Purification and Side Chain Selective Chemical Modifications of Glutamate Dehydrogenase from Bacillus subtilis Natto

Abstract

Glutamate dehydrogenase (GDH) from Bacillus subtilis natto was purified to apparent homogeneity by ammonium sulfate precipitation, ion-exchange chromatography, size exclusion chromatography, and hydroxyapatite (HA) affinity chromatography. The GDH was purified 34-fold, with a yield of 41% of total activity and a specific activity of 34.29U/mg proteins. The molecular weight (Mr) of was measured at 47kDa with sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and 264kDa with high-performance liquid chromatography (HPLC). The optimum pH and temperature for the deammoniation reaction were measured to be 7.5 and 30°C, respectively. The active-site amino acid residues of GDH were investigated by chemical modification. The compounds 2,4,6-trinitrobenzenesulfonic acid (TNBS), phenylglyoxal (PG), and phenylmethanesulfonyl fluoride (PMSF) were used to modify lysine, arginine, and serine active site residues, respectively. After treatment with modifying reagents at concentrations of 1mM, GDH activity fell to 10.7% with TNBS, 83.3% with PG, and 12.8% with PMSF. However, with substrate protection, there was almost no loss in GDH activity following treatment with any modifying reagent. The kinetic parameters K m and V max were determined in each case. K m values for native GDH, 50% TNBS-inactivated GDH, and 50% PMSF-inactivated GDH were 0.037, 0.104, and 0.017mM, respectively. V max values were 0.048, 0.022, and 0.031mM/s, respectively. These results suggest that the active site contains one or more lysine residues that play a role in substrate binding and one or more serine residues that may maintain the enzyme conformation. However, arginine residues played less of a role in the activity of GDH.