The formation of glucose from various precursors, such as pyruvate, certain amino acids (glucogenic amino acids), and intermediates of the TCA cycle. Most of the stages in the pathway are reversals of the reactions involved in glycolysis, with two exceptions. Phosphoenolpyruvate cannot be formed from pyruvate by reversal of the pyruvate kinase reaction of glycolysis. An alternative sequence of reactions involving the formation of oxaloacetate and malate and the input of a molecule each of ATP and GTP is performed to bypass this step. Similarly fructose 1,6-bisphosphate cannot be converted to fructose 6-phosphate by reversal of the glycolytic reaction catalysed by phos-phofructokinase. The reaction is instead catalysed by the enzyme fructose bisphosphatase. In plants glucose is formed predominantly by photosynthesis. Amino acids may be converted to glucose by the TCA and * glyoxylate cycles though, unlike animals, no distinction is made between glucogenic and nonglucogenic amino acids, all being suitable for gluconeogenesis by this pathway. Plants can also bring about the net synthesis of glucose from fatty acids via the succinate produced by the glyoxylate cycle. This process is utilized by germinating fatty seeds. |
