A-ketobutyrate

The ACS function is known only in higher plants. The activity of ACS isozymes is a key regulatory factor of ethylene biosynthesis pathway. In general, microorganisms liberate ethylene but their ethylene synthesis pathways do not involve ACC as an intermediate. Penicillium citrinum is the first reported microorganism that is able to synthesize ACC from SAM and to degrade it into ammonia and a-ketobutyrate, not to ethylene. ACS from P. citrinum shows a 100-fold higher for SAM than its plant counterparts. ... 

Schemes Model of mechanism-based inactivation of ACS by SAM (a-e) Model of mechanism-based inactivation of ACS by L-vinylglycine (l-VG) (f-g-d-e) Conversion of l-VG to a-ketobutyrate and ammonia catalyzed by ACS (f-h). Notations are as in Scheme 2. For details see text.

Biological. Bacterial degradation of 2-methylphenol may introduce a hydroxyl group producing 3-methylcatechol (Chapman, 1972). In phenol-acclimated activated sludge, metabolites identified include 3-methylcatechol, 4-methylresorcinol, methylhydroquinone, a-ketobutyric acid, dihydrox-ybenzaldehyde, and trihydroxytoluene (Masunaga et al, 1986). 

This pyridoxal-phosphate-dependent enzyme [EC 4.4.1.11], also known as L-methioninase, catalyzes the conversion of L-methionine to methanethiol, ammonia, and a-ketobutyrate (or, 2-oxobutanoate). 

HYDROXYNICOTINATE REDUCTASE a-KETOBUTYRATE SYNTHASE a-KETOGLUTARATE SYNTHASE... 

FATTY ACID SYNTHETASE /3-KETOACYL-ACP SYNTHASE FATTY ACID SYNTHETASE a-KETOBUTYRATE SYNTHASE 2-Keto-3-deoxy-L-arabonate aldolase,... 

Drosopterin (87a) and isodrosopterin (87b) have been prepared by treatment of 7,8-dihydropterin (593) with p-hydroxy-a-ketobutyric acid (595) or a-hy-droxyacetoacetic acid (601) (506). Their initially proposed structures (507) have been revised to 87 (see Scheme 76) on the basis of detailed, H-NMR spectral analysis of 6,7-dimethyldrosopterin (602), which was prepared by treatment of 7-methyl-7,8-dihydropterin (603) with a-hydroxyacetoacetic acid (601) (50S). Structures of other drosopterins (88-90) have not yet been established. 

The route from methionine to homocysteine is described in more detail in Figure 18-18 the conversion of homocysteine to a-ketobutyrate in Figure 22-14 the conversion of propionyl-CoA to succinyl-CoA in Figure 17-11. 

Threonine is converted to pyruvate or to a-ketobutyrate, which forms succinyl CoA. 

Threonine is dehydrated to a-ketobutyrate, which is converted to propionyl CoA, the precursor of succinyl CoA (see p. 191) [Note-Threonine can also be converted to pyruvate.]... 

Cysteine is synthesized by two consecutive reactions in which homocysteine combines with serine, forming cystathionine, which, in turn, is hydrolyzed to a-ketobutyrate and cysteine (see Figure 20.8). Homocysteine is derived from methionine as described on p. 262. Because methionine is an essential amino acid, cysteine synthesis can be sustained only if the dietary intake of methionine is adequate. 

The subsequent cleavage of cystathionine to yield cysteine, a-ketobutyrate and NH4+ is catalyzed by y-cystathionase, a pyridoxal-phosphate-containing enzyme. This transsulfura-tion pathway is one of the routes used for methionine catabolism. 

The first step in valine biosynthesis is a condensation between pyruvate and active acetaldehyde (probably hy-droxyethyl thiamine pyrophosphate) to yield a-acetolactate. The enzyme acetohydroxy acid synthase usually has a requirement for FAD, which, in contrast to most flavopro-teins, is rather loosely bound to the protein. The very same enzyme transfers the acetaldehyde group to a-ketobutyrate to yield a-aceto-a-hydroxybutyrate, an isoleucine precursor. Unlike pyruvate, the a-ketobutyrate is not a key intermediate of the central metabolic routes rather it is produced for a highly specific purpose by the action of a deaminase on L-threonine as shown in figure 21.10. 

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