Galactokinase (EC ) catalyzes the first committed step in the catabolism of galactose. The sugar is phosphor-ylated at position 1 at the expense of ATP. Lack of fully functional galactokinase is one cause of the inherited disease galactosemia, the main clinical manifestation of which is early onset cataracts. Human galactokinase (GALK1) was expressed in and purified fromEscherichia recom-binant enzyme was both soluble and active. Product inhi-bition studies showed that themost likelykineticmechanism of the enzyme was an ordered ternary complex one inwhich ATP is the first substrate to bind. .
