Energy metabolism

Abstract

The major fuels that supply energy within the body are glucose, lactate, fatty acids, ketone bodies and some amino acids. Different cells and tissues show specific patterns of fuel utilization. Notably the brain which normally consumes 120g of glucose per day but if ketone body concentrations are elevated they can replace some the glucose use. Overall, catabolism of fuels provides reducing power as NADH that is oxidized to produce ATP the energy reservoir in cells. The bulk of glucose will be broken down through glycolysis. In some cells the breakdown of glucose only proceeds as far as lactate production which results in a yield of 2 ATP but in most cells the end product of glycolysis is pyruvate that enters the mitochondria to be decarboxylated to acetyl CoA. Fats, derived from the diet or storage in adipose tissue, will be oxidized within mitochondria to produce acetyl CoA and considerable amounts of reducing power (as NADH and FADH2). In the liver the end products of fatty acid oxidation are the ketone bodies, acetoacetate and β-hydroxybutyrate that can be broken down in other tissues. Similarly, the degradation of some amino acids will yield acetyl CoA. Oxidation of acetyl CoA, and other intermediate products, with the Tricarboxylic Acid Cycle yields large amounts of NADH and FADH2 that enter the process of oxidative phosphorylation whereby electrons pass along a series complexes in the inner mitochondrial membrane to the final electron acceptor, molecular oxygen. At key steps in the electron transfer chain protons are pumped out the mitochondrial matrix resulting in a buildup of protons in the inner membrane space. These protons pass back down a gradient into the mitochondrial matrix driving the synthesis of ATP. In addition to catabolism some fuels can be stored to be used during the post-absorptive phase. Glucose will be stored as glycogen in liver to be released as free glucose to maintain plasma glucose levels while glycogen in skeletal muscle is used soley as a local fuel for muscle contraction. Similarly, triglycerides are stores of fatty acids that are mobilized to provide fuel for most tissues in the postabsorptive period. As liver glycogen stores become depleted blood glucose levels are maintained by gluconeogenesis, the synthesis of glucose from non-glucose precursors. The coordination of the tissue specific metabolism of glucose, fatty acids and amino acids ensures the energy needed for key functions of different cells is maintained from the absorptive state after a meal through the post-absorptive phase into starvation.