Glucose is the major form in which dietary sugars are made available to cells of the human body. Its breakdown is a major source of energy for all cells, and is essential for the brain and red blood cells. Glucose utilization begins with its uptake by cells and conversion to glucose 6-phosphate, which cannot traverse the cell membrane. Fates open to cytosolic glucose 6-phosphate include glycolysis to yield pyruvate, glycogen synthesis, and the pentose phosphate pathway. In some tissues, notably the liver and kidney, glucose 6-phosphate can be synthesized from pyruvate by the pathway of gluconeogenesis.
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While there are multiple human glyceraldehyde 3-phosphate dehydrogenase-like pseudogenes, there is only one glyceraldehyde 3-phosphate dehydrogenase gene expressed in somatic tissue (Benham and Povey 1989). Consistent with this conclusion, the homogeneous enzymes purified from various human tissues had indistinguishable physical and immunochemical properties (Heinz and Freimuller 1982), and studies of human erythrocytes of various ages suggested that variant forms of the enzyme arise as a result of post-translational modifications (Edwards et al. 1976). There is, however, an authentic second isoform of glyceraldehyde 3-phosphate dehydrogenase whose expression is confined to spermatogenic cells of the testis (Welch et al. 2000).
At the beginning of this reaction, 1 molecule of 'pPF2K-Pase complex' is present. At the end of this reaction, 1 molecule of 'Orthophosphate', and 1 molecule of 'PF2K-Pase1 homodimer' are present.
This reaction takes place in the 'cytosol' and is mediated by the 'phosphatidate phosphatase activity' of 'PP2A-ABdeltaC complex'.
Activated PKA (protein kinase A) phosphorylates serine 36 of the bifunctional 6-Phosphofructo-2-kinase /Fructose-2,6-bisphosphatase (PFKFB1) enzyme. This phosphorylation inhibits the enzyme's phosphofructokinase (PFK-2) activity while activating its phosphatase activity. As a result, cytosolic levels of Fructose-2,6-bisphosphate (F-2,6-P2) are reduced. F-2,6-P2 in turn is a key positive regulator of the committed step of glycolysis, so the net effect of this phosphorylation event is a reduced rate of glycolysis.
The cytosolic glucokinase (GCK1):glucokinase regulatory protein (GKRP) complex dissociates.However, while GCK1:GKRP complex formation is reversible (Brocklehurst et al. 2004) the major fate of these complexes in vivo appears to be transport to the nucleus (Shiota et al. 1999).
In the presence of high glucose concentrations, the complex of nucleoplasmic glucokinase (GCK1) and glucokinase regulatory protein (GKRP) dissociates (Shiota et al. 1999).
Glucokinase (GCK1) reversibly binds glucokinase regulatory protein (GKRP) to form an inactive complex. Binding is stimulated by fructose 6-phosphate and sorbitol 6-phosphate (hence high concentrations of these molecules tend to reduce GCK1 activity) and inhibited by fructose 1-phosphate (hence a high concentration of this molecule tends to increase GCK1 activity) (Brocklehurst et al. 2004).
Free glucokinase (GCK1) leaves the nucleus via the nuclear pore. While the GCK1 protein contains a nuclear export sequence motif, the molecular details of the GCK1 export process remain to be worked out (Shiota et al. 1999).
The SLC25A11 transport protein in the inner mitochondrial membrane mediates the reversible exchange of mitochondrial malate and cytosolic alpha-ketoglutarate (2-oxoglutarate). Qualitative evidence for this process comes from studies of the human protein (Kabe et al. 2006); kinetic details are inferred from stdies of rat mitochondria under conditions in which only the one transport protein appears to be active (Sluse et al. 1973). The dimeric state of the transport protein is inferred from studies of its bovine homologue (Bisaccia et al. 1996).
Glucose generated within the endoplasmic reticulum is exported from the cell. Several mechanisms for this transport process have been proposed but experimental data remain incomplete and contradictory (e.g., Hosokawa and Thorens 2002; Fehr et al. 2005; Van Schaftingen and Gerin 2002).
Cytosolic malate dehydrogenase catalyzes the reaction of malate and NAD+ to form oxaloacetate and NADH + H+ (Lo et al. 2005). The dimeric structure of the human dehydrogenase is inferred from that established for its well-studied pig homolog (Birktoft et al. 1989).
