BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 16. ENERGY METABOLISM

16.1. Biological Oxidation

16.1.1. Characteristics of High-Energy Phosphates. The ATP — ADP Cycle

Living organisms contain a group of organic phosphates whose Hydrolysis releases a large amount of Free energy. Such compounds are referred to as high-energy phosphates (Table 16.1).

Various phosphorylated compounds possess different reserves of free energy (Table 16.1). In addition to ATP, the group of high-energy phosphates includes enol phosphates, anhydrides, and phosphoguanidines. ATP is the principal compound among them (Fig. 16.2).

Class="center">Table 16.1

Free energy of Hydrolysis for Certain Organic Phosphates

Fig. 16.2. Adenosine triphosphate (ATP)

ATP is an energy-rich molecule because it contains two phosphoanhydride bonds, β and γ (indicated by the tilde symbol ~ in Fig. 16.2). Hydrolysis of the terminal phosphoanhydride bond converts ATP into ADP and inorganic phosphate P_i, with a Standard Free Energy change of 30.51 kJ/mol. Under standard cellular conditions (pH 7.0, Temperature 37 °C), the actual value of ∆G⁰' for hydrolysis is approximately 51 kJ/mol. The magnitude of the free energy of ATP hydrolysis enables its formation from ADP through Phosphate group transfer from high-energy phosphates, such as phosphoenolpyruvate or 1,3-bisphosphoglycerate; conversely, ATP can participate in the phosphorylation of glucose or glycerol. ATP serves as an energy donor in numerous anabolic reactions. Certain BIOSYNTHETIC PROCESSES IN the Organism can utilize other nucleoside triphosphates that are analogues of ATP, including guanosine triphosphate (GTP), uridine triphosphate (UTP), and cytidine triphosphate (CTP). All of these NUCLEOTIDES are, in turn, synthesized using the free energy of the terminal phosphate group of ATP. Finally, the free energy of ATP drives Various Forms of physiological work fundamental to life processes, such as The Biosynthesis of substances, Muscle contraction, and Active Transport across membranes.

Thus, ATP is the primary donor of free energy directly utilized in biological systems. Within The Cell, an ATP molecule is turned over within a minute of its formation.

The utilization of ATP as an energy source is only possible through the continuous synthesis of ATP from ADP, driven by The energy released during The oxidation of Organic compounds (Fig. 16.3). The ATP–ADP cycle is the principal mechanism of energy exchange in biological systems, and ATP Functions as the universal "energy currency".

Fig. 16.3. The ATP–ADP cycle