Biochemical Foundations of Human Vital Activity - Volkov, N. I., & Nesen, E. N. 2000

Biochemical Foundations of Human Vital Activity
Energy Metabolism in the Organism
ATP is the universal source of energy in the organism

Chemical Structure of ATP. Adenosine triphosphate is a nucleotide. It consists of a nitrogenous base, adenine, and a sugar, ribose, which together form adenosine, and three phosphoric acid residues (Fig. 13). The first phosphoric acid residue is attached to the ribose via a standard ester bond, while the subsequent two are linked by high-energy phosphoanhydride bonds (~). Within The Cell, the ATP molecule contains negatively charged phosphate groups that bind to cations, most commonly Mg²⁺, forming the Mg²⁺-ATP complex.

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Fig. 13 STRUCTURE OF THE ATP molecule

ATP Hydrolysis. ATP performs its energetic function through The breakdown of its molecule involving H₂O (hydrolysis). Typically, the terminal phosphate group is cleaved from ATP, yielding ADP and orthophosphoric acid, which can also be denoted as P_i. Under standard conditions, this releases approximately 30 kJ · mol^-1 of energy and increases the concentration of hydrogen protons (H⁺) in the medium:

This reaction is catalyzed by specific Enzymes known as ATPases (adenosine triphosphatases).

ATP hydrolysis can also proceed with The formation of AMP (adenosine monophosphate) and pyrophosphate, releasing approximately 30 kJ of energy:

АТФ + H₂O → АМФ + Н₄Р₂O₇ (∆Q⁰ = -30 кДж)

ADP is likewise a high-energy compound and can serve as an energy source. Its hydrolysis yields AMP and orthophosphate, releasing approximately 30 kJ of energy:

АДФ + Н₂O → АМФ + Н₃РO₄ (∆Q = -7,3 ккал)

ADP can be converted into ATP through the action of the enzyme myokinase (adenylate kinase). In this process, two ADP molecules yield one ATP and one AMP molecule:

2АДФ → АТФ + АМФ

The AMP molecule contains no high-energy bonds. Although rarely used to regenerate ATP, it plays a vital role in regulating METABOLISM, particularly ATP metabolism.

ATP acts as a store and carrier of Free energy. An ATP molecule is synthesized using the free energy released during catabolic reactions, According to the following scheme:

АДФ + Н₃РO₄ + ∆Q → АТФ

Therefore, ATP serves as a battery (a storage form) of free energy, which in the inanimate world would otherwise be dissipated as heat. However, in living Cells, ATP is consumed rapidly because it readily donates its high-energy phosphate to other molecules, functioning as a phosphate group donor. Virtually all Energy Metabolism reactions in cellular organisms proceed via the formation and breakdown of ATP molecules.

Due to thermal motion, ATP molecules are capable of diffusing short distances within cells (up to 10 µm). To transfer energy between cellular compartments, a specialized transport mechanism involving creatine kinase and creatine phosphokinase enzymes is utilized. Consequently, in living cells, ATP Functions not only as a chemical energy source in numerous metabolic reactions, but also as an energy store, donor, and specialized carrier.

Utilization of ATP energy. The chemical energy of ATP is continuously consumed in cells to sustain all energy-requiring biological processes (Fig. 14). For instance, in skeletal Muscles, ATP provides the energy required for contraction and relaxation. During contraction, the energy from ATP hydrolysis powers the interaction and sliding of Actin and Myosin contractile filaments. Contractile Proteins convert chemical energy into the mechanical energy of Muscle contraction. During relaxation, ATP energy is utilized for The Active Transport of Ca²⁺ ions across the reticulum membranes against their concentration gradient (mechanisms of active transport are discussed in Chapter 5).

ATP energy is also utilized in The Nervous system to generate electrical potentials during excitation and to transmit nerve impulses. A significant amount of ATP is expended by cells on The Biosynthesis of various substances, particularly the restoration and accumulation of proteins in Skeletal Muscle. A fraction of ATP energy may also be converted into thermal energy.

Fig. 14 Utilization of ATP energy in the Organism

Thus, within living cells, the chemical energy of ATP is converted into Other forms of energy: kinetic (mechanical), electrical, osmotic, and thermal.

Tissue ATP content. The amount of ATP in human Tissues is relatively small, as it is not stored in reserve. Skeletal muscle contains 5 mmol · kg^-1 of fresh tissue, or 25 mmol · kg^-1 of dry Muscle tissue. In cardiac and smooth muscle, ATP levels are 2.6 and 1.4 mmol · kg^-1 of fresh tissue, respectively. The entire human body contains a total of approximately 50 g of ATP.

ATP is characterized by a high turnover rate, especially during intense Physical Exercise. In skeletal muscle, this rate can reach 0.5 kg · min^-1. Nevertheless, no substantial drop in cellular ATP levels is observed. Even during strenuous muscular activity that induces fatigue, muscle ATP stores decline by only 20–25% over a span of a few seconds, thanks to the continuous operation of regeneration mechanisms. Consequently, the intracellular concentration of ATP is maintained at a relatively constant level. This stability is ensured by a precise balance between ATP synthesis (resynthesis) and utilization. An increase in The rate of ATP consumption automatically triggers the mechanisms responsible for its production:

The balance of these processes is achieved through the presence of specialized regulatory mechanisms for ATP metabolism.