Module 7: Figure glycogenolysis and gluconeogenesis
Liver cell glycogenolysis and gluconeogenesis.
The liver cell can produce glucose through two processes: glycogenolysis (highlighted in the yellow box) and gluconeogenesis (highlighted in the grey box). Glycogen synthesis and hydrolysis are controlled by phosphorylase a and glycogen synthase a respectively. Cyclic AMP stimulates glycogen breakdown by inhibiting glycogen synthase and by enhancing the sensitivity of the Ca2+-sensitive enzyme phosphorylase kinase. Gluconeogenesis is the process for converting various substrates such as lactate (derived from skeletal muscle) or free fatty acids (FFA; from white fat cells) into glucose by reversing many of the steps of glycolysis (red arrows). This process is markedly enhanced by an increase in the expression of some of the key enzymes, such as phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase), and the carnitine palmitoyltransferase 1 (CPT1) that brings FFAs into the mitochondrion. The transcriptional control of these proteins is shown in Module 7: Figure liver cell signalling). Other abbreviations for gluconeogenic intermediates: DHAP, dihydroxyacetone phosphate; G-3-P, glyceraldehyde 3-phosphate; 2PG, 2-phosphoglycerate; 3PG, 3-phosphoglycerate.