Hexokinase is the first glycolysis enzyme.
- Function The hexokinase phosphorylates the glucose at the C6 atom by transferring a phosphoryl group from ATP to glucose-6-phosphate.
- Structure link to E. coli-Genome Database (ECDC): glk.
- The transfer of a phosphoryl group is a fundamental reaction in biochemistry. Enzymes that catalyze this process are called kinases. All kinases require divalent metal ions such as Mg to complex with ATP2+. The hexokinase has two flexible lobes that form a gap. When binding with glucose, this gap is filled. As a result, the lobes shift towards each other (conformational change) and close this gap. The substra-induced closure of the gap (induced fit) is a typical characteristic of kinases. One isozyme is glucokinase (hexokinase IV), which occurs predominantly in the liver and pancreas and has a 50-fold lower affinity for glucose than hexokinase. However, in contrast to hexokinase, the activity of glucokinase is not inhibited by the glucose-6-phosphate formed. Because of the lower affinity of glucokinase for glucose, significant activity is only found at high blood sugar levels. This fact ensures, on the one hand, that the brain and muscles are supplied with glucose and, on the other hand, it prevents glucose from overenriching in the blood. The glucose-6-phosphate molecules formed by glucokinase are converted into glycogen for short- and medium-term storage.
- Enzymatic reaction Glucose + ATP ⇌ Glucose-6-phosphate + ADP