- Probing the chemical mechanism of saccharopine reductase from Saccharomyces cerevisiae using site-directed mutagenesis
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Saccharopine reductase catalyzes the reductive amination of l-α-aminoadipate-δ-semialdehyde with l-glutamate to give saccharopine. Two mechanisms have been proposed for the reductase, one that makes use of enzyme side chains as acid-base catalytic groups, and a second, in which the reaction is catalyzed by enzyme-bound reactants. Site-directed mutagenesis was used to change acid-base candidates in the active site of the reductase to eliminate their ionizable side chain. Thus, the D126A, C154S and Y99F and several double mutant enzymes were prepared. Kinetic parameters in the direction of glutamate formation exhibited modest decreases, inconsistent with the loss of an acid-base catalyst. The pH-rate profiles obtained with all mutant enzymes decrease at low and high pH, suggesting acid and base catalytic groups are still present in all enzymes. Solvent kinetic deuterium isotope effects are all larger than those observed for wild type enzyme, and approximately equal to one another, suggesting the slow step is the same as that of wild type enzyme, a conformational change to open the site and release products (in the direction of saccharopine formation). Overall, the acid-base chemistry is likely catalyzed by bound reactants, with the exception of deprotonation of the α-amine of glutamate, which likely requires an enzyme residue.
- Vashishtha, Ashwani K.,West, Ann H.,Cook, Paul F.
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- Glutamates 78 and 122 in the Active Site of Saccharopine Dehydrogenase Contribute to Reactant Binding and Modulate the Basicity of the Acid-Base Catalysts
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Saccharopine dehydrogenase catalyzes the NAD-dependent oxidative deamination of saccharopine to give L-lysine and α-ketoglutarate. There are a number of conserved hydrophilic, ionizable residues in the active site, all of which must be important to the overall reaction. In an attempt to determine the contribution to binding and rate enhancement of each of the residues in the active site, mutations at each residue are being made, and double mutants are being made to estimate the interrelationship between residues. Here, we report the effects of mutations of active site glutamate residues, Glu78 and Glu122, on reactant binding and catalysis. Site-directed mutagenesis was used to generate E78Q, E122Q, E78Q/E122Q, E78A, E122A, and E78A/E122A mutant enzymes. Mutation of these residues increases the positive charge of the active site and is expected to affect the pKa values of the catalytic groups. Each mutant enzyme was completely characterized with respect to its kinetic and chemical mechanism. The kinetic mechanism remains the same as that of wild type enzymes for all of the mutant enzymes, with the exception of E78A, which exhibits binding of α-ketoglutarate to E and E·NADH. Large changes in V/K Lys, but not V, suggest that Glu78 and Glu122 contribute binding energy for lysine. Shifts of more than a pH unit to higher and lower pH of the pKa values observed in the V/KLys pH-rate profile of the mutant enzymes suggests that the presence of Glu78 and Glu122 modulates the basicity of the catalytic groups.
- Ekanayake, Devi K.,Andi, Babak,Bobyk, Kostyantyn D.,West, Ann H.,Cook, Paul F.
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experimental part
p. 20756 - 20768
(2011/04/18)
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- VITAMIN COMPRISING PYROLOQUINOLINE QUINONE AND USE THEREOF
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It is an object of the present invention to clarify the biochemical role of pyrroloquinoline quinone (PQQ) in living bodies by identifying an enzyme that uses PQQ as a coenzyme in mammals and then by clarifying the oxidation-reduction reaction, with which PQQ is associated as a coenzyme in living bodies. The present invention provides a method of using pyrroloquinoline quinone as a coenzyme for 2-aminoadipate 6-semialdehyde dehydrogenase.
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Page/Page column 20
(2008/06/13)
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- Amino Acids, 6. - Investigations on the Synthesis of L-Saccharopine
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N-Alkyl-5-oxoproline esters 4 or 6 are obtained by alkyl desilylation of N-trimethylsilyl-5-oxoproline esters 2 with 6- or 2-halohexanoates 3, 5 in the presence of KF and crown-6.The higher reactivity of the 2-halo compounds 5 is established from comp
- Burkard, Ulrike,Walther, Ingrid,Effenberger, Franz
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p. 1030 - 1043
(2007/10/02)
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