria uses lV -methyltetrahydrofolate as a methyl donor. Another form of the enzyme present in some bacteria and mammals uses A/ -methyltetrahydro-folate, but the methyl group is first transferred to cobalamin, derived from coenzyme B12, to form methylcobalamin as the methyl donor in methionine formation. This reaction and the rearrangement of L-methyl-malonyl-CoA to succinyl-CoA (see Box 17-2, Fig. la) are the only known coenzyme Bi2-dependent reactions in mammals. In cases of vitamin B12 deficiency, some symptoms can be alleviated by administering not only vitamin B12 but folate. As noted above, the methyl group of methylcobalamin is derived from W -methyltetrahy-drofolate. Because the reaction converting the methylene form to the 7V -methyl form of tetrahydrofo-... 

FIG. 4.2 Malate metabolism in mitochondria from body wall muscle of adult Ascaris smm. (1) Fumarase (2) malic enzyme (3) pyruvate dehydrogenase complex (4) complex I (5) succinate-coenzyme Q reductase (complex II, fumarate reductase) (6) acyl CoA transferase (7) methylmalonyl CoA mutase (8) methyl-malonyl CoA decarboxylase (9) propionyl CoA condensing enzyme (10) 2-methyl acetoacetyl CoA reductase (11) 2-methyl-3-oxo-acyl CoA hydratase (12) electron-transfer flavoprotein (13) 2-methyl branched-chain enoyl CoA reductase (14) acyl CoA transferase. 

Defects in intracellular metabolism of vitamin Bj have been reported in children with methylmalonic aciduria and homocystinuria. Potential mechanisms include an inability of cells to transport vitamin Bj or accumulate the vitamin because of a failure to synthesize an intracellular acceptor, a defect in the formation of deoxyadenosylcobalamin, or a congenital lack of methyl-malonyl CoA isomerase. 

What compounds form succinyl CoA via propionyl CoA and methyl-malonyl CoA ... 

C. Pharmacodynamics Vitamin B is essential in two reactions conversion of methyl-malonyl-CoA to succinyl-CoA and conversion of homocysteine to methionine. The second reaction is linked to folic acid metabolism and synthesis of deoxythymidylate (dTMP Figure 33-2, reaction 2), a precursor required for DNA synthesis. In vitamin B,2 deficiency, folates accumulate as AP-methyltetrahydrofolate the supply of tetrahydrofolate is depleted and the production of red blood cells slows. Administration of folic acid to patients with vitamin Bj deficiency helps refill the tetrahydrofolate pool (Figure 33-2, reaction 3) and partially or fully corrects the anemia. However, the exogenous folic acid does not correct the neurologic defects of vitamin Bj2 deficiency. 

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