Regulation of hexokinase I: crystal structure of recombinant human brain hexokinase complexed with glucose and phosphate.
Hexokinase I, the pacemaker of glycolysis in brain tissue and red blood cells, is comprised of two similar domains fused into a single polypeptide chain. The C-terminal half of hexokinase I is catalytically active, whereas the N-terminal half is necessary for the relief of product inhibition by phosphate. A crystalline complex of recombinant human hexokinase I with glucose and phosphate (2.8 A resolution) reveals a single binding site for phosphate and glucose at the N-terminal half of the enzyme. Glucose and phosphate stabilize the N-terminal half in a closed conformation. Unexpectedly, glucose binds weakly to the C-terminal half of the enzyme and does not by itself stabilize a closed conformation. Evidently a stable, closed C-terminal half requires either ATP or glucose 6-phosphate along with glucose. The crystal structure here, in conjunction with other studies in crystallography and directed mutation, puts the phosphate regulatory site at the N-terminal half, the site of potent product inhibition at the C-terminal half, and a secondary site for the weak interaction of glucose 6-phosphate at the N-terminal half of the enzyme. The relevance of crystal structures of hexokinase I to the properties of monomeric hexokinase I and oligomers of hexokinase I bound to the surface of mitochondria is discussed.