The SLC37A4 transport protein in the endoplasmic reticulum membrane mediates the exchange of cytosolic glucose-6-phosphate and orthophosphate from the endoplasmic reticulum lumen. Defects in this transporter are associated with glycogen storage disease type Ib (Gerin et al. 1997; Chen et al. 2008; Veiga-da-Cunha et al. 1998).
The SLC37A1 and SLC37A2 transport proteins mediate the exchange of glucose-6-phosphate and orthophosphate across lipid bilayer membranes in vitro, and both proteins are located in the endoplasmic reticulum membrane. Their physiological function, however, is unknown (Pan et al. 2011; Chou et al. 2013).
Glucose-6-phosphatase 3 (G6PC3) associated with the endoplasmic reticulum membrane catalyzes the hydrolysis of glucose-6-phosphate to glucose and orthophosphate. This reaction is essentially irreversible (Guionie et al. 2003; Ghosh et al. 2004). In the body, this enzyme is ubiquitously expressed; mutations that inactivate it are associated with severe congenital neutropenia (but not with fasting hypoglycemia or lactic acidemia) (Boztug et al. 2009).
Glucose-6-phosphatase 2 (G6PC2), associated with the endoplasmic reticulum membrane catalyzes the hydrolysis of glucose-6-phosphate to glucose and orthophosphate. This reaction is essentially irreversible. In the body, this enzyme is islet cells of the pancreas (Petrolonis et al. 2004).
Calcium-binding mitochondrial carrier proteins Aralar1 and Aralar2 (SLC25A12 and SLC25A13 respectively), located in the inner mitochondrial membrane, mediate the exchange of cytosolic aspartate and mitochondrial glutamate (Palmieri et al. 2001). The exchange is physiologically irreversible because of the potential across the inner mitochondrial membrane (positive outside, negative inside). In the body, SLC25A12 is found mainly in heart, skeletal muscle, and brain, while SCL25A13 is widely expressed but most abundant in liver (del Arco et al. 2000; Palmieri et al. 2001). Defects in SLC25A13 are associated with type II citrullinemia, characterised by a liver-specific deficiency of the urea cycle enzyme argininosuccinate synthase (Kobayashi et al. 1999, Saheki et al. 2002).
SLC25A1, in the inner mitochondrial membrane, mediates the exchange of cytosolic citrate for mitochondrial phosphoenolpyruvate. The exchange is physiologically irreversible because of the potential of the inner mitochondrial membrane. The gene encoding human SLC25A1 has been cloned and its expression pattern has been characterized (Heisterkamp et al. 1995; Iacobazzi et al. 1997), but the biochemical details of the transport process are inferred from those worked out for the well-characterized rat system (Robinson 1971; Soling et al. 1971; Kleineke et al. 1973).
PCK2 (phosphoenolcarboxykinase), located in the mitochondrial matrix, catalyzes the physiologically irreversible reaction of oxaloacetate and GTP to form phosphoenolpyruvate, GDP, and CO2 (Modaressi et al. 1996, 1998).
SLC25A10, the mitochondrial dicarboxylate carrier protein in the inner mitochondrial membrane, mediates the reversible exchange of mitochondrial malate for cytosolic phosphate (Fiermonte et al. 1999).
The SLC25A11 transport protein in the inner mitochondrial membrane mediates the reversible exchange of mitochondrial alpha-ketoglutarate (2-oxoglutarate) and cytosolic malate. Qualitative evidence for this process comes from studies of the human protein (Kabe et al. 2006); kinetic details are inferred from studies of rat mitochondria under conditions in which only the one transport protein appears to be active (Sluse et al. 1973). The dimeric state of the transport protein is inferred from studies of its bovine homologue (Bisaccia et al. 1996).
Glucose phosphorylation is a central event in cellular metabolism. ADP-dependent glucokinase (ADPGK) can phosphorylate glucose (Glc) using ADP as the phosphate donor to glucose 6-phopshate (G6P). To date, it has not been established whether this phosphorylation supports a significant role in priming glucose for a metabolic fate other than glycolysis (Richter et al. 2012). Stdies of metabolic changes during T cell activation suggest a role for it there (Kaminski et al. 2012).
