Rat brain hexokinase: location of the allosteric regulatory site in a structural domain at the N-terminus of the enzyme.
After denaturation in 0.6 M guanidine hydrochloride, rat brain hexokinase becomes highly susceptible to proteolysis by trypsin. Glucose 6-phosphate (Glc-6-P) and its analog, 1,5-anhydroglucitol 6-phosphate, selectively protect the N-terminal half of the molecule from proteolysis. These compounds do not protect the C-terminal half of the molecule, nor do they protect enzyme activity; the Glc analog, N-acetylglucosamine, does protect the C-terminal domain and catalytic activity, but does not prevent proteolysis of the N-terminal half of the molecule. These results are consistent with previous work [M. Nemat-Gorgani and J. E. Wilson (1986) Arch. Biochem. Biophys. 251, 97-103; D. M. Schirch and J. E. Wilson (1987) Arch. Biochem. Biophys. 254, 385-396] demonstrating that binding sites for both hexose and nucleotide substrates, and thus catalytic function, are associated with a 40-kDa domain located at the C-terminus of the enzyme. They further demonstrate that the binding site for the allosteric effector, Glc-6-P, lies in the N-terminal half of the molecule and is distinct from the catalytic site. Using protection against proteolysis as a reflection of binding, it is shown that the Glc-6-P binding site in the N-terminal region has all the characteristics described for the allosteric effector site on this enzyme in terms of affinity for Glc-6-P, specificity, and synergistic interactions with the hexose binding site in the C-terminal region of the molecule. This disposition of catalytic and regulatory functions in discrete halves of the molecule is consistent with suggestions by several investigators that mammalian hexokinases evolved by a process of duplication and fusion of an ancestral gene coding for a hexokinase similar to the present-day yeast enzyme, with the regulatory site of mammalian hexokinases having evolved from what was originally a catalytic site.
Department of Biochemistry, Michigan State University, East Lansing 48824.
T K
TK
White
J E
JE
Wilson
D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000201:enzymology*; D019298:Glucose-6-Phosphate; D005958:Glucosephosphates / Q000378:metabolism; D019791:Guanidine; D006146:Guanidines / Q000494:pharmacology; D006593:Hexokinase / Q000032:analysis* / Q000037:antagonists & inhibitors; D006601:Hexoses / Q000378:metabolism; D010449:Peptide Mapping; D051381:Rats; D014508:Urea / Q000494:pharmacology
D005958:Glucosephosphates; D006146:Guanidines; D006601:Hexoses; D019298:Glucose-6-Phosphate; D014508:Urea; D006593:Hexokinase; D019791:Guanidine