Hexokinase is a protein which is classified under the main grouping of a transferase enzyme. Its structure was first determined from yeast by Tom Steitz at Yale University. Hexokinase is the first enzyme in the glycolytic pathway, and it converts glucose into glucose-6-phosphate. It uses ATP to phosphorylate the 6-hydroxyl group of glucose, and it is inhibited by its product, glucose-6-phosphate. It is also allosterically relieved of product inhibition by phosphate. Thus, hexokinase governs the flow of glucose into energy metabolism of the bain and red blood cells. Glucose-6-phosphate and glucose bind synergistically to hexokinase, as do glucose and innorganic phosphate. Phosphate plays only a small role in the regulation of hexokinase during normal respiration, since glycolysis is limited by the supply of glucose (McDonald, 1979). During peroids of oxygen deprivation more ATP must come from glycolysis since pyruvate forms lactic acid instead of entering the Kreb's cycle. When ATP phosphrylates glucose to glucose-6-phosphate, it increase the Gibbs free energy from -31.0 KJ to 14.3 KJ (Aleshin, 98). This makes the reaction thermodynamically favorable. When the concentration of extracellular glucose is approximately 5mM, flux through the glycolytic pathway increase up to 100 % capacity. This occurs provided the glucose transporter of brain tissue raises intracellular concentrations of glucose by 50-fold and a mechanism exists to offset the inhibitory effects of glucose-6-phosphate on hexokinase.
The stoichiometry of bound glucose to hexokinase is approximately 1:1 with a Kd of 67 µM. Initial velocity kinetics provides a Km for glucose approximately 50 µM and no evidence of cooperativity in catalysis (Aleshin, 98). These findings have lead to one conclusion: one molecule of glucose binds tightly to the C-terminal half of the enzyme. However, based on the current structure glucose binds with high affinity to the N-terminal half under conditions similar to those of equilibrium binding studies. It is believed that the inability to form a salt link in the N-terminal half may destablize the open conformer compared to the closed conformer. Moreover, the catalytic half may require the presence of a second ligand (most likely ATP) to induce a closed conformation. This explains why in kinetic studies with ATP present, the Km for glucose represents the dissociation of the ligand from the closed conformer of the C-terminal half, in equilibrium binding studies (no ATP) the Kd for glucose represents the dissociation of the ligand from the N-terminal half (Zeng, 96). This suggests that only when glucose and ATP are bound together does a conformational change occur.
It should be mentioned that as with all other enzymes, hexokinase's complex structure has distinct contacts within it's active site which bind a specific substrate closely. This is viewed as a Lock and key model since it determines enzyme specificity. What best fits an enzymes active site is a substrate induced to adopt a conformation resembling the distortion of the enzyme and the substrate . This conformational change is either local or more widespread. Hexokinase exists in various forms within different organisms but is generally characterised by an extensive specificity for sugars and also a low Km for the sugar substrate. As the enzyme plays a major role in the first step of glycolysis the enzyme referred to must be present outside of the mitochondria membrane within the cytosol of eukaryotic cells. As intracellular glucose levels are usually far higher than the KM value for hexokinase, the enzyme often functions in vivo at saturating substrate concentrations (McDonald, 79).
[Colleges & Dept's] [Chemistry Dept.] [Chem 406]