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Full-length hexokinase (HK; ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1), a truncate form of the enzyme lacking the first 11 amino acids (HK-11aa) and the 50 kDa C-terminal half ('mini'-HK) containing the catalytic domain, were overexpressed and purified to homogeneity to investigate the influence of the N-terminal region of human hexokinase type I (HK) on its regulatory properties. All forms of the enzyme are catalytically active with the HK-11aa being the most active. All the forms of HK showed the same affinity for glucose and MgATP and were also inhibited by glucose 6-phosphate (Glc 6-P) competitively vs. MgATP with similar Kis (28.5-37 microM). Glucose 1,6-bisphosphate (Glc 1,6-P2) was also a strong inhibitor of all HKs without significant differences among the different truncate forms of the enzyme (Kis 49.5-59 microM). At low concentrations (0-3 mM), Pi was able to reverse the sugar phosphate inhibition of the full-length HK and HK-11aa but not of the 'mini'-HK. In contrast, at high concentrations Pi was an inhibitor of all the hexokinases investigated. These findings confirm that Pi has a low affinity binding site on the C-terminal of HK while counteracts glucose 6-phosphate inhibition by binding to or requiring the N-terminal half of the enzyme. The first 11 N-terminal amino acids influence the specific activity of HK but are unable to affect the kinetic properties investigated.

Human hexokinase type I, one of the four isozymes consisting of a single polypeptide chain of about l 00 kDa, has been cloned in the pET expression plasmid in a truncated form lacking a segment of 11 apolar amino acids at the N-terminus. The protein has been overexpressed in E.coii and purified to homogeneity. Truncated hexokinase I has been crystallized in the presence of glucose 6-phosphate and of the ATP analogue AMP-PNP. The crystals belong to the monoclinic space group P21, with unit cell constants: a= 84.2 A, b = 177.7 A, c = 88.2 A, 13 = 90.8° and diffract to 3.0 A resolution.

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