Bisphosphoglycerate mutase (BPGM) is an erythrocyte-specific trifunctional enzyme. One of its functions is the isomerisation of 1,3-bisphosphoglycerate (1,3BPG) to 2,3-bisphosphoglycerate (2,3BPG) (Rose 1968). In red blood cells, 2,3BPG is the main allosteric effector of hemoglobin, binding preferentially to the deoxygenated hemoglobin tetramer, thus reducing oxygen affinity (Arnone 1972).
Glucosamine-6-phosphate isomerases 1 and 2 (GNPDA1, 2) catalyse the reversible deamination and with an aldo/keto isomerisation of D-glucosamine 6-phosphate (GlcN6P) to D-fructose 6-phosphate (Fru(6)P) and ammonia (NH3). GNDPA1 and 2 function as homohexamers in the cytosol. This reaction could provide a source of energy from catabolic pathways of hexosamines found in glycoproteins and glycolipids (Wolosker et al. 1998, Zhang et al. 2003, Arreola et al. 2003).
The transfer of a high-energy phosphate bond from GTP to oxaloacetate, to form phosphoenolpyruvate, GDP, and CO2, is catalyzed by cytosolic phosphoenolpyruvate carboxykinase (Dunten et al. 2002). This reaction is irreversible under physiological conditions.
The fructose 2,6-bisphosphatase activity of cytosolic phosphofructokinase 2/fructose-2,6-bisphosphatase catalyzes the hydrolysis of fructose 2,6 bisphosphate to form fructose 6-phosphate. Fructose 2,6 bisphosphate is a key allosteric regulator of PFK1. In its absence the activity of PFK1 is reduced while fructose 1,6-bisphosphatase is activated, thus inhibiting glycolysis and favoring gluconeogenesis (Pilkis et al. 1995).
Cytosolic glucokinase and the three isoforms of hexokinase catalyze the irreversible reaction of glucose and ATP to form glucose 6 phosphate and ADP. In the body glucokinase is found only in hepatocytes and pancreatic beta cells. Glucokinase and the hexokinase enzymes differ in that glucokinase has a higher Km than the hexokinases and is less readily inhibited by the reaction product. As a result, glucokinase should be inactive in the fasting state when glucose concentrations are low but in the fed state should have an activity proportional to glucose concentration. These features are thought to enable efficient glucose uptake and retention in the liver, and to function as a sensor of glucose concentration coupled to insulin release in pancreatic beta cells (Thorens 2001). Glucokinase mutations are associated with MODY2, a heritable early onset form of type II diabetes (Tanizawa et al. 1991; Takeda et al. 1993). Three human hexokinase enzymes have been characterized, HK1 (Aleshin et al. 1998), HK2 (Lehto et al. 1995), and HK3 (Rijksen at al. 1982).
Cytosolic glyceraldehyde 3-phosphate dehydrogenase catalyzes the reversible reaction of glyceraldehyde 3-phosphate, orthophosphate, and NAD+ to form NADH + H+ and 1,3-bisphosphoglycerate, the first energy rich intermediate of glycolysis. The biochemical details of this reaction were worked out by C and G Cori and their colleagues (Taylor et al. 1948; Cori et al. 1948).
While there are multiple human glyceraldehyde 3-phosphate dehydrogenase-like pseudogenes, there is only one glyceraldehyde 3-phosphate dehydrogenase gene expressed in somatic tissue (Benham and Povey 1989; Heinz and Freimuller 1982; Ercolani et al. 1988), and studies of aged human erythrocytes suggest that variant forms of the enzyme arise as a result of post-translational modifications (Edwards et al. 1976). There is, however, an authentic second isoform of glyceraldehyde 3-phosphate dehydrogenase whose expression is confined to spermatogenic cells of the testis (Welch et al. 2000).
Cytosolic triose phosphate isomerase catalyzes the freely reversible interconversion of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate (Lu et al. 1984). The active form of the enzyme is a homodimer (Kinoshita et al. 2005).
