Hexokinase ( ATP- dependent D-hexose-6-phosphotransferase) ( EC 2.7.1.1) is a cytoplasmic enzyme of the transferase class, a subclass of phosphotransferases , the first enzyme of the glycolysis pathway . Unlike glucokinase , the Michaelis constant of hexokinase is 0.1 mmol / L, therefore, hexokinase, localized in the cells of most tissues of the human body, literally "captures" glucose from blood plasma, while glucokinase catalyzes the phosphorylation of glucose only at high concentrations. Accordingly, glucokinase and hexokinase redistribute the flow of glucose in the body: during the absorption of nutrients from the intestine, the concentration of glucose in the blood plasma increases and glucose is sent to the liver, where it is exposed to glucokinase; at the end of digestion - against the background of a decrease in glucose concentration - it is sent to the skeletal muscles, where hexokinase acts on it.
Hexokinase phosphorylates six-carbon sugars ( hexose ) using ATP energy:
- D-hexose + ATP → D-hexose-6-phosphate + ADP.
Hexoses such as D-glucose , D-mannose , D-fructose , sorbitol and D-glycosamine can be a phosphorylation substrate.
Of greatest importance is the phosphorylation of D-glucose, the reaction has the form:
- D-glucose + ATP → D- glucose-6-phosphate + ADP.
Hexokinase is found in bacteria , plants, and in all animals , including humans . In mammals , four isoenzymes, or isotypes, of hexokinase (I-IV) are known. Type IV is also called glucokinase . This hexokinase isotype is found primarily in hepatocytes .
A number of factors act on the activity of hexokinase in animal cells. Hexokinases I – III are inhibited by the reaction product glucose-6-phosphate. Insulin , adrenaline , thyroid hormones increase hexokinase activity in cells, while glucocorticoids , somatotropin reduce it.
See also
- glucokinase
- glycolysis
- insulin