Glutamine transferase

Histamine is synthesised by decarboxylation of histidine, its amino-acid precursor, by the specific enzyme histidine decarboxylase, which like glutaminic acid decarboxylase requires pyridoxal phosphate as co-factor. Histidine is a poor substrate for the L-amino-acid decarboxylase responsible for DA and NA synthesis. The synthesis of histamine in the brain can be increased by the administration of histidine, so its decarboxylase is presumably not saturated normally, but it can be inhibited by a fluoromethylhistidine. No high-affinity neuronal uptake has been demonstrated for histamine although after initial metabolism by histamine A-methyl transferase to 3-methylhistamine, it is deaminated by intraneuronal MAOb to 3-methylimidazole acetic acid (Fig. 13.4). A Ca +-dependent KCl-induced release of histamine has been demonstrated by microdialysis in the rat hypothalamus (Russell et al. 1990) but its overflow in some areas, such as the striatum, is neither increased by KCl nor reduced by tetradotoxin and probably comes from mast cells. 

Glycine A-benzoyltransferase [EC 2.3.1.71] catalyzes the reaction of benzoyl-CoA with glycine to produce coenzyme A and A-benzoylglycine. This enzyme is not identical with glycine A-acyltransferase or glutamine A-acyl-transferase [EC 2.3.1.68]. 

HOOC-CH-CH2-CO-COOH 1 OH Glutamin-Oxalo- essigsaure-Amino- transferase NH 1 HO2S— CH2—CH COOH HsO 20% 9 h erythro-... + threo-4-Hydroxy-glutaminsdure 43 43 98 98 4 vergl. 2... 

FIGURE 18-26 Catabolic pathways for arginine, histidine, glutamate, glutamine, and proline. These amino acids are converted to a-ketoglutarate. The numbered steps in the histidine pathway are catalyzed by histidine ammonia lyase, urocanate hydratase, imida-zolonepropionase, and glutamate formimino transferase. 

Glutamine is one of the principal combined forms of ammonia that is transported throughout the body (Chapter 24). Glucosamine 6-phosphate synthase, which catalyzes the reaction of Eq. 20-5, is an amido-transferase of the N-terminal nucleophile hydrolase superfamily (Chapter 12).31 It hydrolyzes the amide... 

Neither glutamine nor glutamyl naphthylamide are substrates. The specificity requirements for the acceptor are considerably more demanding in that acyl transfer to water (hydrolysis) or to hydroxylamine is not observed under any conditions. The transferase was separated chromatographically from an enzyme capable of hydrolyzing glutamyl naphthylamide which conceivably is related to the previously described transpeptidase. 

Two acyl-CoA amino acid A-acyltransferases have been purified from liver mitochondria of cattle, Rhesus monkeys, and humans. One is a benzoyltransferase CoA that utilizes benzyl-CoA, isovaleryl-CoA, and tiglyl-CoA, but not phenylacetyl CoA, malonyl-CoA, or indolacetyl-CoA. The other is a phenylacetyl transferase that utilizes phenylacetyl-CoA and indolacetyl-CoA but is inactive toward benzoyl-CoA. Neither is specific for glycine, as had been supposed from studies using less defined systems both also utilize asparagine and glutamine, although at lesser rates than glycine. 

Reliable enzymatic assays for SeMet are not available as specific SeMet metabolizing enzymes have not been identified and enzymes such as glutamine transaminase react with Met equally as well as with SeMet (Blazon et al., 1994). However, with some enzymes reaction rates for SeMet and Met differ sufficiently to be of some use in SeMet analysis. For example, SeMet is a better substrate than Met for the a,y-elimination by i.-methionine y-lyase of Pseudomonas putida (Esaki et al., 1979). The adenosyl methionine transferase from rat liver reacts with L-SeMet at 51% of the rate with L-Met, and with the corresponding D-isomers at only 13 and 10% of the rate of L-Met (Pan and Tarver, 1967). Other adenosyl methionine transferases, such as that from yeast, react with SeMet more rapidly and with higher stereoselectivity than with Met, providing an indirect means for SeMet determination (Mudd and Cantoni, 1957 Sliwkowski, 1984 Uzar and Michaelis, 1994). 

FIGURE 10.19 Deduced amino acid sequence for fructan fructan transferase (FFT) and sucrose sucrose 1-fructosyltransferase (SST). (From van der Meer, I.M. et al., Plant /., 15, 489-500, 1998.) Amino acids in bold represent homology between FFT and SST. Symbols A = alanine C = cysteine D = aspartate E = glutamate F = phenylalanine G = glycine H = histidine I = isoleucine K = lysine L = leucine M = methionine N = asparagine P = proline Q = glutamine R = arginine S = serine T = threonine V = valine W = tryptophan and Y = tyrosine. 

The reactions are catalyzed by acyl-CoA amino acid A-acyltransferase, of which two distinct A-acyltransferases exist in mammalian mitochondria. The predominant transferase conjugates medium-chained fatty acyl CoA and substituted benzoic acid derivatives with glycine and is termed an aralkyl-CoA glycine A-acyltransferase, while the other enzyme conjugates arylacetic acid derivatives with glycine, glutamine, or arginine and is an arylacetyl-CoA amino acid A-transferase. 

OH, NH2, SH groups glutamine synthetase plasmalemma ATPase coupling factor 1 mitochondrial membrane aspartate carbamoyl-transferase... 

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