Cytosolic phosphofructokinase 1 catalyzes the reaction of fructose 6-phosphate and ATP to form fructose 1,6-bisphosphate and ADP. This reaction, irreversible under physiological conditions, is the rate limiting step of glycolysis. Phosphofructokinase 1 activity is allosterically regulated by ATP, citrate, and fructose 2,6-bisphosphate.
Phosphofructokinase 1 is active as a tetramer (although higher order multimers, not annotated here, may form in vivo). Two isoforms of phosphofructokinase 1 monomer, L and M, are widely expressed in human tissues. Different tissues can contain different homotetramers or heterotetramers: L4 in liver, M4 in muscle, and all possible heterotetramers, L4, L3M, L2M2, LM3, and M4, in red blood cells, for example (Raben et al. 1995; Vora et al. 1980, 1987; Vora 1981). A third isoform, P, is abundant in platelets, where it is found in P4, P3L, P2L2, and PL3 tetramers (Eto et al. 1994; Vora et al. 1987).
Cytosolic phosphoglucose isomerase catalyzes the reversible interconversion of glucose 6-phosphate and fructose 6-phosphate (Tsuboi et al. 1958; Noltmann 1972; Bloxham and Lardy 1973). The active form of the enzyme is a homodimer (Read et al. 2001). Mutations in the enzyme are associated with hemolytic anemia (Xu and Beutler 1994).
The reversible isomerization of fructose-6-phosphate to form glucose-6-phosphate is catalyzed by cytosolic phosphoglucose isomerase (Noltman 1972; Xu and Beutler 1994; Tsuboi et al. 1958).
Cytosolic fructose-1,6-bisphosphatase catalyzes the physiologically irreversible hydrolysis of fructose-1,6-bisphosphate to form fructose-6-phosphate and orthophosphate. In the body, two isoforms of the enzyme are expressed, one ubiquitous and one muscle-specific (Kikawa et al. 1997; Tillmann and Eschrich 1998).
The reversible conversion of glyceraldehyde-3-phosphate to dihydroxyacetone phosphate is catalyzed by cytosolic triose phosphate isomerase (Watanabe et al. 1996; Lu et al. 1984).
The reversible reduction of 1,3-bisphosphoglycerate to form glyceraldehyde-3-phosphate is catalyzed by cytosolic glyceraldehyde-3-phosphate dehydrogenase tetramer.
There are multiple human glyceraldehyde 3-phosphate dehydrogenase-like pseudogenes, but only one glyceraldehyde 3-phosphate dehydrogenase gene expressed in somatic tissue (Benham and Povey 1989). Consistent with this conclusion, the homogeneous enzymes purified from various human tissues had indistinguishable physical and immunochemical properties (Heinz and Freimuller 1982), and studies of human erythrocytes of various ages suggested that variant forms of the enzyme arise as a result of post-translational modifications (Edwards et al. 1976). There is, however, an authentic second isoform of glyceraldehyde 3-phosphate dehydrogenase whose expression is confined to spermatogenic cells of the testis (Welch et al. 2000).
Cytosolic phosphoglycerate kinase complexed with magnesium catalyzes the reversible phosphorylation of 3-phosphoglycerate to form 1,3-bisphosphoglycerate (Huang et al. 1980a, 1980b).
Cytosolic enolase dimer catalyzes the reversible reaction of phosphoenolpyruvate and water to form 2-phosphoglycerate. Three enolase isozymes have been purified and biochemically characterized. The alpha isoform is widely expressed (Giallongo et al. 1986). The beta isoform is expressed in muscle. Evidence for its function in vivo in humans comes from studies of a patient in whom a point mutation in the gene encoding the enzyme was associated specifically with reduced enolase activity in muscle extracts, and with other symptoms consistent with a defect in glycolysis (Comi et al. 2001). The gamma isoform of human enolase is normally expressed in neural tissue. It is not known to have distinctive biochemical functions, but is of possible clinical interest as a marker of some types of neuroendocrine and lung tumors (McAleese et al. 1988). Verma and Kurl (1993) identifed a possible fourth isoform, a "lung-specific" enolase whose expression is increased in response to dexamethasone treatment. The protein has not been biochemically characterized, however, nor have the levels of mRNA and protein in other tissues been examined. Thus, the observation that this protein is particularly similar in its predicted amino acid sequence to a duck crystallin (Wistow et al. 1988) raises the possibility that its normal function is unrelated to glycolysis.
