Isomerases, coenzymes

SERINE PALMITOYLTRANSFERASE SUCCINYL-CoA SYNTHETASE TRANSCARBOXYLASE VINYLACETYL-CoA ISOMERASE COENZYME Q (Ubiquinone)... 

Figure 4.9 (a) Triose phosphate isomerase (TIM), has a (3-a-(3 structure made up of eight P-a motifs terminating in a final a-helix, which form a barrel-like structure, (b) An open twisted P-sheet with helices on both sides, such as the coenzyme-binding domain of many dehydrogenases. (From Branden and Tooze, 1991. Reproduced by permission of Garland Publishing, Inc.)... 

Adenosylcobalamin (coenzyme 812) carries a covalently bound adenosyl residue at the metal atom. This is a coenzyme of various isomerases, which catalyze rearrangements following a radical mechanism. The radical arises here through homolytic cleavage of the bond between the metal and the adenosyl group. The most important reaction of this type in animal metabolism is the rearrangement of methylmalonyl-CoAto form succinyl-CoA, which completes the breakdown of odd-numbered fatty acids and of the branched amino acids valine and isoleucine (see pp. 166 and 414). 

Oxidation of unsaturated fatty acids requires two additional enzymes enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase. Odd-number fatty acids are oxidized by the /3-oxidation pathway to yield acetyl-CoA and a molecule of propionyl-CoA This is carboxylated to methylmalonyl-CoA, which is isomerized to succinyl-CoA in a reaction catalyzed by methylmalonyl-CoA mutase, an enzyme requiring coenzyme B12. 

The mercapturic acids and related compounds can then be exported from cells by an ATP-dependent export pump. Glutathione is a coenzyme for glyoxalase (Eq. 13-33), maleylacetoacetate isomerase (Eq. 13-20), and DDT dehydrochlorinase. The latter enzyme catalyzes elimination of HC1 from molecules of the insecticide and is especially active in DDT-resistant flies.3 Glutathione is said to be the specific factor eliciting the feeding reaction of Hydra that is, the release of glutathione from injured cells causes the little animal to engulf food. 

Other enzymes in the aconitase family include isopropylmalate isomerase and homoaconitase enzymes functioning in the chain elongation pathways to leucine and lysine, both of which are pictured in Fig. 17-18.90 There are also iron-sulfur dehydratases, some of which may function by a mechanism similar to that of aconitase. Among these are the two fumarate hydratases, fumarases A and B, which are formed in place of fumarase C by cells of E. coli growing anaerobically.9192 Also related may be bacterial L-serine and L-threonine dehydratases. These function without the coenzyme pyridoxal phosphate (Chapter 14) but contain iron-sulfur centers.93-95 A lactyl-CoA... 

RNS, ribonuclease LZM, lysozyme SNS, staphylococcal nuclease LZ4, T4 lysozyme PAP, papain TLS, thermolysin, TRX, thioiedoxin FLN, flavodoxin ADH, alcohol dehydrogenase coenzyme domain AKN, adenyl kinase MDG, malate dehydrogenase TIM, triosephosphate isomerase SUB, subtilisin CPA, carboxypeptidase LDH, lactate dehydrogenase PGK, phosphoglycerate kinase GPD, glyceraldehyde 3-phosphate dehydrogenase, HKN, hexokinase. 

Co corrinoids play central roles in the two classes of enzymic reactions, i.e. methyl transfer mediated by vitamin B,2 and mutase or isomerase reactions catalyzed by coenzyme B. 253 Though there remain many ambiguities, the former is considered to be a combination of Scheme 100, i and its reverse process, and the latter to be represented by Scheme 103. 

Fig. 1. Simplified diagram of the phenylpropanoid and flavonoid biosynthetic pathways. Enzymes that catalyze the reactions are placed on the left-hand side, and transcription factors on the right-hand side of the arrows. Both transcription factors for which their control over the enzymatic steps has been genetically proven, as well as transcription factors that have been shown to interact with promoters of the structural genes, are shown. PAL Phenylalanine ammonia lyase C4H cinnamate 4-hydroxylase 4CL 4-coumaroyl-coenzyme A ligase CHS chalcone synthase CHI chalcone-flavanone isomerase F3H flavanone 3(3-hydroxylase DFR dihydroflavonol 4-reductase AS anthocyanin synthase UFGT UDP glucose-flavonol glucosyl transferase RT anthocyanin rhamnosyl transferase...

Kiang, H., Luo, L., Taylor, K. L., and Dunaway-Maariano, D. (2000) Interexchange of catalytic activity within the 2-enoyl-coenzyme A hydratase/isomerase super family based on a common active site template,... 

In addition to serving as structural motifs, enols and enolates are involved in diverse biological processes. Several enol/enolate intermediates have been proposed to be involved in glycolysis (Section IV.A), wherein c/ -enediol 21 is proposed to be an intermediate in the catalytic mechanism of phosphohexose isomerase and an enol-containing enamine intermediate (22) has been proposed in the catalytic pathway of class I aldolase. In the case of glucose-fructose (aldose-ketose) isomerization, removal of the proton on Cl-OH produces the aldose while deprotonation of C2-OH yields the ketose, which is accompanied by protonation at the C2 and Cl positions, respectively. There are several cofactors that are involved in various biological reactions, such as NAD(H)/NADP(H) in redox reaction and coenzyme A in group transfer reactions. Pyridoxal phosphate (PLP, 23) is a widely distributed enzyme cofactor involved in the formation of a-keto acids, L/D-amino... 

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