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

Chapter II. GENERAL PRINCIPLES OF METABOLISM

CHAPTER 8. METABOLISM: CATABOLISM, ANABOLISM

METABOLISM is the sum of biochemical Reactions Involving the transformation of chemical compounds (metabolites) in living organisms. Continuous exchange of matter and energy with the environment is the defining characteristic of a living Cell, maintaining its thermodynamically steady state and countering the increase in Entropy within any biological system.

8.1. GENERAL PRINCIPLES OF METABOLISM

Metabolism in humans and higher animals consists of several consecutive stages, including:

- the intake of bioorganic substances (nutrients) — Proteins, Lipids, CARBOHYDRATES, Vitamins, minerals, and Water — into the body via food;

- The breakdown of nutrients (proteins, Polysaccharides, fats) in the digestive tract into simpler compounds (Amino Acids, Monosaccharides, Fatty acids, glycerol) capable of being absorbed by the epithelial mucosa of The Stomach and intestines;

- the biotransport of nutrient Digestion products via Blood AND Lymph, and their delivery across vascular and cellular membranes to specific Organs and Tissues (Liver, Muscles, Brain, Kidneys, adipose tissue, etc.);

- the intracellular metabolism of Biomolecules within organs and tissues (Intermediary Metabolism, or metabolism in the strict sense);

- the excretion from the body — via the kidneys, Lungs, Skin, and intestines — of metabolic end products (carbon dioxide, ammonia, urea, water, and conjugates or oxidation products of certain organic molecules).

Intracellular metabolic reactions encompass the following biochemical transformations:

a) the breakdown of bioorganic molecules (glucose, fatty acids, amino acids, glycerol) into intermediary metabolism end products (carbon dioxide, water, ammonia) with the release of chemical energy and its accumulation in the form of adenosine triphosphate (ATP), other high-energy phosphates, or proton motive force, which meets the energy demands of vital physiological processes. The collective processes of biomolecule degradation coupled with energy release are termed Catabolism;

b) the Synthesis of specific biomolecules genetically characteristic of a given Organism (proteins, Nucleic Acids, polysaccharides, lipids, BIOREGULATORS, etc.) required for The formation of cellular and extracellular structures. These processes are termed anabolism and require energy input in the form of ATP.

c) the utilization of energy (in the form of ATP or proton motive force) to drive cellular physiological processes such as the function of contractile structures (Muscle contraction, cytoskeletal dynamics, Cilia and flagella motility, etc.), exocytosis and endocytosis, Membrane Potential generation, and The Active Transport of metabolites and inorganic ions.

The enzyme-catalyzed conversion of certain metabolites (biomolecules) into others constitutes metabolic pathways.

The reaction sequences that comprise Metabolic pathways can be linear, branched, or cyclic:

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Metabolic pathways are classified into:

- Catabolic pathways, which collectively constitute catabolism — the breakdown reactions (such as Hydrolysis and oxidation) of bioorganic substances acquired from the environment via food (carbohydrates, lipids, proteins, etc.) as well as biomolecules that make up The Structure of the organism's Cells and Tissues;

- anabolic pathways, which constitute anabolism — the synthesis reactions of complex bioorganic compounds that form various Structural components of the organism (specifically, Biopolymers such as proteins, nucleic acids, and polysaccharides) and ensure its functioning (lipids, monosaccharides, amino acids, NUCLEOTIDES, vitamins, Coenzymes, Hormones, etc.).

- amphibolic pathways, which lie at the "crossroads" of catabolism and anabolism; metabolites comprising amphibolic pathways can be utilized in both Catabolic and anabolic processes; a prime example is The Tricarboxylic Acid Cycle.

Metabolism in the living organism is directly coupled with Energy Metabolism. Metabolic reactions are broadly categorized into exergonic and endergonic reactions.

Exergonic reactions (processes) are those accompanied by the release of chemical energy necessary to sustain living organisms.