The carboxylation of pyruvate to form oxaloacetate, catalyzed by mitochondrial pyruvate carboxylase, is an irreversible and allosterically regulated reaction (Jitrapakdee and Wallace 1999).
Cytosolic aspartate aminotransferase (glutamate oxaloacetate transaminase 1 - GOT1) catalyzes the reversible reaction of aspartate and 2-oxoglutarate (alpha-ketoglutarate) to form oxaloacetate and glutamate (Doyle et al. 1990). Unpublished crystallographic data (PBD 3IIO) suggest the enzyme is a homodimer).
Mitochondrial aspartate aminotransferase catalyzes the reversible reaction of oxaloacetate and glutamate to form aspartate and 2-oxoglutarate (alpha-ketoglutarate) (Martini et al. 1985). The active form of the enzyme is inferred to be a dimer with one molecule of pyridoxal phosphate associated with each monomer.
Cytosolic phosphoglycerate mutase dimer catalyzes the reversible isomerisation of 2- and 3-phosphoglycerate. There are two isoforms of this enzyme, PGAM1 (isoform B) and PGAM2 (isoform M). In the body, erythrocytes express only PGAM1, while skeletal muscle expresses only PGAM2. Other tissues express both isoforms (Repiso et al. 2005; Tsujino et al. 1993).
In this freely reversible cytosolic reaction, dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate react to form D-fructose 1,6-bisphosphate. The active form of aldolase, the enzyme that catalyzes the reaction, is a homotetramer. Three aldolase isozymes have been identified which differ in their patterns of expression in various adult tissues and during development but are otherwise functionally indistinguishable (Ali and Cox 1995; Dunbar and Fothergill-Gilmore 1988).
Cytosolic aldolase catalyzes the cleavage of D-fructose 1,6-bisphosphate to yield dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate. The active form of aldolase is a homotetramer. Three aldolase isozymes have been identified which differ in their patterns of expression in various adult tissues and during development but are otherwise functionally indistinguishable (Ali and Cox 1995; Freemont et al. 1984, 1988).
Cytosolic phosphoglycerate mutase catalyzes the reversible isomerisation of 3- and 2-phosphoglycerate. The active form of the enzyme is a dimer. There are two isoforms of this enzyme, PGAM1 (isoform B, widely expressed in non-muscle tissue) and PGAM2 (isoform M, expressed in muscle) (Blouquit et al. 1988; Omenn and Cheung 1974; Repiso et al. 2005; Tsujino et al. 1993).
Cytosolic enolase catalyzes the reversible reaction of 2 phosphoglycerate to form phosphoenolpyruvate and water, elevating the transfer potential of the phosphoryl group.
Enolase is a homodimer and requires Mg++ for activity. Three isozymes have been purified and biochemically characterized. The alpha isoform is expressed in many normal human tissues (Giallongo et al. 1986). The beta isoform is expressed in muscle. Evidence for its function in vivo in humans comes from studies of a patient in whom a point mutation in the gene encoding the enzyme was associated specifically with reduced enolase activity in muscle extracts and with other symptoms consistent with a defect in glycolysis (Comi et al. 2001). The gamma isoform of human enolase is normally expressed in neural tissue and is of possible clinical interest as a marker of some types of neuroendocrine and lung tumors (McAleese et al. 1988). Biochemical studies of the homologous rat proteins indicate that both homo- and heterodimers of enolase form and are enzymatically active (Rider and Taylor 1974).
Cytosolic pyruvate kinase catalyzes the transfer of a high-energy phosphate from phosphoenolpyruvate to ADP, forming pyruvate and ATP. This reaction, an instance of substrate-level phosphorylation, is essentially irreversible under physiological conditions.
