BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 14. OXIDATIVE PHOSPHORYLATION

NADH and FADH₂ produced during Glycolysis, Fatty acid oxidation, and The Tricarboxylic Acid Cycle are energy-rich molecules, as each contains a pair of electrons with a high transfer potential. The transfer of these electrons to molecular oxygen releases a large amount of energy, which can be harnessed to generate ATP. Oxidative Phosphorylation is the process in which ATP formation is coupled to the flow of electrons along a chain of carriers from NADH or FADH₂ to O₂. In aerobic organisms, this process serves as the principal source of ATP. For instance, oxidative phosphorylation accounts for the generation of 32 out of the 36 ATP molecules produced during the Complete oxidation of glu

cose to CO₂ and H₂O. The most salient features of this process are outlined below.

1. Oxidative phosphorylation is carried out by respiratory assemblies localized in The inner mitochondrial membrane. The tricarboxylic acid cycle and the fatty acid oxidation pathway, which supply the bulk of NADH and FADH₂, take place in the adjacent mitochondrial matrix.

2. The oxidation of NADH yields 3 ATP, whereas the oxidation of FADH₂ yields 2 ATP. Oxidation and phosphorylation are coupled processes.

Respiration is a process of ATP generation in which an inorganic compound (such as O₂) serves as the terminal electron acceptor. The electron donor may be either an organic or an inorganic compound.

Class="center">Fig. 14.1. Electron micrograph of a mitochondrion

3. Respiratory assemblies comprise numerous electron carriers, notably Cytochromes. The multistep transport of electrons from NADH or FADH₂ to O₂ via these carriers results in the extrusion of protons from the mitochondrial matrix and the generation of a Membrane Potential (proton-motive force). Protons are pumped by Three types of electron-transfer complexes. The reentry of protons into the mitochondrial matrix mediated by

an enzyme complex drives the synthesis of ATP. Thus, the Coupling of oxidation and phosphorylation is mediated by a proton gradient across the inner mitochondrial membrane.

14.1. Oxidative Phosphorylation Takes Place in Mitochondria

Cell/35.html">Mitochondria are oval-shaped Organelles, typically about 2 µm in length and 0.5 µm in diameter. The method for isolating mitochondria was developed in the late 1940s. Eugene Kennedy and Albert Lehninger then discovered the presence of respiratory assemblies, tricarboxylic acid cycle Enzymes, and fatty acid oxidation enzymes within mitochondria. Electron Microscopy studies performed by George Palade and Fritjof Sjöstrand revealed that mitochondria possess two membrane systems: an outer membrane and an extensively folded inner membrane. The latter forms numerous deep invaginations called cristae. Consequently, mitochondria contain two distinct compartments: the intermembrane space (between the outer and inner membranes) and the matrix, enclosed by the inner membrane (Fig. 14.2). The respiratory assemblies are an integral part of the inner mitochondrial membrane, whereas most of the Reactions of the tricarboxylic acid cycle and fatty acid oxidation occur in the matrix.

The outer membrane is freely permeable to most small molecules and ions. In contrast, the inner membrane is impermeable to nearly all ions and most uncharged molecules. Specific transport Proteins exist to shuttle molecules such as ATP and long-chain Fatty acids across the inner mitochondrial membrane.

Fig. 14.2. Schematic diagram of a mitochondrion