Active-site modification of glycerol-3-phosphate dehydrogenase by pyridoxal-5'-phosphate

X-ray studies of nucleotide binding and pyridoxal phosphate labeling of the gene 5 DNA unwinding protein

X-ray diffraction studies have been carried out using difference Fourier methods to evaluate the reaction or interaction of an affinity label and 5'-phosphate nucleotides with the gene 5 DNA binding protein in the crystalline state. In the first case the crystalline protein was reacted with pyridoxal phosphate. Pyridoxal phosphate, which has served as an affinity label for nucleotide binding sites on other enzymes, demonstrated a major site of substitution at the center of the protein's DNA binding cleft adjacent to lysine 46 as well as two other reaction sites near residues implicated in DNA binding. Difference Fourier maps of crystals exposed to 5'-dAMP, 5'-dCMP and 5'-dTMP indicated that phosphate groups were associated with most lysine and arginine side-chains on the surface of the protein but that the nucleoside portion of the ligands were generally disordered. In several cases, however, more specific binding of the nucleotides appeared to have occurred and these sites were primarily within the proposed DNA binding cleft of the protein. In particular, binding was observed near tyrosine 34, phenylalanine 73 and within the curl of the DNA binding loop containing tyrosine 26.

Chemical modification of NADPH-cytochrome P-450 reductase. Presence of a lysine residue in the rat hepatic enzyme as the recognition site of 2'-phosphate moiety of the cofactor

Chemical modification of rat hepatic NADPH-cytochrome P-450 reductase by sodium 2,4,6-trinitrobenzenesulfonate (TNBS) resulted in a time-dependent loss of the reducing activity for cytochrome c. The inactivation exhibited pseudo-first-order kinetics with a reaction order approximately one, and a second-order constant of 4.8 min-1 X M-1. The reducing activities for 2,6-dichloroindophenol and K3Fe(CN)6 were also decreased by TNBS. Almost complete protection of the NADPH-cytochrome P-450 reductase from inactivation by TNBS was achieved by NADP(H), while partial protection was obtained with a high concentration of NADH. NAD, FAD and FMN showed no effect against the inactivation. 3-Acetylpyridine-adenine dinucleotide phosphate, adenosine 2',5'-bisphosphate and 2'AMP protected the enzyme against the chemical modification. Stoichiometric studies showed that the complete inactivation was caused by modification of three lysine residues per molecule of the enzyme. But, under the conditions where the inactivation was almost protected by NADPH, two lysine residues were modified. From those results, we propose that one residue of lysine is located at the binding site of the 2'-phosphate group on the adenosine ribose of NADP(H), and plays an essential role in the catalytic function of the NADPH-cytochrome P-450 reductase.