Acetyltransferase reactions

Manufacturing processes for cephalosporin C and benzylpenicilhn are broadly similar. In common with mai other antibiotic fermentations, no specific precursor feed is necessary for cephalosporin C. There is sufficient acetyl group substrate for the terminal acetyltransferase reaction available fiom the organism s metabolic pool. 

CATHARANTHUS ROOT-AND SHOOT SPECIFIC O-ACETYLTRANSFERASE REACTIONS... 

The metabolism of xenobiotics proceeds at different rates for different individuals. This is because of genetic variations. Two examples demonstrate this point. CYP450 enzyme production (required for Phase I metabolism) varies by as much as 30% in healthy individuals. TV-acetyltransferase reaction rates (an example of a Phase II metabolism reaction) vary widely. Some individuals acetylate rapidly and others slowly, with the slow acety-lators having lower toxic thresholds. 

Fig. 31.29 Different metabolic pathways of p-amino-salicylic acid. Several conjugating enzymes are involved glycine TV-acyltransferases (reaction 1), UDP-glucuro-nosyltransferases (acyl-, ether-, A/-glucuronides, reactions 2, 4 and 6, respectively), sulfotransferases (reactions 3 and 5), A/-acetyltransferases (reaction 7).

Ketone body synthesis occurs only in the mitochondrial matrix. The reactions responsible for the formation of ketone bodies are shown in Figure 24.28. The first reaction—the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA—is catalyzed by thiolase, which is also known as acetoacetyl-CoA thiolase or acetyl-CoA acetyltransferase. This is the same enzyme that carries out the thiolase reaction in /3-oxidation, but here it runs in reverse. The second reaction adds another molecule of acetyl-CoA to give (i-hydroxy-(i-methyl-glutaryl-CoA, commonly abbreviated HMG-CoA. These two mitochondrial matrix reactions are analogous to the first two steps in cholesterol biosynthesis, a cytosolic process, as we shall see in Chapter 25. HMG-CoA is converted to acetoacetate and acetyl-CoA by the action of HMG-CoA lyase in a mixed aldol-Claisen ester cleavage reaction. This reaction is mechanistically similar to the reverse of the citrate synthase reaction in the TCA cycle. A membrane-bound enzyme, /3-hydroxybutyrate dehydrogenase, then can reduce acetoacetate to /3-hydroxybutyrate. 

Step 1 of Figure 27.7 Claisen Condensation The first step in mevalonate biosynthesis is a Claisen condensation (Section 23.7) to yield acetoacetyl CoA, a reaction catalyzed by acetoacetyl-CoA acetyltransferase. An acetyl group is first bound to the enzyme by a nucleophilic acyl substitution reaction with a cysteine —SH group. Formation of an enolate ion from a second molecule of acetyl CoA, followed by Claisen condensation, then yields the product. 

The metabolism of foreign compounds (xenobiotics) often takes place in two consecutive reactions, classically referred to as phases one and two. Phase I is a functionalization of the lipophilic compound that can be used to attach a conjugate in Phase II. The conjugated product is usually sufficiently water-soluble to be excretable into the urine. The most important biotransformations of Phase I are aromatic and aliphatic hydroxylations catalyzed by cytochromes P450. Other Phase I enzymes are for example epoxide hydrolases or carboxylesterases. Typical Phase II enzymes are UDP-glucuronosyltrans-ferases, sulfotransferases, N-acetyltransferases and methyltransferases e.g. thiopurin S-methyltransferase. 

The reaction of choline with mitochondrial bound acetylcoenzyme A is catalysed by the cytoplasmic enzyme choline acetyltransferase (ChAT) (see Fig. 6.1). ChAT itelf is synthesised in the rough endoplasmic reticulum of the cell body and transported to the axon terminal. Although the precise location of the synthesis of ACh is uncertain most of that formed is stored in vesicles. It appears that while ChAT is not saturated with either acetyl-CoA or choline its synthesising activity is limited by the actual availability of choline, i.e. its uptake into the nerve terminal. No inhibitors of ChAT itself have been developed but the rate of synthesis of ACh can, however, be inhibited by drugs like hemicholinium or triethylcholine, which compete for choline uptake into the nerve. 

Office of Prevention, Protection and Toxic Substances Phosphinothricin acetyltransferase Phenoxybenzoic acid Polychlorinated biphenyl Polymerase chain reaction Polygalacturonase Acid dissociation constant... 

