Isolation and characterization of the discrete N- and C-terminal halves of rat brain hexokinase: retention of full catalytic activity in the isolated C-terminal half

Abstract

Selective stabilization of either the N- or C-terminal half (by ligands binding to these regions) of rat brain hexokinase against partial denaturation with guanidine hydrochloride and subsequent digestion with trypsin has provided a means for isolating these regions, referred to as N fragment and C fragment, respectively, in quantities adequate for characterization. The N fragment (mol wt 52 kDa) is devoid of catalytic activity. In contrast, the C fragment (mol wt 51 kDa) has a specific activity of about 110 U/mg, nearly twice that (60 U/mg) of the intact 100-kDa enzyme, indicating that the kappa cat is virtually identical for both species. Unlike the parent enzyme, the C fragment is quite sensitive to inhibition by Pi (competitive vs ATP, noncompetitive vs Glc); sulfate and arsenate, but not acetate, inhibit with effectiveness similar to that seen with Pi. The Glc-6-P analog, 1,5-anhydroglucitol-6-P, also inhibits the C fragment (competitive vs ATP, uncompetitive vs Glc). Both N and C fragments bind to Affi-Gel Blue, an affinity matrix bearing a covalently attached analog of ATP, and are eluted by hexose 6-phosphates competitive with nucleotide binding to the parent enzyme. Based on the ability of various hexoses and hexose 6-phosphates (and analogs) to protect against guanidine-induced denaturation and subsequent proteolysis it is concluded that both fragments contain discrete sites for hexoses and hexose 6-phosphates, with specificities resembling those seen for the binding of these ligands to the parent enzyme. Synergistic interactions between the hexose and hexose-6-P binding sites, previously seen with the parent enzyme, are also observed with the C fragment but not the N fragment. The existence of binding sites for hexoses and hexose 6-phosphates on both halves conflicts with previous binding studies demonstrating a single hexose binding site and a single hexose 6-phosphate binding site on the intact 100-kDa enzyme, leading to the conclusion that one of each pair of sites must be latent in the intact enzyme, becoming manifest only in the isolated discrete halves. Several investigators have previously suggested that the 100-kDa mammalian hexokinases evolved by duplication and fusion of a gene encoding an ancestral 50-kDa Glc-6-P-insensitive hexokinase, similar to the present-day yeast enzyme, with sensitivity to Glc-6-P resulting from evolution of a duplicated catalytic site into a regulatory site.(ABSTRACT TRUNCATED AT 400 WORDS)