Coenzymes specificity

Several classes of vitamins are related to, or are precursors of, coenzymes that contain adenine nucleotides as part of their structure. These coenzymes include the flavin dinucleotides, the pyridine dinucleotides, and coenzyme A. The adenine nucleotide portion of these coenzymes does not participate actively in the reactions of these coenzymes rather, it enables the proper enzymes to recognize the coenzyme. Specifically, the adenine nucleotide greatly increases both the affinity and the speeifieity of the coenzyme for its site on the enzyme, owing to its numerous sites for hydrogen bonding, and also the hydrophobic and ionic bonding possibilities it brings to the coenzyme structure. 

They stated further that, the new adaptive enzyme catalyzing Reaction 3 appears to be similar to the malic enzyme of pigeon liver, although strictly DPN (instead of TPN)-specific. The coenzyme specificity explains the ready occurrence of Reaction 1. Therefore, the authors showed that exogenous NAD was required for the overall reaction (malic acid -> lactic acid), but because this activity was measured manometrically, they never demonstrated the formation of reduced NAD. Similarly, they did not attempt to show that pyruvic acid was the intermediate between L-malic acid and lactic acid. Instead, the formation of pyruvic acid was inferred from the NAD requirement and because the malic acid dissimilation activity remained constant during purification while the lactate dehydrogenase activity decreased (14). In fact, attempts to show any appreciable amounts of pyruvic acid intermediate failed (22). 

Table 16,1 The coenzyme specificity of some dehydr ogenases...

N. Holmberg, U. Ryde, and L.Bulow, Redesign of the coenzyme specificity in L-lactate dehydrogenase from Bacillus stearothermophilus using site-directed mutagenesis and media engineering,... 

T. Yaoi, K. Miyazaki, T. Oshima, Y. Komukai, and M. Go, Conversion of the coenzyme specificity of isodtrate dehydrogenase by module replacement, J. Biochem. (Tokyo) 1996, 119, 1014-1018. 

In order to overcome limitations derived from the protein structure such as lacking stability, substrate or coenzyme specificity, protein engineering methods... 

RD Chen, A Greer, JM Hurley, AM Dean. Engineering secondary structure to invert coenzyme specificity in isopropylmalate dehydrogenase. In DR Marshak, ed. Techniques in Protein Chemistry VIII. New York Academic Press, 1997, pp 809-816. 

RD Chen, AF Greer, AM Dean. A highly active decarboxylating dehydrogenase with rationally inverted coenzyme specificity. Proc Natl Acad Sci (USA) 92 11666-11670, 1995. 

RD Chen, A Greer, AD Dean. Structural constraints in protein engineering the coenzyme specificity of Escherichia coli isocitrate dehydrogenase. Eur J Biochem 250 578-582, 1997. 

In this study we combined two strategies common in protein engineering. Substitutions based on rational design within the nucleotide-binding pocket were used to convert E. coli IDH coenzyme specificity from NADP to NAD, while substitutions improving overall performance were identified by partial random mutagenesis at sites outside the nucleotide-binding pocket [5,12],... 

Enzymes are highly selective of the substrates with which they interact and in the reactions that they catalyze. This selective nature of enzymes collectively known as enzyme specificity can be best illustrated with oxidoreductases (dehydrogenases), which display substrate and bond specificities (e.g., acting on —CHOH—, versus —CHO versus —CH—CH— versus —CHNH2, and cis versus trans for unsaturated substrates), coenzyme specificity (e.g., NAD(H) versus NADP(H)), chiral stereospecificity (d- versus l- or R- versus S-stereoisomers), and prochiral stereospecificity (A versus B corresponding to proR- versus proS isomers and re face versus si face, respectively). The table lists some dehydrogenases and their coenzyme, substrate, product and stereospecificities (You, 1982) ... 

Apply Microsoft Access to design a database appended with queries for retrieving groups of dehydrogenases according to their coenzyme specificity and stereospecificity. 

Although numerous enzymatic reactions requiring vitamin B12 have been described, and 10 reactions for adenosylcobalamin alone have been identified, only three pathways in man have so far been recognized, one of which has only recently been identified (PI). Two of these require the vitamin in the adenosyl form and the other in the methyl form. These cobalamin coenzymes are formed by a complex reaction sequence which results in the formation of a covalent carbon-cobalt bond between the cobalt nucleus of the vitamin and the methyl or 5 -deoxy-5 -adenosyl ligand, with resulting coenzyme specificity. Adenosylcobalamin is required in the conversion of methylmalonate to succinate (Fig. 2), while methylcobalamin is required by a B12-dependent methionine synthetase that enables the methyl group to be transferred from 5-methyltetrahydrofolate to homocysteine to form methionine (Fig. 3). 

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