BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 16. ENERGY METABOLISM

16.2. Oxidative Phosphorylation of ADP

16.2.4. Thermoregulatory Function of the ETC

Approximately 40-45 % of the total energy of electrons transferred along the ETC is utilized for ATP synthesis, while about 25 % is spent on transporting substances across the membrane. The remaining energy is dissipated as heat and used by warm-blooded animals to maintain body Temperature. In addition, extra heat can be generated through the Uncoupling of Respiration and phosphorylation. The uncoupling of Oxidative Phosphorylation can be biologically advantageous. It allows for heat generation to maintain body temperature in newborns, hibernating animals, and all mammals during cold adaptation. Newborns and hibernating animals possess a specialized tissue dedicated to heat production via the uncoupling of respiration and phosphorylation, known as brown adipose tissue (brown fat). This tissue is rich in Cell/35.html">Mitochondria. The mitochondrial membrane contains a large excess of respiratory Enzymes relative to ATP synthase. About 10 % of all mitochondrial Proteins consist of the so-called uncoupling protein 1 (UCP1), or thermogenin. Brown fat is present in infants, but virtually absent in adult humans. In recent years, evidence has emerged indicating the existence of uncoupling proteins structurally similar to brown fat UCP1 in the mitochondria of various mammalian Organs and Tissues. Structurally, thermogenin is closely related to the ATP/ADP antiporter, yet it is unable to transport NUCLEOTIDES, although it has retained The ability to carry fatty acid anions, which act as uncoupling agents.

On the outer side of the membrane, a fatty acid anion binds a proton and crosses the membrane in this form; on the inner side, it dissociates by releasing a proton into the matrix, thereby diminishing the proton gradient. The resulting anion returns to the outer side of the membrane via the ATP/ADP antiporter.

Exposure to cold stimulates the release of norepinephrine from sympathetic nerve terminals. This leads to the activation of lipase in adipose tissue and the mobilization of fat from fat depots. The resulting free Fatty acids serve not only as "fuel" but also as a critical regulator of the uncoupling between respiration and phosphorylation.