The most crucial exergonic processes in living systems are oxidation Reactions Catalyzed by oxidoreductases in the mitochondrial inner membrane. The energy released during oxidation is largely conserved in the form of high-energy compounds.

Endergonic reactions (processes) are those that require an input of energy to proceed; these include enzymatic synthesis and reduction reactions:

- synthesis reactions lead to the formation of new chemical bonds and increased complexity of biomolecule structures (simple compounds and biopolymers); high-energy bonds of ATP serve as the energy source for the majority of synthesis reactions;

- reduction reactions involve The addition of hydrogen atoms to bioorganic compounds (mostly to unsaturated bonds); reduced nicotinamide adenine dinucleotide phosphate (NADPH) molecules are used as hydrogen Donors in reduction reactions.

The Coupling of Endergonic processes with exergonic ones is achieved through the synthesis, during the exergonic reaction, of high-energy potential compounds that are subsequently utilized in endergonic reactions, thereby ensuring The transfer of chemical energy from the exergonic to the endergonic process.

High-energy potential compounds are biomolecules (organic phosphates) characterized by a high Standard Free energy of terminal Phosphate group transfer (specifically, hydrolysis) ΔG° (or ΔG° at pH = 7.0). Such biomolecules and their corresponding chemical bonds are referred to as high-energy (macroergic) bonds. The presence of a high-energy phosphate bond in these compounds is denoted by the symbol ~ (tilde), i.e., ~ Ф or ~ P (Eng.).

The primary high-energy compound in living organisms is the ATP molecule. The ATP molecular structure contains two high-energy bonds, whereas the ADP molecule contains one high-energy bond:

High-energy compounds also include such intermediates of carbohydrate, lipid, and Amino acid metabolism as phosphoenolpyruvate, 1,3-bisphosphoglycerate, and phosphocreatine, which serves as an important energy reserve in muscles.

The release of chemical energy occurs during the hydrolysis of ATP and ADP or the transfer of high-energy phosphate groups to other acceptors:

The reverse conversion of ADP into ATP involving inorganic phosphate (P_i), i.e., the phosphorylation of ADP to ATP, requires a corresponding expenditure of chemical energy:

In accordance with these energetic principles, cyclic conversions of ATP into ADP link processes that generate ~ Ф with processes that consume ~ Ф. Thus, in all biological systems, ATP is the primary compound that transfers energy from exergonic to endergonic processes (biochemical reactions and physiological Functions).

The scheme illustrating the coupling of exergonic (A—B) and endergonic (C—D) reactions via the ATP—ADP system is presented in Fig. 8.1.

Fig. 8.1. The Role of the ATP—ADP system in coupling exergonic and endergonic processes.

During biological evolution on Earth, two Types of Metabolism emerged—autotrophic and heterotrophic—which differ depending on the form in which organisms obtain carbon and energy from the environment to sustain life and build biostructures.

Autotrophic cells (organisms) can utilize atmospheric CO₂ as their sole carbon source, from which they are capable of synthesizing all their carbon-containing components. Autotrophs include the photosynthesizing cells of higher green plants and cells of certain prokaryotes, such as blue-green Algae, green Bacteria, and purple bacteria. Autotrophs obtain energy for their endergonic reactions from sunlight, which is captured and transformed into chemical energy by specialized light-sensitive proteins, notably chlorophylls in the Chloroplasts of green plants.

The overall equation of Photosynthesis:

Heterotrophic cells (organisms) obtain the carbon necessary to build their own molecules and biostructures in the form of complex bioorganic compounds (carbohydrates, lipids, proteins, etc.) found in food. The energy source for metabolic processes in heterotrophs is provided by Biological Oxidation REACTIONS of metabolites produced during the catabolism of monosaccharides, lipids, and Certain amino acids.

Heterotrophs include Eukaryotic cells of animal organisms, in which the main exergonic processes accompanied by the release of chemical energy take place in Mitochondria.