Synthesis of acetyl coenzyme A from carbon monoxide, methyltetrahydrofolate, and coenzyme A by enzymes from Clostridium thermoaceticum

Abstract

Two purified fractions from Clostridium thermoaceticum are shown to catalyze the following reaction: CO + CH3THF + CoA ATPCH3COCoA + THF. The methyltetrahydrofolate (CH3THF) gives rise to the methyl group of the acetylcoenzyme A (CoA) and the carbon monoxide (CO) and CoA to its carboxyl thio ester group. The role of ATP is unknown. One of the protein fractions (F2) is a methyltransferase, whereas the other fraction (F3) contains CO dehydrogenase and a methyl acceptor which is postulated to be a corrinoid enzyme. The methyltransferase catalyzes the transfer of the methyl group to the methyl acceptor, and the CO is converted to a formyl derivative by the CO dehydrogenase. By a mechanism that is as yet unknown, the formyl derivative in combination with CoA and the methyl of the methyl acceptor are converted to acetyl-CoA. It is also shown that fraction F3 catalyzes the reversible exchange of 14C from rl-14C]acetyl-CoA into 14CO and that ATP is required, but not the methyltransferase. It is proposed that these reactions are part of the mechanism which enables certain autotrophic bacteria to grow on CO. It is postulated that CH3THF is synthesized from CO and tetrahydrofolate which then, as described above, is converted to acetyl-CoA. The acetyl-CoA then serves as a precursor in other anabolic reactions. A similar autotrophic pathway may occur in bacteria which grow on carbon dioxide and hydrogen.The fermentation of glucose by Clostridium thermoaceticum occurs by a homoacetate pathway in which 3 mol of acetate is formed per mol of glucose; for a recent review, see Ljungdahl and Wood (22). The fermentation as outlined below in reactions 1 to 5 is considered to occur via the Embden-Meyerhof glycolytic pathway yielding 2 mol of pyruvate (reaction 1). One of the pyruvate molecules is converted to CO2 and acetate; the resulting CO2 is reduced to formate, which combines with tetrahydrofolate (THF) and is reduced to methyltetrahydrofolate (CH3THF) (reactions 2, 3, and 4). Thus, this portion of the pathway yields acetate and CH3THF. In the succeeding steps, the CH3THF and the second molecule of pyruvate are converted to two molecules of acetate (reaction 5).

Old Acetogens, New Light

“…Resolution of enzmes required for synthesis of acetyl-CoA from pyruvate and methyltetrahydrofolate (Drake et al 253 ) 1981-1982 Demonstration that CO replaces the carboxyl-group of pyruvate and undergoes an exchange reaction with acetyl-CoA (Drake et al, 254 Hu et al 61 ) 1982…”

“…Acetyl-CoA synthase not only catalyzes the reduction of CO 2 to CO and the synthesis of acetyl-CoA as shown in FIGURE 5, but can also oxidizes CO to CO 2 ; this latter reaction is the historical basis for referring to acetyl-CoA synthase as CO dehydrogenase. [58][59][60][61][62] It is now understood that the different subunits of acetyl-CoA synthase are capable of catalyzing different reactions. The enzymological features of the acetyl-CoA pathway that was resolved from M. thermoacetica can be found in several reviews and articles.…”

The acetyl-CoA pathway of autotrophic growth

“…The CO-DH from anaerobes contains Ni and Fe-sulfur centers [18], whereas that from aerobes contains Mo instead of Ni [19]. The importance of this enzyme in the metabolism of C. thermoaceticum became apparent when it was found that CO could replace pyruvate as a source of the carbonyl group of acetyl-CoA and that acetyl-CoA was synthesized from CH3THF, CoASH and CO [20]. At that time.…”

CO2fixation in acetogenic bacteria: Variations on a theme

“…CO dehydrogenase was purified and confirmed to contain Ni [130]. Most important of all, they found that that fraction F 3 and fraction F2, the methyltransferase, catalyzed the formation of acetyl CoA from CO, CH3-H4folate and coenzyme A [131]. Thus, CO can replace pyruvate as the source of the carboxyl in the formation of acetate.…”

“…Before the discovery that acetyl CoA could be synthesized from CO and CH3-H4folate [131], acetate synthesis could only be studied from pyruvate and CH3-H4folate (reaction 6b) in a complex assay [108]. The system became much simpler when acetyl CoA or acetyl phosphate formation from CO, CH3-H4folate, CoA, and ATP was demonstrated.…”