page_title: Glycolysis
section_title: Hexokinase and glucokinase
hierarchical_section_title: Glycolysis / Regulation / Regulation of the rate limiting enzymes / Hexokinase and glucokinase
caption_attribution_description: English: By Richard Wheeler (Zephyris) 2006. For the metabolic pathways wikiproject.
context_page_description: Glycolysis is the metabolic pathway that converts glucose C₆H₁₂O₆, into pyruvate, CH₃COCOO⁻, and a hydrogen ion, H⁺. The free energy released in this process is used to form the high-energy molecules ATP and NADH. Glycolysis is a sequence of ten enzyme-catalyzed reactions. Most monosaccharides, such as fructose and galactose, can be converted to one of these intermediates. The intermediates may also be directly useful rather than just utilized as steps in the overall reaction. For example, the intermediate dihydroxyacetone phosphate is a source of the glycerol that combines with fatty acids to form fat.
  Glycolysis is an oxygen-independent metabolic pathway. The wide occurrence of glycolysis indicates that it is an ancient metabolic pathway. Indeed, the reactions that constitute glycolysis and its parallel pathway, the pentose phosphate pathway, occur metal-catalyzed under the oxygen-free conditions of the Archean oceans, also in the absence of enzymes.
  In most organisms, glycolysis occurs in the cytosol. The most common type of glycolysis is the Embden–Meyerhof–Parnas, which was discovered by Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas.
context_section_description: All cells contain the enzyme hexokinase, which catalyzes the conversion of glucose that has entered the cell into glucose-6-phosphate (G6P). Since the cell membrane is impervious to G6P, hexokinase essentially acts to transport glucose into the cells from which it can then no longer escape.  Hexokinase is inhibited by high levels of G6P in the cell. Thus the rate of entry of glucose into cells partially depends on how fast G6P can be disposed of by glycolysis, and by glycogen synthesis (in the cells which store glycogen, namely liver and muscles).
  Glucokinase, unlike hexokinase, is not inhibited by G6P. It occurs in liver cells, and will only phosphorylate the glucose entering the cell to form glucose-6-phosphate (G6P), when the sugar in the  blood is abundant. This being the first step in the glycolytic pathway in the liver, it therefore imparts an additional layer of control of the glycolytic pathway in this organ.
caption: Yeast hexokinase B (PDB: 1IG8​)
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