Specificity of ligand binding to yeast hexokinase PII studied by STD-NMR

Hexokinase catalyzes the phosphorylation of glucose and is the first enzyme in glycolysis. To investigate enzyme-ligand interactions in yeast hexokinase isoform PII under physiological conditions, we utilized the technique of Saturation Transfer Difference NMR (STD NMR) to monitor binding modes and binding affinities of different ligands at atomic resolution. These experiments clearly show that hexokinase tolerates several changes at C-2 of its main substrate glucose, whereas epimerization of C-4 significantly reduces ligand binding. Although both glucose anomers bind to yeast hexokinase, the alpha-form is the preferred form for the phosphorylation reaction. These findings allow mapping of tolerated and prohibited modification sites on the ligand. Furthermore, competitive titration experiments show that mannose has the highest binding affinity of all examined sugars. As several naturally occurring sugars in cells show binding affinities in a similar range, hexokinase may be considered as an 'emergency enzyme' in yeast cells. Taken together, our results represent a comprehensive analysis of ligand-enzyme interactions in hexokinase PII and provide a valuable basis for inhibitor design and metabolic engineering.