Four isozymes of human pyruvate kinase have been described, L, R, M1 and M2. Isozymes L and R are encoded by alternatively spliced transcripts of the PKLR gene; isozymes M1 and M2 are encoded by alternatively spliced transcripts of PKM2. In the body, L pyruvate kinase is found in liver (Tani et al. 1988), R in red blood cells (Kanno et al. 1991), M1 in muscle, heart and brain (Takenaka et al. 1991), and M2 in early fetal tissues and tumors (e.g., Lee et al. 2008). In all cases, the active form of the enzyme is a homotetramer, activated by fructose 1,6-bisphosphate (Valentini et al. 2002; Dombrauckas et al. 2005). Mutations in PKLR have been associated with hemolytic anemias (e.g., Zanella et al. 2005).
Mitochondrial malate dehydrogenase catalyzes the reversible reaction of oxaloacetate and NADH + H+ to form malate and NAD+ (Luo et al. 2006). Unpublished crystallographic data indicate that the protein is a dimer (PDB 3E04).
The 6-phosphofructo-2-kinase activity of cytosolic PFKFB (6-phosphofructo-2-kinase/fructose-2,6-biphosphatase) homodimer catalyzes the reaction of fructose 6-phosphate and ATP to form fructose 2,6-bisphosphate and ADP (Pilkis et al. 1988). Fructose 2,6-bisphosphate is not itself on the pathway of glycolysis. Rather, it acts as a positive allosteric effector of phosphofructokinase 1, greatly increasing the rate of synthesis of fructose 1,6-bisphosphate and hence the overall rate of glycolysis. The conversion of PFKFB between its dephosphorylated form, which catalyzes the synthesis of fructose 2,6-bisphosphate as described here, and its phosphorylated form, which catalyzes the hydrolysis of fructose 2,6-bisphosphate to fructose 6-phosphate and orthophosphate plays a central role in the short-term regulation of glycolysis (Pilkis et al. 1995).
Four isoforms of PFKFB protein encoded by four different genes exhibit tissue-specific expression patterns. PFKFB1 is expressed in the liver (Algaier and Uyeda 1988), PFKFB2 is expressed in the heart (Hirata et al. 1998), PFKFB3 is ubiquitously expressed (Manes and el-Maghrabi 2005), and PFKFB4, originally described as a testis-specific gene product (Manzano et al. 1999), may also be expressed in several kinds of tumors.
Glucose-6-phosphatase (G6PC) associated with the inner face of the endoplasmic reticulum membrane catalyzes the hydrolysis of glucose-6-phosphate to glucose and orthophosphate. This reaction is essentially irreversible (Lei et al. 1993, Ghosh et al. 2002). Defects in glucose-6-phosphatase are the cause of glycogen storage disease type 1a (Lei et al. 1993, 1995, Chou and Mansfield 2008).
Cytosolic phosphoglycerate kinase (PGK) catalyzes the reaction of ADP and 1,3-bisphosphoglycerate (1,3BPG) to form D glyceraldehyde 3-phosphate (3PG) and ATP. The active form of the enzyme is a monomer and requires Mg++ (Yoshida and Watanabe 1972; Huang et al. 1980a,b). This is the first substrate level phosphorylation reaction in glycolysis. Two PGK isoforms are known: PGK1 is widely expressed in the body while PGK2 (Chen et al. 1976; McCarrey & Thomas 1987) appears to be confined to sperm cells.
1,3-bisphosphoglycerate (1,3BPG) is the first energy rich intermediate of glycolysis. Cytosolic glucose 1,6-bisphosphate synthase (PGM2L1) utilises 1,3BPG as a phosphate donor to phosphorylate a series of 1-phosphate sugars. Although 5- and 6-phosphate sugars are poor substrates for PGM2L1, glucose 6-phosphate (G6P) is the exception (Maliekal et al. 2007, Veiga-da-Cunha et al. 2008). PGM2L1 complexed with Mg2+ as cofactor, phosphorylates G6P to glucose 1,6-bisphosphate (G1,6BP), a cofactor for phosphomutases and a putative regulator of glycolysis. PGM2L1 is mainly expressed in brain where its activity is particularly high (Maliekal et al. 2007).
Glycerol-3-phosphate (aka 3-phospho-D-glycerate, 3PG) is a metabolic intermediate of glucose, lipid and energy metabolism. Its cellular levels may be regulated by cytosolic glycerol-3-phosphate phosphatase (PGP aka G3PP), which hydrolyses 3PG to glycerol. PGP functions as a homodimer, binding one Mg2+ ion per subunit. The function of the human protein is inferred from rat Pgp characterisation and functional studies (Mugabo et al. 2016).
