Kinetic mechanism of octopine dehydrogenase from the muscle of the sea mollusc, Concholepas concholepas

A structural basis for substrate selectivity and stereoselectivity in octopine dehydrogenase from Pecten maximus

Octopine dehydrogenase [N(2)-(D-1-carboxyethyl)-L-arginine:NAD(+) oxidoreductase] (OcDH) from the adductor muscle of the great scallop Pecten maximus catalyzes the reductive condensation of l-arginine and pyruvate to octopine during escape swimming. This enzyme, which is a prototype of opine dehydrogenases (OpDHs), oxidizes glycolytically born NADH to NAD(+), thus sustaining anaerobic ATP provision during short periods of strenuous muscular activity. In contrast to some other OpDHs, OcDH uses only l-arginine as the amino acid substrate. Here, we report the crystal structures of OcDH in complex with NADH and the binary complexes NADH/l-arginine and NADH/pyruvate, providing detailed information about the principles of substrate recognition, ligand binding and the reaction mechanism. OcDH binds its substrates through a combination of electrostatic forces and size selection, which guarantees that OcDH catalysis proceeds with substrate selectivity and stereoselectivity, giving rise to a second chiral center and exploiting a "molecular ruler" mechanism.

Purification, characterization, and cDNA cloning of opine dehydrogenases from the polychaete rockworm Marphysa sanguinea

Alanopine dehydrogenase (AlDH) and three isoforms of strombine/alanopine dehydrogenase (St/AlDH) were purified from muscle tissue of the polychaete rockworm Marphysa sanguinea. The four enzymes, which can be distinguished by the isoelectric point, are monomeric 42 kDa proteins, possess similar pH-activity profiles, and display specificity for pyruvate and NAD(H). The three isoforms of St/AlDH show equivalent Km and Vmax for glycine and L-alanine and for D-strombine and meso-alanopine. Free amino acid levels in the muscle and D-strombine accumulation in vivo during muscle activity suggest that St/AlDHs function physiologically as StDH. AlDH shows specificity for L-alanine and meso-alanopine, but not for glycine or D-strombine. The amino acid sequences of AlDH and one of the St/AlDH isoforms were determined by a combination of amino acid sequence analysis and cDNA cloning. St/AlDH cDNA consisted of 1586 bp nucleotides that encode a 399-residue protein (43,346.70 Da), and AlDH cDNA consisted of 1587 bp nucleotides that encode a 399-residue protein (43,886.68 Da). The two amino acid sequences deduced from the cDNA displayed 67% amino acid identity, with greatest similarity to that of tauropine dehydrogenase from the polychaete Arabella iricolor.

Chemical modification of octopine dehydrogenase by thiol-specific reagents: evidence for the presence of an essential cysteine at the catalytic site

Thiol-specific reagents, p-chloromercuricphenyl sulfonic acid (PCMS), 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) and N-ethyl-maleimide (NEM), incubated with octopine dehydrogenase resulted in the loss of catalytic activity. The kinetic profile of PCMS inactivated enzyme showed that the reaction followed pseudo-first-order kinetics during the initial phase of inactivation. The reversal of enzyme activity was obtained by thiol-containing reagents. The loss of enzyme activity was prevented only by NADH and not by other substrates. The dissociation constant for NADH calculated from the decrease in the enzyme inactivation rate was 45 microM. Cyanolysis of the DTNB-modified enzyme with KCN led to the release of 5-thio-2-nitrobenzoate (TNB) accompanied by the formation of thio-cyano enzyme. By correlating the enzyme activity with the formation of thio-cyano derivative it was found that no activity was recovered after KCN treatment. These evidences clearly established the critical involvement of the thiol group in catalysis. Double inhibition studies with PCMS and NEM on octopine dehydrogenase showed that the inactivating reagents bind to the same functional thiol present at the catalytic center. pH-dependent inactivation by PCMS indicated that a group with a pKa value of 7.4 is involved in the loss of enzyme activity. These approaches suggested that at least one thiol group present at the coenzyme-binding domain is essential for the catalytic reaction of octopine dehydrogenase.

Analysis of the kinetic mechanism of halophilic NADP-dependent glutamate dehydrogenase

The amination of 2-oxoglutarate catalyzed by NADP-specific glutamate dehydrogenase (EC 1.4.1.4, L-glutamate:NADP+ oxidoreductase (deaminating)) from Halobacterium halobium has been analyzed by initial rate, graphical analysis, and product and competitive inhibition studies. Initial rate and graphical analysis reveal that a B term (representing 2-oxoglutarate) is not statistically necessary for an initial rate equation. However, the absence of a B term does not distinguish between ordered and random binding of NADPH and ammonia. The patterns of product inhibition by NADP+ and L-glutamate, and competitive inhibition by hydroxylamine and succinate permit deduction of the kinetic mechanism as ordered, with NADPH, 2-oxoglutarate and ammonia added in that order, and L-glutamate release preceding NADP+ release.

Kinetic mechanism of Halobacterium halobium NAD+-glutamate dehydrogenase

The kinetic mechanism of Halobacterium halobium NAD+-glutamate dehydrogenase (EC 1.4.1.3) has been investigated at pH 9.0, 3 M NaCl and 40 degrees C in both directions, by initial rate and inhibition studies. The results of the initial rate studies indicate that the mechanism is sequential with respect to substrate addition. The inhibition patterns obtained with halophilic NAD+-glutamate dehydrogenase are not consistent with a simple ordered mechanism without modification. They can, however, be reconciled with this type of mechanism by postulating an appropriate abortive complex.