|Topic:||Regulation of Glycolytic Flux by Pyruvate Kinase .|
|Details:|| Regulation of glycolysis also occurs at the step catalyzed by pyruvate kinase, (PK). There are four distinct isoforms of PK in human tissues encoded for by two different genes. One is located on chromosome 1, identified as the PKLR gene, and it encodes the liver (PKL or L-PK) and erythrocyte (PKR or R-PK) pyruvate kinase proteins. Expression of PKL or PKR is dependent upon the use of tissue-specific promoter elements in the PKLR gene. The other pyruvate kinase gene is located on chromosome 15 and encodes two proteins identified as PKM1 and PKM2. The designation PKM reflects that fact that the enzyme was originally thought to be muscle specific in is expression. The two PKM isoforms result from alternative splicing of the PKM gene. It is now known that most tissues express either the PKM1 of the PKM2 isoform. PKM1 is found in numerous normal differentiated tissues, whereas, PKM2 is expressed in most proliferating cells. All cancers that have been examined for PK expression pattern show expression of the PKM2 isoform. Indeed, expression of PKM2 allows for a unique pathway of enhanced glucose oxidation to lactate in cancer cells.
The liver isoform (PKL or L-PK) has been most studied in vitro. This enzyme is inhibited by ATP and acetyl-CoA and is activated by F1,6BP. The inhibition of PK by ATP is similar to the effect of ATP on PFK-1. The binding of ATP to the inhibitor site reduces its affinity for PEP. The liver enzyme is also controlled at the level of synthesis. Increased carbohydrate ingestion induces the synthesis of L-PK resulting in elevated cellular levels of the enzyme. Regulation of L-PK is characteristic of a gluconeogenic tissue being regulated via phosphorylation by PKA. Whereas the M-type isozymes are unaffected by PKA. As a consequence of these differences, blood glucose levels and associated hormones can regulate the balance of liver gluconeogenesis and glycolysis while for instance, muscle metabolism remains unaffected.
In erythrocytes, the fetal PK isozyme has much greater activity than the adult isozyme; as a result, fetal erythrocytes have comparatively low concentrations of glycolytic intermediates. Because of the low steady-state concentration of fetal 1,3BPG, the 2,3BPG shunt (see diagram above) is greatly reduced in fetal cells and little 2,3BPG is formed. Since 2,3BPG is a negative effector of hemoglobin affinity for oxygen, fetal erythrocytes have a higher oxygen affinity than maternal erythrocytes. Therefore, transfer of oxygen from maternal hemoglobin to fetal hemoglobin is favored, assuring the fetal oxygen supply. In the newborn, an erythrocyte isozyme of the M-type with comparatively low PK activity displaces the fetal type, resulting in an accumulation of glycolytic intermediates. The increased 1,3BPG levels activate the 2,3BPG shunt, producing 2,3BPG needed to regulate oxygen binding to hemoglobin.
Genetic diseases of adult erythrocyte PK are known in which the kinase is virtually inactive. The erythrocytes of affected individuals have a greatly reduced capacity to make ATP and thus do not have sufficient ATP to perform activities such as ion pumping and maintaining osmotic balance. These erythrocytes have a short half-life due to easy lysis. Pyruvate kinase deficiency is the most common cause of inherited non-spherocytic hemolytic anemia and the second most common cause of inherited hemolytic anemia behind glucose-6-phosphate dehydrogenase (G6PD) deficiencies.
The liver PK isozyme is regulated by phosphorylation, allosteric effectors, and modulation of gene expression. The major allosteric effectors are F1,6BP, which stimulates PK activity by decreasing its Km for PEP, and for the negative effector, ATP. Expression of the liver PK gene is strongly influenced by the quantity of carbohydrate in the diet, with high-carbohydrate diets inducing up to 10-fold increases in PK concentration as compared to low carbohydrate diets. Liver PK is phosphorylated and inhibited by PKA, and thus it is under hormonal control similar to that described earlier for PFK-2.
Muscle PK (PKM1) is not regulated by the same mechanisms as the liver enzyme. Extracellular conditions that lead to the phosphorylation and inhibition of liver PK, such as low blood glucose and high levels of circulating glucagon, do not inhibit the muscle enzyme. The result of this differential regulation is that hormones such as glucagon and epinephrine favor liver gluconeogenesis by inhibiting liver glycolysis, while at the same time, muscle glycolysis can proceed in accord with needs directed by intracellular conditions.
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