Acetyl coenzyme methyl carbon

Acetyl coenzyme A synthase of C. thermoaceticum is able to catalyze the whole reaction for reductive synthesis of acetyl coenzyme A from carbon dioxide, a methylated corrinnoid/iron-sulfur protein, and coenzyme A. The enzyme catalyzes several exchange reactions [147-149] ... 

Barondeau, D. P., and Lindahl, P. A., 1997, Methylation of carbon monoxide dehydrogenase from Clostridium thermoaceticum and mechanism of acetyl coenzyme A synthesis, J. Am. Chem. Soc. 119(17) 3959n3970. 

The catalysis of the transfer of a methyl group is an important role of enzyme-bound vitamin B12 derivatives in human, animal, and bacterial metabolism. The known enzyme-controlled methyl group-transfer reactions are key steps in the cobamide-dependent methylations of homocysteine to methionine, in the metabolic formation of methane from other Ci-compounds in methanogenic bacteria, and in the fixation of carbon dioxide via the acetyl coenzyme A pathway of some bacterial autotrophs (Figure 10). ... 

In acetate-catabohzing methanogens, enzyme-bound methylcorrinoids are also indicated to serve as intermediates in methyl-group transport. There, methane and carbon dioxide are formed from acetic acid (via acetyl coenzyme A, whose acetyl group is spht into carbon monoxide and a corrin-bound methyl group). Accordingly, the methyl-group... 

Often, the same enzyme also acts as an acetyl coenzyme-A synthase carbon monoxide, a methyl group, and coenzyme-A are bound at the active site and the condensation reaction to form acetyl coenzyme-A is catalyzed (81),... 

Fig. 11. Proposed function of electrochemical and Na potentials in energy conservation coupled to acetate fermentation to CH4 and CO2. The Na /H antiporter is involved in the generation of A/iH from A/iNa. CH3CO-S-C0A, acetyl-coenzyme A [CO], CO bound to carbon monoxide dehydrogenase CH3-H4MPT, methyl-tetrahydromethanopterin CH3-S-C0M, methyl-coenzyme M. The hatched boxes indicate membrane-bound electron transport chains or membrane-bound methyl-transferase catalyzing either IT or Na translocation (see Figs. 5, 6 and 12). It is assumed that enzyme-bound [CO] is energetically equal to free CO. ATP is synthesized via membrane-bound H -translocating ATP synthase. The stoichiometries of translocation were taken from refs. [107,234] n, X, y and z are unknown stoichiometric factors.

Concomitant to the enhancement of the signal for C-3 in the C NMR spectrum, the signals for H2 and H4 decreased in the H-NMR spectrum of all the tropine moieties compared with those in authentic samples. This indicates the incorporation of deuterium at C2 and C4 derived from C Hj CXX)". As the diminution of the signals for H2 and H4 was not identical at both positions, we suppose a sequential incorporation of labeled acetate (example given in Fig. 15b). From these data we assume that the biosynthesis from N-methyl-pyrrolidinium ion to tropine is a two-step process which does not involve a four carbon unit (acetoacetyl coenzyme A) but two units of acetyl coenzyme A, which were added sequentially as has been suggested for the biosynthesis of cocaine [28, 37]. On the other hand it has been reported that l,2- C2-acetate was incorporated with an equal efficiency at C2 and C4 by non-transformed root cultures of H. albus [36]. Further investigations are required to clarify this matter. 

Coenzyme A is a complex molecule which contains a free sulfydryl (-SH) group. This group can react with a carboxyl group to form a thioester. In acetyl CoA, the thioester linkage can activate the methyl carbon as well as the acetyl group. 

The interconversion of CO and CO2 is carried out by carbon monoxide dehydrogenases (COdH s). These enzymes are major components in the global carbon cycle. They are often, but not exclusively, found in bifunctional enzymes together with ACS, which synthesizes acetyl coenzyme A from CO, a methyl group, and the thiol coenzyme A. Scheme 7 demonstrates the relevance of COdH and ACS in the early stages of carbon assimilation of anaerobic organisms. ... 

The biosynthesis of saphenic acid (27) and derivatives thereof is based on addition of a one-carbon unit to phenazine-1,6-carboxylic acid (Iq, Scheme 3). The transfer of a methyl group from C2 of acetate is a well-known biosynthetic transformation and occurs when the thioester of acetyl coenzyme A is converted by acetyl-CoA carboxylase to malonyl-CoA. Malonyl-CoA undergoes a decarboxylative Claisen condensation with a mono-CoA thioester of phenazine-1,6-... 

The bacterial metalloenzyme acetyl coenzyme A synthase/CO dehydrogenase (ACS/CODH) catalyzes two very important biological processes, namely the reduction of atmospheric CO2 to CO and the synthesis of acetyl coerrzyme A from CO, a methyl from a methylated corrinoid iron-sulfur protein, and the thiol coenzyme A [166-168]. This bifunctional errzyme is the key to the Wood-Ljiungahl pathway of anaerobic CO2 fixation (Scheme 1.18) and a major component of the global carbon cycle. Reactions catalyzed by CODH and ACS are shown in Eqs. (1.4) and (1.5) below. 

Citrate synthase catalyzes the condensation reaction by bringing the substrates into close proximity, orienting them, and polarizing certain bonds. Two histidine residues and an aspartate residue are important players (Figure 1711). One of the histidine residues (His 274) donates a proton to the carbonyl oxygen of acetyl CoA to promote the removal of a methyl proton by Asp 375. Oxaloacetate is activated by the transfer of a proton from His 320 to its carbonyl carbon atom. The concomitant attack of the enol of acetyl CoA on the carbonyl carbon of oxaloacetate results in the formation of a carbon-carbon bond. The newly formed citryl CoA induces additional structural changes in the enzyme. The active site becomes completely enclosed. His 274 participates again as a proton donor to hydrolyze the thioester. Coenzyme A leaves the enzyme, followed by citrate, and the enzyme returns to the initial open conformation. 

Fig. 10. Proposed pathway of methanogenesis from acetate (Methanosarcina, Methanothrix) and from pyruvate (Methanosarcina) Intermediates, enzymes and a site for Na dependence. CoA, coenzyme A PP, pyrophosphate CH3-H4MPT, methyl-tetrahydromethanopterin, CH3-S-C0M, methyl-coenzyme M [CO], CO bound to carbon monoxide dehydrogenase. Numbers in circles refer to enzymes involved (1) acetyl-CoA synthetase (2) pyrophosphatase (3) acetate kinase (4) phosphate acetyltransferase (5) pyruvate ferredoxin oxidoreductase (6) carbon monoxide dehydrogenase (7) CH3-H4MPT FI-S-CoM methyltransferase (8) methyl-coenzyme M reductase (9) heterodisulfide...

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. 

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