Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism: Energy-Yielding Processes
Principles of Metabolism

METABOLISM is generally understood as the exchange of matter and energy that takes place within Cells. This highly integrated process comprises two closely interrelated components—Catabolism and anabolism—that cannot function without each other. Catabolic reactions involve The breakdown of complex and diverse substances into simpler ones (building blocks) whose variety is relatively limited. In most cases, these processes are accompanied by The oxidation of substances, resulting in the generation of reducing equivalents. Additionally, catabolism releases Free energy that can be stored in the high-energy bonds of ATP.

Anabolic reactions operate in the reverse direction. They use building blocks to synthesize complex substances of remarkable diversity, THE SPECTRUM OF which is specific to the cells of different organisms or the Organs (Tissues) of a multicellular Organism. Biosynthetic processes require an influx of energy as well as reducing equivalents, since the complex substances being synthesized generally have a lower oxidation state than their precursors. This process is accompanied by the regeneration of the oxidized forms of reducing equivalent carriers.

Often, a third (intermediate) component of metabolism is distinguished—amphibolism, which refers to the reactions of The Tricarboxylic Acid Cycle (TCA cycle). Amphibolism involves transformations that lead, on the one hand, to the Complete oxidation of substances to carbon dioxide and Water, accompanied by energy release and the generation of reducing equivalents, and on the other hand, to The formation of intermediates used as substrates for Biosynthesis.

Thus, catabolic reactions function on the funnel principle, whereas anabolic reactions operate on the inverted-funnel principle (Fig. 8.1). However, in terms of time, these Two Types of reactions are not necessarily separated and can occur synchronously, a process greatly facilitated by their different compartmentalization in Eukaryotic cells.

Complex organic substances utilized by cells as nutritional substrates are most commonly represented by Polysaccharides, Proteins, and Lipids. Their breakdown yields primarily hexoses and pentoses, 20 types of Amino Acids, several predominant Fatty acids, and glycerol. These molecules can either be used directly as substrates for biosynthesis or undergo further degradation coupled with energy release. Most of them can be converted into pyruvic acid and acetyl-CoA—Key Intermediates and substrates for various Types of Fermentation and the TCA cycle.

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Fig. 8.1. Interrelationship between Catabolic and anabolic reactions

Characterizing the scheme presented in Fig. 8.1, two further dimensions of metabolism can be distinguished: constructive and Energy Metabolism. Constructive metabolism refers to The interconversions of substances involving the breakdown and formation of chemical bonds mediated by Enzymes. Energy metabolism entails the interconversions of energy within The Cell, achieved primarily through the action of coupling agents, such as the most common phosphate group carrier, ADP (Chapter 7). Constructive and energy metabolism are likewise intimately linked.

An important feature of metabolism is its universality: in the majority of living organisms, the main Metabolic pathways are similar, and the principles of energy storage via substrate-level, oxidative, and Photophosphorylation are uniform. As a result, the diverse variants of metabolic transformation across different organisms can be reduced to a general scheme. Another common feature is that metabolic transformations proceed enzymatically, in a stepwise manner (via the formation of intermediate products—metabolites), through the exchange of multi-atom groups rather than atom-by-atom assembly.

The metabolic process can be viewed as an orderly system of Chemical Reactions characterized by strict subordination, maintained through regulatory mechanisms. It is precisely this complex, multi-level REGULATION OF METABOLISM that makes it possible to coordinate the rates of catabolic and anabolic processes, matching them to changing environmental conditions.