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Hexokinase is an enzyme that irreversibly phosphorylates hexoses (six-carbon sugars), forming hexose phosphate. Hexokinase II (HK-II) is the predominant isoform found in insulin-sensitive tissues such as skeletal muscle, heart, and adipose tissues. Hexokinase II is also located at the mitochondria outer membrane, which gives it direct access to ATP. Hexokinase II plays an essential role in glucose utilization by cells and is increasingly recognized as a component of a survival signaling nexus.
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What You'll Learn
- Hexokinase II is the predominant isoform in muscle and heart
- Hexokinase I and II are the two isoforms found in human skeletal muscle
- Hexokinase is a rate-limiting glycolytic enzyme
- Hexokinase II is found in increased amounts in many cancers
- Hexokinase I and II can bind to the outer surface of mitochondria
Hexokinase II is the predominant isoform in muscle and heart
Hexokinase is a tissue-specific isoenzyme that catalyzes the phosphorylation of glucose to glucose-6-phosphate (G6P). Hexokinase possesses the ability to transfer an inorganic phosphate group from ATP to a substrate. This is the first step in glycolysis, which is also rate-limiting. G6P serves as the starting point for the sugar to enter the glycolic pathway or the PPP, or for glycogen synthesis.
Hexokinase has four isoforms: I, II, III, and IV. Hexokinase II (HK-II) is the predominant isoform in insulin-sensitive tissues such as the heart, skeletal muscle, and adipose tissues. It is also upregulated in many types of tumors associated with enhanced aerobic glycolysis in tumor cells, known as the Warburg effect. HK-II is found mainly in skeletal muscle and adipose tissues.
The relative specific activity of hexokinase II increases with pH, at least in a pH range from 6.9 to 8.5. Hexokinase II is also located at the mitochondria outer membrane so it can have direct access to ATP. Mammalian skeletal muscles contain two hexokinase isozymes: hexokinase type I and type II. Hexokinase II activity was largely bound to the particulate fraction (72%), but 28% was found within the cytosolic fraction.
The hormonal regulation of hexokinase II expression suggests a specific role in glucose metabolism. Binding of hexokinase isozymes to mitochondria and other subcellular structures has been considered as a potential mechanism for the regulation of enzyme activity and is therefore potentially important for the regulation of glucose metabolism.
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Hexokinase I and II are the two isoforms found in human skeletal muscle
Hexokinase is an enzyme that catalyses the initial phosphorylation of glucose to form glucose 6-phosphate. Hexokinase is found in several cellular compartments, including the outer mitochondrial membrane and in mitochondria. It is known to be present in muscle and heart tissue.
Hexokinase has four isoforms, designated HK I, HK-II, HK III, and HK IV (glucokinase). Hexokinase I and II are the two isoforms found in human skeletal muscle. They are also found in the heart. Hexokinase I is found in all mammalian tissues and is considered a "housekeeping enzyme", unaffected by most physiological, hormonal, and metabolic changes. Hexokinase II is the predominant isoform in insulin-sensitive tissues such as heart, skeletal muscle, and adipose tissues. It is also upregulated in many types of tumors and is associated with enhanced aerobic glycolysis in tumor cells, known as the Warburg effect.
The activity and subcellular distribution of hexokinase isozymes in human skeletal muscle have been studied using ion-exchange chromatography and a highly sensitive high-performance liquid chromatography–based hexokinase assay. The results suggest that insulin can regulate hexokinase II expression. However, the intracellular distribution of these hexokinase isoforms in human skeletal muscle is still not entirely clear.
Hexokinase plays an essential role in glucose utilization by cells. Glucose 6-phosphate, the product of the hexokinase reaction, is a key intermediate in glucose conversion in metabolic pathways, including ATP and NADPH regeneration and glycogen synthesis. Hexokinase might also be considered part of the glucose transport system as it maintains a high transmembrane gradient for free glucose.
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Hexokinase is a rate-limiting glycolytic enzyme
The enzyme is found in several isoforms, designated as hexokinase I, II, III, and IV, or A, B, C, and D. These isoforms have distinct functions and are distributed across different tissues and cell types. Hexokinase I/A, for example, is considered a "housekeeping enzyme" and is present in all mammalian tissues. On the other hand, hexokinase II/B is the predominant isoform found in muscles, the heart, and adipose tissues. It is also associated with tumour progression and is often overexpressed in cancer cells.