The role of N-sulfonyloxy arylamines as ultimate carcinogens appears to be limited. For N-hydroxy-2-naphthylamine, conversion by rat hepatic sulfotransferase to a N-sulfonyloxy metabolite results primarily in decomposition to 2-amino-l-naphthol and 1-sulfonyloxy-2-naphthylamine which are also major urinary metabolites and reaction with added nucleophiles is very low, which suggests an overall detoxification process (9,17). However, for 4-aminoazobenzene and N-hydroxy-AAF, which are potent hepatocarcinogens in the newborn mouse, evidence has been presented that strongly implicates their N-sulfonyloxy arylamine esters as ultimate hepatocarcinogens in this species (10,104). This includes the inhibition of arylamine-DNA adduct formation and tumorigenesis by the sulfotransferase inhibitor pentachlorophenol, the reduced tumor incidence in brachymorphic mice that are deficient in PAPS biosynthesis (10,115), and the relatively low O-acetyltransferase activity of mouse liver for N-hydroxy-4-aminoazobenzene and N-OH-AF (7,114,115). 

Acetyltransferases catalyze the acetylation of amino, hydroxyl, and thiol functional groups. Acetylation of hydroxy and thiol groups is comparatively rare and of much less importance in alkaloid metabolism than reactions with amino functional groups. The types of amines that are acetylated include arylamines (the major route of metabolism in many species), aliphatic amines, hydrazines, sulfonamides, and the a-amino group of cysteine conjugates. The purification, physical properties, and specificity of the N-acetyltransfereases have been reviewed (116-118). 

Synthesis of noradrenaline (norepinephrine) is shown in Figure 4.7. This follows the same route as synthesis of adrenaline (epinephrine) but terminates at noradrenaline (norepinephrine) because parasympathetic neurones lack the phenylethanolamine-N-methyl transferase required to form adrenaline (epinephrine). Acetylcholine is synthesized from acetyl-Co A and choline by the enzyme choline acetyltransferase (CAT). Choline is made available for this reaction by uptake, via specific high-affinity transporters, within the axonal membrane. Following their synthesis, noradrenaline (norepinephrine) or acetylcholine are stored within vesicles. Release from the vesicle occurs when the incoming nerve impulse causes an influx of calcium ions resulting in exocytosis of the neurotransmitter. 

Acetylcholine is synthesized from choline and acetyl-SCoA in a reaction catalyzed by choline acetyltransferase ... 

Figure 11.5 Reactions of the fatty acid synthase complex. A single multi-subunit enzyme is responsible for the conversion of acetyl-CoA to palmitate. The subunits in the enzyme are (i) acetyltransferase, (ii) malonyltransferase, (iii) oxoacyl synthase, (iv) oxoacyl reductase, (v) hydroxyacyl dehydratase, (vi) enoyl reductase. Finally, a separate enzyme, thioester hydrolase, hydrolyses palmitoyl-CoA to produce palmitate (vii).

Histone acetylation is a reversible amidation reaction involving defined e-amino groups of lysine residues (see Fig. 6) at the N-terminal tails of core histones. The highly dynamic equilibrium between the acetylated and non-acetylated states of lysine is maintained by two enzymatic groups, referred to as histone acetyltransferases (HATs) and histone deacetylases (HDACs). 

Histone acetyltransferases (H ATs) catalyze the transfer of an acetyl moiety from acetyl-CoA to the E-amino group of certain lysine residues within core histone proteins. This transferase reaction produces acetylated histones and the deacetylated cofactor CoA-SH. As HATs are important enzymes in the regulation of gene expression, there are also a number of assays available to detect acetyltransferases activity. 

This enzyme [EC 2.3.1.1], also referred to as amino-acid A-acetyltransferase and acetyl-CoA glutamate N-acetyltransferase, catalyzes the reaction of acetyl-CoA with glutamate to form coenzyme A and A-acetylgluta-mate. The enzyme will also acts on aspartate and, more slowly, with some other amino acids. The mammalian enzyme is activated by L-arginine. See also Glutamate Acetyltransferase... 

This enzyme [EC 2.3.1.87], also known as serotonin acetyltransferase and serotonin acetylase, catalyzes the reaction of acetyl-CoA and arylalkylamine to generate coenzyme A and A-acetylarylalkylamine. The enzyme exhibits a rather narrow specificity toward other aryl-alkylamines. This enzyme is distinct from arylamine acetyltransferase. 

Pyruvate dehydrogenase (lipoamide) [EC 1.2.4.1], which requires thiamin pyrophosphate, catalyzes the reaction of pyruvate with lipoamide to produce 5-acetyldihydroli-poamide and carbon dioxide. It is a component of the pyruvate dehydrogenase complex (which also includes dihydrolipoamide dehydrogenase [EC 1.8.1.4] and dihy-drolipoamide acetyltransferase [EC 2.3.1.12]). Pyruvate dehydrogenase (cytochrome) [EC 1.2.2.2] catalyzes the... 

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