Effects of bienzyme complex formation of cysteine synthetase from Escherichia coli on some properties and kinetics

Some properties and kinetics of the free and bound serine acetyltransferase s (SATs) and O-acetylserine sulfhydrylase-As (OASS-As) from Escherichia col i were investigated. In some cases, SAT Delta C20, deleting 20 amino acid r esidues from the C-terminus of the wild-type SAT (Biosci. Biotechnol. Bioch em., 63, 168-179 (1999)) was tested for comparison. The optimum pH and stab ility against some reagents for the free and bound wild-type SATs were simi lar except for the resistance to cold inactivation. The kinetics for the wi ld-type SAT and SAT Delta C20 followed a Ping-Pong Bi Bi mechanism with a m ixed-type inhibition by L-cysteine. The kinetics and kinetic constants for the wild-type SAT were not changed by the complex formation with OASS-A. Th e optimum pH for OASS-A was shifted towards an alkaline pH by the complex f ormation. Thermal stability and stability against some reagents for the fre e and bound OASS-As were almost the same. On the other hand, the maximum ve locity for OASS-A was lowered and dissociation constants for the substrates and products were increased by forming the complex: with the wild-type SAT , although the kinetics for the free and bound enzymes followed the same Pi ng-Pong Bi Bi mechanism. From comparisons of computed courses of L-cysteine formation from L-serine using SAT (wildtype SAT and SAT Delta C20) and OAS S-A with the experimental results and changes in the stability of the wild- type SAT by the complex formation, we discuss the role and significance of a complex formation for the cysteine synthetase.