Characterization of the allosteric anion-binding site of O-acetylserine sulfhydrylase.
A new crystal structure of the A-isozyme of O-acetylserine sulfhydrylase-A (OASS) with chloride bound to an allosteric site located at the dimer interface has recently been determined [Burkhard, P., Tai, C.-H., Jansonius, J. N., and Cook, P. F. (2000) J. Mol. Biol. 303, 279-286]. Data have been obtained from steady state and presteady-state kinetic studies and from UV-visible spectral studies to characterize the allosteric anion-binding site. Data obtained with chloride and sulfate as inhibitors indicate the following: (i) chloride and sulfate prevent the formation of the external aldimines with L-cysteine or L-serine; (ii) chloride and sulfate increase the external aldimine dissociation constants for O-acetyl-L-serine, L-methionine, and 5-oxo-L-norleucine; (iii) chloride and sulfate bind to the allosteric site in the internal aldimine and alpha-aminoacrylate external aldimine forms of OASS; (iv) sulfate also binds to the active site. Sulfide behaves in a manner identical to chloride and sulfate in preventing the formation of the L-serine external aldimine. The binding of chloride to the allosteric site is pH independent over the pH range 7-9, suggesting no ionizable enzyme side chains ionize over this pH range. Inhibition by sulfide is potent (K(d) is 25 microM at pH 8) suggesting that SH(-) is the physiologic inhibitory species.
Tai
C H
CH
Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman, Oklahoma 73019, USA.
Burkhard
P
P
Gani
D
D
Jenn
T
T
Johnson
C
C
Cook
P F
PF
D000495:Allosteric Site; D000838:Anions / Q000737:chemistry*; D001426:Bacterial Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism; D001667:Binding, Competitive; D002712:Chlorides / Q000737:chemistry; D003545:Cysteine / Q000096:biosynthesis; D003547:Cysteine Synthase / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D009646:Norleucine / Q000031:analogs & derivatives / Q000737:chemistry; D011485:Protein Binding; D013056:Spectrophotometry, Ultraviolet; D013431:Sulfates / Q000737:chemistry; D013440:Sulfides / Q000737:chemistry
D000838:Anions; D001426:Bacterial Proteins; D002712:Chlorides; D004791:Enzyme Inhibitors; D013431:Sulfates; D013440:Sulfides; D009646:Norleucine; D003547:Cysteine Synthase; D003545:Cysteine