Textbook - BIOLOGICAL CHEMISTRY - Hubsky Y.I. - 2000

Chapter II. GENERAL PRINCIPLES OF METABOLISM

CHAPTER 8. METABOLISM: CATABOLISMS, ANABOLISM

8.3. STAGES OF BIOMOLECULE CATABOLISMS

The enzymatic breakdown of complex bioorganic compounds in the body involves three main stages, which are common to the Catabolism of various Biomolecules.

Stage 1. In The First stage of catabolism, complex molecules (macromolecules of CARBOHYDRATES, Proteins, Nucleic Acids, and lipid molecules) are broken down into simple components:

- Polysaccharides into Monosaccharides (primarily glucose, fructose, galactose);

- Lipids (triacylglycerols) into Fatty acids and glycerol;

- proteins into Amino Acids;

- nucleic acids into NUCLEOTIDES.

The Reactions of the first stage of catabolism are hydrolytic in mechanism and are catalyzed by Hydrolases of the digestive tract (Stomach, intestines). These reactions are not accompanied by a significant release of chemical energy.

Stage 2. In the second stage of catabolism, several dozen metabolites formed in the first stage undergo enzymatic breakdown reactions with the release of a certain amount of chemical energy, which is accumulated in high-energy (macroergic) ATP bonds.

The reactions of the Second Stage of catabolism occur intracellularly (in the Cytoplasm and partially in the Cell/35.html">Mitochondria). The main reactions include:

for monosaccharides — Glycolysis, the end metabolite of which is pyruvic acid (Pyruvate), which is subsequently oxidized to the active form of acetic acid — acetyl-coenzyme A (acetyl-CoA);

for fatty acids — β-oxidation, the end product of which is acetyl-CoA;

for glycerol — breakdown to pyruvate, which is converted into acetyl-CoA;

for Amino Acids and nucleotides — deamination with the release of ammonia and The breakdown of nitrogen-free molecular skeletons into two- and three-carbon carboxylic acids and their derivatives; most of these metabolites ultimately also form an acetyl radical in the form of acetyl-CoA.

Thus, acetyl-CoA is the universal final product of the second stage of intracellular catabolism of carbohydrates, lipids, and amino acids.

Stage 3. The Third Stage of catabolism involves The oxidation of acetyl-CoA to end metabolites — carbon dioxide and Water. This stage takes place in the mitochondria and consists of two processes:

- the Tricarboxylic Acid Cycle (TCA cycle, Krebs cycle), the functioning of which yields CO₂, while hydrogen atoms are used to reduce the Coenzymes nicotinamide adenine dinucleotide (NAD⁺) and flavin adenine dinucleotide (FAD);

- the electron transport system in mitochondrial membranes, in which hydrogen atoms (protons and electrons) are transferred to oxygen to form H₂O; this system is coupled with Oxidative Phosphorylation, whereby the energy of Biological Oxidation REACTIONS is used to synthesize ATP molecules — the primary direct energy supplier for all biological endergonic processes.

The metabolic map of general pathways and stages of catabolism is shown in Fig. 8.2:

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Fig. 8.2. Schematic Overview of the General Pathways of biomolecule catabolism.