The activity and distribution of hexokinase isozymes vary between different tissues. In human skeletal muscle, hexokinase I and II account for the majority of hexokinase activity, with hexokinase II exhibiting a strong binding affinity for mitochondria. The regulation of hexokinase II expression is influenced by factors such as insulin and pH levels.
Hexokinase plays a vital role in glucose utilisation by cells. The conversion of glucose to G6P is a crucial step in metabolic pathways, including ATP and NADPH regeneration, as well as glycogen synthesis. Hexokinase helps maintain a high transmembrane gradient for free glucose, facilitating its transport into cells.
Furthermore, hexokinase is involved in the regulation of insulin and glucagon release in pancreatic cells. Hexokinase IV, also known as glucokinase, is present in the liver, pancreas, hypothalamus, and small intestine. It serves as a glucose sensor, responding to changes in ambient glucose levels and regulating carbohydrate metabolism.
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Hexokinase II is found in increased amounts in many cancers
Hexokinase is an enzyme that irreversibly phosphorylates hexoses (six-carbon sugars) to form hexose phosphate. Hexokinase II (HK-II) is one of the four isoforms of hexokinase, which is found in skeletal muscle and adipose tissues. HK-II is the predominant isoform in the heart.
HK-II is found in increased amounts in many cancers, particularly the most aggressive ones. This overexpression of HK-II is associated with a poor prognosis. It is a key hallmark of cancer, providing cancer cells with a distinct competitive edge over normal cells. The elevated levels of HK-II in cancer cells support the highly glycolytic phenotype, which is critical for the rapid growth of cancer cells. This is achieved through the rapid entry of glucose into cancer cells via the glucose transporter, followed by phosphorylation by HK-II to produce glucose-6-phosphate at an elevated rate. This critical metabolite serves as a biosynthetic precursor for cell proliferation and lactic acid production.
The abundant amounts of HK-II in cancer cells are bound to the outer mitochondrial membrane via the voltage-dependent anion channel (VDAC). This binding provides HK-II with direct access to ATP, one of its substrates. The binding of HK-II to mitochondria is crucial for the regulation of enzyme activity and glucose metabolism. This mitochondrial-bound HK-II has been shown to drive the extremely high glycolytic rates observed in tumor cells, also known as the Warburg effect.
The specific role of HK-II in glucose metabolism and its increased presence in many cancers make it a potential target for cancer treatment. By inhibiting HK-II activity or targeting its interactions with mitochondria, it may be possible to disrupt the highly glycolytic phenotype and impede cancer cell growth and metastasis.
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Hexokinase I and II can bind to the outer surface of mitochondria
Hexokinase is an enzyme that catalyses the phosphorylation of glucose to form glucose 6-phosphate. This is the first step in glycolysis, a metabolic pathway. Hexokinase is found in several cellular compartments, including the outer mitochondrial membrane and in mitochondria.
Hexokinase has four isoforms, designated HK I, HK-II, HK III, and HK IV (glucokinase). HK-II is the predominant isoform in insulin-sensitive tissues such as the heart, skeletal muscle, and adipose tissues. It is also found in the brain. HK-II is upregulated in many types of tumors and is associated with enhanced aerobic glycolysis in tumor cells, known as the Warburg effect.
HK-II is located at the mitochondria outer membrane, where it has direct access to ATP. It binds to voltage-dependent anion channel 1 (VDAC1), an outer mitochondrial membrane protein, which interacts with the adenine nucleotide translocase (ANT). This forms a contact site between the outer and inner mitochondrial membranes. The mitochondrially bound HKs provide facilitation of coupling between glycolysis and oxidative phosphorylation through preferential access to ATP generated by mitochondria.
HK-I also binds to the outer mitochondrial membrane via a mitochondrial binding motif at the N-terminal. This high affinity of HK-I for mitochondria is consistent with its known function.
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Frequently asked questions
Hexokinase is found in skeletal muscle and cardiac muscle.
Hexokinase type I and type II are found in skeletal muscle.
Hexokinase is an enzyme that phosphorylates hexoses, which are sugars with six carbons, resulting in hexose phosphate.
Hexokinase catalyses the initial phosphorylation of glucose to form glucose 6-phosphate. This is the first step of glycolysis.
The tertiary structure of hexokinase includes an ATP-binding domain, composed of five beta sheets and three alpha helices.
