title: Ion-pair formation as a source of enhanced reactivity of the essential thiol group of D-glyceraldehyde-3-phosphate dehydrogenase.
abstract: The reactivity and the mode of activation of the essential--SH group (Cys-149) of D-glyceraldehyde-3-phosphate dehydrogenase have been studied by means of a spectrophotometric method [Polgár, L., FEBS Lett. 38, 187-190 (1974)], capable of detecting the dissociated form of the thiol group in proteins. Alkylations of Cys-149 of NAD-free D-glyceraldehyde-3-phosphate dehydrogenase with iodoacetamide and iodoacetate were investigated. The corrected absorbance change on alkylation at 250 nm (which is a direct parameter of the dissociation of the thiol group) and the alkylation rate were determined as a function of pH. The pH profiles of both dissociation and alkylation rate of Cys-149 conform to doubly sigmoid curves. All these curves implicate two ionizing groups (pK1 equals 5.5, pK2 equals 8.2). It is concluded that there are two reactive forms of the--SH group in the apoenzyme between pH 5 and 10. One reactive form corresponds to the free mercaptide ion. The other can be identified with an ion-pair composed of a mercaptide ion and some base, possibly the imidazolium group of His-176. The ion-pair has lower molar absorption coefficient and nucleophilicity than the free mercaptide ion. The two reactive forms are transformed into each other with pK2 equals 8.2. The ion-pair decomposes to a nondissociated thiol group and a protonated base with pK1 equals 5.5. In the presence of NAD, only the pH-rate profile of alkylation of D-glyceraldehyde-3-phosphate dehydrogenase was measured (at 370 nm). Using iodoacetamide as alkylating agent we also obtained a doubly sigmoid curve. A slight downward shift on pK1 and an upward shift in pK2 indicate that the ion-pair exists in a somewhat wider pH-range in the enzyme-coenzyme complex. An increase in the ionic strength of the reaction mixture from 0.09 to 0.45 M does not abolish the doubly sigmoid character of the curves determined either in the presence or in the absence of NAD.
mesh_terms: D000478:Alkylation; D000818:Animals; D001051:Apoenzymes / Q000378:metabolism; D003545:Cysteine / Q000378:metabolism; D004789:Enzyme Activation; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007477:Ions; D007700:Kinetics; D009132:Muscles / Q000201:enzymology; D009243:NAD / Q000494:pharmacology; D013056:Spectrophotometry, Ultraviolet; D013438:Sulfhydryl Compounds / Q000378:metabolism*; D013552:Swine
chemical_list: D001051:Apoenzymes; D007477:Ions; D013438:Sulfhydryl Compounds; D009243:NAD; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases; D003545:Cysteine