Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Processes Leading to Energy Storage
Catabolic Pathways

The breakdown of complex nutrients occurs within Cells through a series of sequential enzymatic reactions that can be grouped into three stages. In The First stage, polymer molecules are degraded into monomers (Chapter 8). During the Second Stage of Catabolism, these monomers (hexoses, pentoses, Amino Acids, Fatty acids, glycerol) are converted into even simpler and smaller molecules, predominantly Pyruvate, acetyl-CoA, a-ketoglutarate, oxaloacetate, succinate, and fumarate. Finally, in the Third Stage, these building blocks can enter The Tricarboxylic Acid Cycle in aerobic cells, where they undergo ultimate oxidation to CO₂ and H₂O. Free energy is released and reduction equivalents are formed during the second and third stages. The latter can enter the Respiratory Chain, where a significant amount of Energy is stored in the form of high-energy ATP bonds via Oxidative Phosphorylation.

Under anaerobic conditions, the building blocks formed In the second stage can undergo various Types of Fermentation. This process yields fermentation products (primarily organic acids, alcohols, and CO₂) that are excreted from The Cell. Energy is conserved only in a few Chemical Reactions during fermentation through substrate-level phosphorylation.

This chapter examines the core Catabolic pathways that lead to the release of substantial amounts of free energy: ß-Oxidation of Fatty acids and hexose catabolism. These specific substrates drive the highest levels of cellular METABOLISM by serving as universal sources of energy and carbon. In addition, many cells are capable of the oxidative degradation of amino acids (a process discussed in the chapter "Nitrogen Compound Metabolism"), as well as utilizing less common or even unique chemical compounds as substrates.