Pyruvate kinase catalyses the formation of pyruvate from phosphoenolpyruvate in the glycolysis pathway, with the production of two molecules of ATP. People with a relatively common inherited deficiency of pyruvate kinase demonstrate haemolytic anaemia due to the inadequate production of ATP by glycolysis to meet the erythrocytes’ energy needs.
Since the cell has no other method of producing ATP, it is unable to maintain the structural integrity of its cell membrane. This leads to changes in the shape of the red blood cell and hence its destruction by phagocytosis.
Glucokinase catalyses the first reaction of glycolysis in the liver – the phosphorylation of glucose. Glucokinase differs from hexokinase, the enzyme used in all other tissues, in that it requires a higher concentration of glucose to become half-saturated (high Km). It only functions when glucose concentration in the liver is very high, in other words directly after a carbohydrate-rich meal. This allows the liver to remove glucose from the portal blood effectively preventing hyperglycaemia.
Glucose enters cells by facilitated diffusion through specific transporter proteins. The first reaction of glycolysis, catalysed by hexokinase, phosphorylates glucose to glucose 6-phosphate. In the third reaction of the pathway, phosphofructokinase catalyses the transfer of another phosphate group using ATP, to give the product fructose 1,6-bisphosphate. The phosphorylated intermediates are trapped in the cytosol as they cannot diffuse across the cell membrane.
The next reaction, catalysed by aldolase, splits the hexose sugar to give dihydroxyacetone phosphate and glyceraldehyde phosphate (both triose sugars) so that eventually two molecules of pyruvate (or lactate) are produced from each glucose molecule.
Glycolysis has one oxidation reaction in which NAD+ is converted to NADH, and this cofactor is oxidized by the electron transport chain if oxygen is available, or regenerated back to NAD+ under anaerobic conditions by the conversion of pyruvate to lactate. For cells that have no mitochondria, erythrocytes, for example, glycolysis provides the only source of ATP.
Hexokinase is inhibited by glucose 6-phosphate, the product of the reaction it catalyses. Phosphofructokinase is the key rate regulator of glycolysis. When ATP and citrate are present in high concentrations, there is reduced need for sugars to be sent through glycolysis and on to the citric acid cycle.
ATP and citrate allosterically inhibit phosphofructokinase, and any consequent build up of glucose 6-phosphate can be used for glycogen synthesis. Aldolase is not a rate controlling step of glycolysis and is not an allosteric enzyme.
The three physiologically irreversible steps of glycolysis are catalysed by the enzymes hexokinase, phosphofructokinase and pyruvate kinase. If it were not for these steps, glucose could be generated from pyruvate by a simple reversal of the glycolytic reactions. Gluconeogenesis bypasses these steps by using different enzymes.
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