How does the role of glucose-6-phosphate in gluconeogenesis differ from that in glycolysis? (4 marks)
One of the steps that differs gluconeogenesis fromglycolysis is the conversion of glucose-6-P to glucose with the enzymeglucose-6-phosphatase. This enzyme islocated in the endoplasmic reticulum.
The amounts and activities of the distinctive enzymes of each pathway are controlled so that both pathways are not highly active at the same time. The rate of glycolysis is determined by the concentration of glucose, and the rate of gluconeogenesis by the concentrations of lactate and other precursors of glucose (such as glucose-6-phosphate).
The conversion of glucose-6-P to glucose with use of glucose-6-phosphatase is controlled by substrate level regulation. The metabolite glucose-6-P is responsible for this type of regulation. As levels of glucose-6-P increase,glucose-6-phosphatase increases activity and more glucose is produced. Thus glycolysis is unable to proceed. Hexokinase is inhibited by high levels of its product, glucose-6-phosphate (in case of glycolysis).This is a very important regulatory step, since it prevents the consumption of too much cellular ATP to form G6P when glucose is not limiting. When glycolysis is inhibited through phosphofructokinase, glucose-6-phosphate builds up, shutting down hexokinase.