2.1.1. Structure of mitochondria

The typical human cell has several hundred mitochondria, cytoplasmic organelles that convert energy to forms that can be used to drive cellular reactions. Without them cells would be dependent on anaerobic glycolysis for all their adenosine triphosphate (ATP). The mitochondria have a characteristic double membrane structure, in which the outer membrane contains large channel-forming proteins (called porin) and is permeable to all molecules of 5000 daltons or less, while the inner membrane is impermeable to most small ions and is intricately folded, forming structures called cristae. The large surface area of the inner mitochondrial membrane accommodates respiratory chain and ATP synthase enzymes involved in the process of oxidative phosphorylation (OXPHOS). The mitochondrial matrix contains hundreds of enzymes, including those required for the oxidation of pyruvate and fatty acids and those active in the tricarboxylic acid (TCA) cycle. The matrix also contains several identical copies of the mitochondrial DNA, mitochondrial ribosomes, tRNAs and various enzymes required for the transcription and translation of mitochondrial genes (see Alberts et al. 1994).

2.1.2. Energy production by mitochondria

Only a small fraction of the total free energy potentially available from glucose is released in glycolysis. The metabolism of carbohydrates is completed in the mitochondria when pyruvate is imported and oxidized by molecular oxygen (O₂) to CO₂ and water. The energy released is harnessed so efficiently that about 30 molecules of ATP are produced for each molecule of glucose oxidized, whereas only 2 molecules of ATP are produced by glycolysis alone. Oxidative metabolism in mitochondria is fuelled not only by pyruvate produced from carbohydrates by glycolysis in the cytosol but also by fatty acids. Pyruvate and fatty acids (from triglycerides) are selectively transported from the cytosol into the mitochondrial matrix, where they are broken down into the two-carbon acetyl group on acetyl coenzyme A (acetyl CoA) by the pyruvate dehydrogenase complex and the β -oxidation pathway, respectively. The acetyl group is then fed into the tricarboxylic acid cycle for further degradation, and the process ends with the passage of acetyl-derived high-energy electrons along the respiratory chain.

The proteins involved in OXPHOS are located within the mitochondrial inner membrane and include the electron transport chain (ETC) components (complexes I to IV), F_OF₁ ATP synthase and the adenine nucleotide translocator (ANT) (Figure 1). High-energy electrons from TCA are combined with molecular oxygen by means of the ETC to generate water. These electrons, borne on NADH (nicotinamide adenine dinucleotide), are transferred to respiratory complex I (NADH dehydrogenase) and then to coenzyme Q₁₀ (CoQ), while the electrons from succinate are transferred to complex II (succinate dehydrogenase, SDH) and CoQ. From CoQ, they are passed to complex III, and then to cytochrome c (cyt c), complex IV (cytochrome c oxidase, COX) and finally to O₂ to give H₂O. The energy released is used to pump protons (H⁺) out of the mitochondrial matrix, creating an electrochemical gradient across the inner membrane that is positive and acidic on the outside and negative and alkaline on the mitochondrial matrix side. This gradient creates a capacitor that can be depolarized by the transport of protons back into the matrix through a proton channel in the F_O membrane component of ATP synthase. This proton flux drives the condensation of adenosine diphosphate (ADP) and inorganic phosphate (P_i) to make ATP, which is then exported to the cytosol in exchange for ADP by the ANT (Figure 1). In this way oxygen consumption by the ETC is coupled to ADP phosphorylation by ATP synthase through the electrochemical gradient (DiMauro & Bonilla 1997, Wallace 1997).