Chapter 13 - Carbohydrate Metabolism I: Glycolysis and TCA Cycle

Abstract

Carbohydrates are metabolized by several metabolic pathways, each with different functions. Glucose can be stored as glycogen, which, in turn, can be broken down to glucose, synthesized from noncarbohydrate sources, converted to nonessential amino acids, used in the formation of other carbohydrates or their derivatives, and other noncarbohydrate metabolites, converted to fatty acids and stored as triacylglycerols, used in the biosynthesis of glycoconjugates, or catabolized to provide energy for cellular function. Glycolysis is common to most organisms, and in humans, it occurs in virtually all tissues. Glycolytic enzymes can be classified into six groups based on the type of reaction catalyzed, which include kinase, mutase, dehydrogenase, cleaving enzyme, isomerase, and enolase. The glycolytic pathway is also utilized by fructose, galactose, mannose, glycogen, and glycerol. In tissues that lack mitochondria or function under limiting conditions of oxygen, glycolysis is the predominant pathway providing ATP. The tricarboxylic acid (TCA) cycle, also called the citric acid cycle or Krebs cycle, consists of eight enzymes that oxidize acetyl-CoA with the formation of carbon dioxide, reducing equivalents in the form of NADH and FADH 2 , and guanosine triphiosphate (GTP). The reducing equivalents are transferred to the electron transport chain, ultimately to reduce oxygen and generate ATP through oxidative phosphorylation. The TCA cycle is the major final common pathway of oxidation of carbohydrates, lipids, and proteins, as their oxidation yields acetyl-CoA. Acetyl-CoA also serves as precursor in the synthesis of fatty acids, cholesterol, and ketone bodies. All enzymes of the cycle are located in the mitochondrial matrix except for succinate dehydrogenase, which is embedded in the inner mitochondrial membrane.