May 1, 1984

Chemical mechanism of saccharopine dehydrogenase (NAD+, L-lysine-forming) as deduced from initial rate pH studies

Archives of Biochemistry and Biophysics
M Fujioka


The variation of kinetic parameters with pH has been determined so as to gain insight into the chemical mechanism of the saccharopine dehydrogenase (NAD+,L-lysine-forming)-catalyzed reaction. In the direction of reductive condensation of lysine and alpha-ketoglutarate (reverse reaction), the V/K profile for lysine shows a group with a pK of 6.3 must be unprotonated and a group with a pK of 8.0 must be protonated for activity. Similar pK's are obtained in the pKi profile for ornithine, which acts as a linear competitive inhibitor with respect to lysine. Temperature and solvent perturbation studies show that these groups are probably histidines. The V/K profile for alpha-ketoglutarate reveals a single group with pK = 8.4 (probably lysine) that must be protonated. It is proposed that one of the histidines is involved in the binding of the epsilon-amino group of the substrate lysine and the positively charged lysine residue hydrogen bonds to the carbonyl oxygen of alpha-ketoglutarate. In the direction of saccharopine cleavage, the V/K profile for saccharopine shows that two groups with pK values of 6.0 and 7.1, possibly a histidine and lysine, must be unprotonated for its reaction with the enzyme X NAD+ complex. The log V-pH plots ...Continue Reading

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Mentioned in this Paper

Saccharopine dehydrogenase (NADP+, lysine-forming)
Alpha Ketoglutarate
Oxidoreductases Acting on CH-NH Group Donors
Cytokinesis of the Fertilized Ovum
Urine Lysine Measurement
Hydrogen-Ion Concentration

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