Carbonyl groups of acetyl-CoA

Fraction F3 catalyzes an isotope exchange reaction between CO and the carbonyl group of acetyl-CoA. ... 

CO is the precursor of the carbonyl group of acetyl-CoA (discussed earlier). An identical EPR signal is observed when other precursors of the carbonyl group of acetyl-CoA are reacted with CODH/ACS for example, CO2 in the presence of reductant 145) and pyruvate 164). Incubation with acetyl-CoA itself also gives rise to this EPR signal 185). 

Is the paramagnetic adduct between CO and Cluster A a kinetically intermediate in acetyl-CoA synthesis Questions have been raised about whether this adduct is a catalytic intermediate in the pathway of acetyl-CoA synthesis 187, 188) (as shown in Fig. 13), or is formed in a side reaction that is not on the main catalytic pathway for acetyl-CoA synthesis 189). A variety of biochemical studies have provided strong support for the intermediacy of the [Ni-X-Fe4S4l-CO species as the precursor of the carbonyl group of acetyl-CoA during acetyl-CoA synthesis 133, 183, 185, 190). These studies have included rapid ffeeze-quench EPR, stopped flow, rapid chemical quench, and isotope exchange. 

The amino group of the lysine residue, formed from the protonated form by a base, attacks the carbonyl group of acetyl CoA to generate a tetrahedral intermediate. This intermediate collapses to form the amide bond and release CoA. 

Although no O2 is introduced into the TCA cycle, the two molecules of CO2 produced have more oxygen than the acetyl group. These oxygen atoms are ultimately derived from the carbonyl group of acetyl CoA, two molecules of water added by fumarase and citrate synthase, and the PO/ added to GDP. 

When these labeled oxaloacetates enter a second turn of the cycle, both of the carboxyl carbons are lost as CO2, but the methylene and carbonyl carbons survive through the second turn. Thus, the methyl carbon of a labeled acetyl-CoA survives two full turns of the cycle. In the third turn of the cycle, one-half of the carbon from the original methyl group of acetyl-CoA has become one of the carboxyl carbons of oxaloacetate and is thus lost as CO2. In the fourth turn of the cycle, further scrambling results in loss of half of the remaining labeled carbon (one-fourth of the original methyl carbon label of acetyl-CoA), and so on. 

The first reaction of the TCA cycle is catalyzed by citrate synthase and involves a carbanion formed at the methyl group of acetyl-CoA that undergoes aldol condensation with the carbonyl carbon atom of the oxaloacetate ... 

Carbonyl condensations are among the most widely used reactions in the biological world for the assembly of new carbon-carbon bonds in such important biomolecules as fatty acids, cholesterol, and steroid hormones. One source of carbon atoms for the synthesis of these biomolecules is acetji-CoA, a thioester of acetic acid and the thiol group of coenzyme A. The function of the coenzyme A group of acetyl-CoA is to anchor the acetyl group on the surface of the enzyme systems that catalyze the reactions we examine in this section. In the discussions that follow, we will not be concerned with the mechanism by which each enzyme-catalyzed reaction occurs. Rather, our concern is with recognizing the type of reaction that takes place in each step. 

Problem 11.13. As shown in Scheme 11.89, if the methyl group of acetyl-CoA becomes the pw-S carbon in citrate, has the addition occurred to the re- or i-face of the carbonyl ... 

Thus, it seems that the methyl group of acetyl-CoA must be the a carbon of the original fatty acid and the P carbon of the fatty acid becomes the carbonyl carbon of myristate (Fig. 17.61). We will come back to this reaction in Chapter 19, but you should be able to appreciate both the extent and difficulty of the task that Nature faces here—How to break the a-P bond in palmitate That s no simple task, and we shall need the material in Chapter 19 to solve it. 

FIGURE 24.17 The mechanism of the thiolase reaction. Attack by an enzyme cysteine thiolate group at the /3-carbonyl carbon produces a tetrahedral intermediate, which decomposes with departure of acetyl-CoA, leaving an enzyme thioester intermediate. Attack by the thiol group of a second CoA yields a new (shortened) acyl-CoA. 

A histidine deprotonates the acetyl-CoA enol, which adds to the ketone carbonyl group of oxaloacetate in an aldol-like reaction. Simultaneously, an acid N-H proton of another histidine protonates the carbonyl oxygen, producing (S)-citryl CoA. 

The mechanism of the C02 transfer reaction with acetyl CoA to give mal-onyl CoA is thought to involve C02 as the reactive species. One proposal is that loss of C02 is favored by hydrogen-bond formation between the A -carboxy-biotin carbonyl group and a nearby acidic site in the enzyme. Simultaneous deprotonation of acetyl CoA by a basic site in the enzyme gives a thioester eno-late ion that can react with C02 as it is formed (Figure 29.6). 

Note that the hydroxyl-bearing carbon of citrate is a prochirality center and contains two identical "arms." Because the initial aldol reaction of acetyl CoA to oxaloacetate occurs specifically from the Si face of the ketone carbonyl group, the pro-S arm of citrate is derived from acetyl CoA and the pro-R arm is derived from oxaloacetate. 

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