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This reaction takes place in the 'cytosol' and is mediated by the 'phosphatidate phosphatase activity' of 'PP2A-ABdeltaC complex'.
While there are multiple human glyceraldehyde 3-phosphate dehydrogenase-like pseudogenes, there is only one glyceraldehyde 3-phosphate dehydrogenase gene expressed in somatic tissue (Benham and Povey 1989; Heinz and Freimuller 1982; Ercolani et al. 1988), and studies of aged human erythrocytes suggest that variant forms of the enzyme arise as a result of post-translational modifications (Edwards et al. 1976). There is, however, an authentic second isoform of glyceraldehyde 3-phosphate dehydrogenase whose expression is confined to spermatogenic cells of the testis (Welch et al. 2000).
Phosphofructokinase 1 is active as a tetramer (although higher order multimers, not annotated here, may form in vivo). Two isoforms of phosphofructokinase 1 monomer, L and M, are widely expressed in human tissues. Different tissues can contain different homotetramers or heterotetramers: L4 in liver, M4 in muscle, and all possible heterotetramers, L4, L3M, L2M2, LM3, and M4, in red blood cells, for example (Raben et al. 1995; Vora et al. 1980, 1987; Vora 1981). A third isoform, P, is abundant in platelets, where it is found in P4, P3L, P2L2, and PL3 tetramers (Eto et al. 1994; Vora et al. 1987).
There are multiple human glyceraldehyde 3-phosphate dehydrogenase-like pseudogenes, but only one glyceraldehyde 3-phosphate dehydrogenase gene expressed in somatic tissue (Benham and Povey 1989). Consistent with this conclusion, the homogeneous enzymes purified from various human tissues had indistinguishable physical and immunochemical properties (Heinz and Freimuller 1982), and studies of human erythrocytes of various ages suggested that variant forms of the enzyme arise as a result of post-translational modifications (Edwards et al. 1976). There is, however, an authentic second isoform of glyceraldehyde 3-phosphate dehydrogenase whose expression is confined to spermatogenic cells of the testis (Welch et al. 2000).
Enolase is a homodimer and requires Mg++ for activity. Three isozymes have been purified and biochemically characterized. The alpha isoform is expressed in many normal human tissues (Giallongo et al. 1986). The beta isoform is expressed in muscle. Evidence for its function in vivo in humans comes from studies of a patient in whom a point mutation in the gene encoding the enzyme was associated specifically with reduced enolase activity in muscle extracts and with other symptoms consistent with a defect in glycolysis (Comi et al. 2001). The gamma isoform of human enolase is normally expressed in neural tissue and is of possible clinical interest as a marker of some types of neuroendocrine and lung tumors (McAleese et al. 1988). Biochemical studies of the homologous rat proteins indicate that both homo- and heterodimers of enolase form and are enzymatically active (Rider and Taylor 1974).
Four isozymes of human pyruvate kinase have been described, L, R, M1 and M2. Isozymes L and R are encoded by alternatively spliced transcripts of the PKLR gene; isozymes M1 and M2 are encoded by alternatively spliced transcripts of PKM2. In the body, L pyruvate kinase is found in liver (Tani et al. 1988), R in red blood cells (Kanno et al. 1991), M1 in muscle, heart and brain (Takenaka et al. 1991), and M2 in early fetal tissues and tumors (e.g., Lee et al. 2008). In all cases, the active form of the enzyme is a homotetramer, activated by fructose 1,6-bisphosphate (Valentini et al. 2002; Dombrauckas et al. 2005). Mutations in PKLR have been associated with hemolytic anemias (e.g., Zanella et al. 2005).
Four isoforms of PFKFB protein encoded by four different genes exhibit tissue-specific expression patterns. PFKFB1 is expressed in the liver (Algaier and Uyeda 1988), PFKFB2 is expressed in the heart (Hirata et al. 1998), PFKFB3 is ubiquitously expressed (Manes and el-Maghrabi 2005), and PFKFB4, originally described as a testis-specific gene product (Manzano et al. 1999), may also be expressed in several kinds of tumors.