S-Acetyl CoA

Methylthiobutyl glucosinolate derives from L-methionine by a complex elongation process leading to dihomomethionine. Four of the five carbons of methionine are retained, one being lost in a decarboxylation. The two necessary additional carbons each derive from a methyl group of acetyl-S-CoA by a complex, multi-step condensation mechanism (Equation 11) ... 

Initial rate patterns for Escherichia coli NAD+-dependent coenzyme A-linked aldehyde dehydrogenase (Reaction NAD+ + CoA-SFI + acetaldehyde = NADFI + acetyl-S-CoA + FI+). The results of each of three experiments are shown as a single double-reciprocal plot, and the nonvaried substrate concentrations for each curve are indicated above the data points. 

Figure 2. Illustration of the importance of the choice of reaction conditions on the determination of initial velocity. Shown are four conditions applied to examine the rate behavior of Escherichia coli NAD+-dependent coenzyme A-linked aldehyde dehydrogenase (Reaction NAD+ + CoA-SH + Acetaldehyde = NADH + Acetyl-S-CoA + H+). All assay mixtures contained enzyme, 0.5 mM NAD+, 8 /jlW CoA-SFI, 16 mM acetaldehyde, and 22.5 mM Tris buffer at pFI 8.1. (a) Time-course observed when enzyme was added to the standard assay (b) time-course observed when enzyme was added to standard assay augmented with 10 mM 2-mercaptoethanol (c) time-course observed when enzyme was first preincubated for 15 min with 8 /jlW CoA-SH, 16 mM acetaldehyde, 10 mM 2-mercaptoethanol, and 22.5 mM Tris buffer at pH 8.1, and the reaction was initiated by addition of NAD+ (d) time-course observed when enzyme was preincubated with lOmM 2-mercaptoethanol for 15 min andthen addedtostandard assay augmented with 10 mM 2-mercaptoethanol. The data are most compatible with the idea that the enzyme has an active-site thiol group that must be reduced to express full catalytic activity during assay.

K+. Pyruvate kinase is inhibited by 3-5 mM ATP, as well as by acetyl-S-CoA and long-chain fatty acids. Rabbit muscle pyruvate kinase is typically used in ATP regeneration. 

The cleavage of citrate to acetate and oxalacetate has a AG of — 680 cal/mole. The of the citrate synthetase reaction is 3.2 X 10. From this information, calculate the standard free energy of hydrolysis of acetyl-S-CoA and the Ai, for the hydrolysis. 

The most common reactions of acylation are in fact acetylations of xenobiotics containing a primary amino group. The cofactor of acetylation is acetylcoenzyme A (acetyl-S-CoA), the reaction being catalysed by a variety of A-acetyltransferases. Arylamine Af-acetyltransferases (NAT-1 and -2) are the most important enzyme, but aromatic-hydroxylamine O-acetyltransferase and N-hydroxyarylamine >-acetyltransferase are also involved in the acetylation of some aromatic amines and hydroxyl-amines. 

Isoleucine can give its amino group to a-ketoglutarate in a transamination reaction and then be oxidatively decarboxylated and dehydrogenated to form the corresponding (a,(3)-unsaturated acyl-CoA derivative. Further reactions (see the figure on p. 424) then are identical to fatty acid oxidation until the carbon skeleton is split into acetyl-S-CoA and propionyl-S-CoA. The three subsequent steps for the conversion of the (odd-chain) propionyl-S-CoA to succinyl-S-CoA have been discussed for the oxidation of odd-chain fatty acids (see Chapter 22). 

Ethyl acetates of fatty acids, mainly ethyl caproate and caprylate, are produced by yeast during alcoholic fermentation. They are synthesized from forms of the acids activated by the coenzyme A (HS-CoA), acyl-S-CoA. Acetyl-S-CoA, from pyruvic acid, may be involved in a Claisen reaction with malonyl-S-CoA, producing a new acyl-S-CoA with two additional carbon atoms (Figure 2.9). Acetyl-S-CoA thus produces butyryl-S-CoA, then hexanyl-S-CoA, etc. Specific enzymes then catalyze the alcoholysis of acyl-S-CoA into ethyl acetates of fatty acids. At the same time, the coenzyme A is regenerated. 

The overall reaction produces, from one molecule of pyruvic acid, one molecule of COg, one of acetylated Coenzyme A (acetyl-S-CoA) and one of NADH. 

This reaction is essentially irreversible and the thioester bond of acetyl-S-CoA (CHgCO-SCoA) has a high free energy of hydrolysis (AG° = —31 kJ —7 5 kcal). This energy is utilized in a condensation reaction of acetyl CoA with the enol form of oxaloacetic acid to produce citric acid and CoASH is liberated. The enzyme mediating this reaction, citrate synthase (condensing enzyme), is the first enzyme of the tricarboxylic acid cycle (Krebs cycle) (Fig. 17.4). 

The product of fatty acid oxidation is acetyl-S-CoA which is used by the glyoxylate bypass enzymes. 

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