ch12.qxd 2/13/2003 2:46 PM Page 92 The Respiratory Chain & Oxidative Phosphorylation 12 Peter A. Mayes, PhD, DSc, & Kathleen M. Botham, PhD, DSc BIOMEDICAL IMPORTANCE trapping the liberated free energy as high-energy phos- phate, and the enzymes of β-oxidation and of the citric Aerobic organisms are able to capture a far greater pro- acid cycle (Chapters 22 and 16) that produce most of portion of the available free energy of respiratory sub- the reducing equivalents. strates than anaerobic organisms. Most of this takes place inside mitochondria, which have been termed the “powerhouses” of the cell. Respiration is coupled to the Components of the Respiratory Chain generation of the high-energy intermediate, ATP, by Are Arranged in Order of Increasing oxidative phosphorylation, and the chemiosmotic Redox Potential theory offers insight into how this is accomplished. A number of drugs (eg, amobarbital) and poisons (eg, Hydrogen and electrons flow through the respiratory chain (Figure 12–3) through a redox span of 1.1 V cyanide, carbon monoxide) inhibit oxidative phos- + phorylation, usually with fatal consequences. Several in- from NAD /NADH to O2/2H2O (Table 11–1). The herited defects of mitochondria involving components respiratory chain consists of a number of redox carriers of the respiratory chain and oxidative phosphorylation that proceed from the NAD-linked dehydrogenase sys- have been reported. Patients present with myopathy tems, through flavoproteins and cytochromes, to mole- and encephalopathy and often have lactic acidosis. cular oxygen. Not all substrates are linked to the respi- ratory chain through NAD-specific dehydrogenases; some, because their redox potentials are more positive SPECIFIC ENZYMES ACT AS MARKERS (eg, fumarate/succinate; Table 11–1), are linked di- rectly to flavoprotein dehydrogenases, which in turn are OF COMPARTMENTS SEPARATED BY linked to the cytochromes of the respiratory chain (Fig- THE MITOCHONDRIAL MEMBRANES ure 12–4). Mitochondria have an outer membrane that is perme- Ubiquinone or Q (coenzyme Q) (Figure 12–5) able to most metabolites, an inner membrane that is links the flavoproteins to cytochrome b, the member of selectively permeable, and a matrix within (Figure the cytochrome chain of lowest redox potential. Q ex- 12–1). The outer membrane is characterized by the ists in the oxidized quinone or reduced quinol form presence of various enzymes, including acyl-CoA syn- under aerobic or anaerobic conditions, respectively. thetase and glycerolphosphate acyltransferase. Adenylyl The structure of Q is very similar to that of vitamin K kinase and creatine kinase are found in the intermem- and vitamin E (Chapter 45) and of plastoquinone, brane space. The phospholipid cardiolipin is concen- found in chloroplasts. Q acts as a mobile component of trated in the inner membrane together with the en- the respiratory chain that collects reducing equivalents zymes of the respiratory chain. from the more fixed flavoprotein complexes and passes them on to the cytochromes. An additional component is the iron-sulfur protein THE RESPIRATORY CHAIN COLLECTS (FeS; nonheme iron) (Figure 12–6). It is associated & OXIDIZES REDUCING EQUIVALENTS with the flavoproteins (metalloflavoproteins) and with cytochrome b. The sulfur and iron are thought to take Most of the energy liberated during the oxidation of part in the oxidoreduction mechanism between flavin carbohydrate, fatty acids, and amino acids is made and Q, which involves only a single e− change, the iron available within mitochondria as reducing equivalents atom undergoing oxidoreduction between Fe2+ and (H or electrons) (Figure 12–2). Mitochondria con- Fe3+. tain the respiratory chain, which collects and trans- Pyruvate and α-ketoglutarate dehydrogenase have ports reducing equivalents directing them to their final complex systems involving lipoate and FAD prior to reaction with oxygen to form water, the machinery for the passage of electrons to NAD, while electron trans- 92 ch12.qxd 2/13/2003 2:46 PM Page 93 THE RESPIRATORY CHAIN & OXIDATIVE PHOSPHORYLATION /93 Electrons flow from Q through the series of cyto- chromes in order of increasing redox potential to mole- cular oxygen (Figure 12–4). The terminal cytochrome aa3 (cytochrome oxidase), responsible for the final com- Phosphorylating bination of reducing equivalents with molecular oxy- complexes gen, has a very high affinity for oxygen, allowing the respiratory chain to function at maximum rate until the tissue has become depleted of O2. Since this is an irre- MATRIX versible reaction (the only one in the chain), it gives di- rection to the movement of reducing equivalents and to the production of ATP, to which it is coupled. Functionally and structurally, the components of Cristae the respiratory chain are present in the inner mitochon- drial membrane as four protein-lipid respiratory chain complexes that span the membrane. Cytochrome c is the only soluble cytochrome and, together with Q, INNER seems to be a more mobile component of the respira- MEMBRANE tory chain connecting the fixed complexes (Figures 12–7 and 12–8). OUTER MEMBRANE THE RESPIRATORY CHAIN PROVIDES MOST OF THE ENERGY CAPTURED DURING CATABOLISM ADP captures, in the form of high-energy phosphate, a Figure 12–1. Structure of the mitochondrial mem- significant proportion of the free energy released by branes. Note that the inner membrane contains many catabolic processes. The resulting ATP has been called folds, or cristae. the energy “currency” of the cell because it passes on this free energy to drive those processes requiring en- ergy (Figure 10–6). fers from other dehydrogenases, eg, L(+)-3-hydroxyacyl- There is a net direct capture of two high-energy CoA dehydrogenase, couple directly with NAD. phosphate groups in the glycolytic reactions (Table The reduced NADH of the respiratory chain is in 17–1), equivalent to approximately 103.2 kJ/mol of turn oxidized by a metalloflavoprotein enzyme—NADH glucose. (In vivo, ∆G for the synthesis of ATP from dehydrogenase. This enzyme contains FeS and FMN, ADP has been calculated as approximately 51.6 kJ/mol. is tightly bound to the respiratory chain, and passes re- (It is greater than ∆G0′ for the hydrolysis of ATP as ducing equivalents on to Q. given in Table 10–1, which is obtained under standard FOOD ATP Fat Fatty acids + β-Oxidation Glycerol O2 Citric Carbohydrate Glucose, etc acid Acetyl – CoA 2H H2O cycle Respiratory chain Protein Amino acids Digestion and absorption MITOCHONDRION ADP Extramitochondrial sources of reducing equivalents Figure 12–2. Role of the respiratory chain of mitochondria in the conversion of food energy to ATP. Oxidation of the major foodstuffs leads to the generation of reducing equivalents (2H) that are collected by the respiratory chain for oxidation and coupled generation of ATP. ch12.qxd 2/13/2003 2:46 PM Page 94 94 / CHAPTER 12 + 3+ AH2 NAD FpH2 2Fe H2O Substrate Flavoprotein Cytochromes 2+ 1 Figure 12–3. Transport of reducing A NADH Fp 2Fe /2O 2 equivalents through the respiratory H+ H+ 2H+ 2H+ chain. concentrations of 1.0 mol/L.) Since 1 mol of glucose dent that the respiratory chain is responsible for a large yields approximately 2870 kJ on complete combustion, proportion of total ATP formation. the energy captured by phosphorylation in glycolysis is small. Two more high-energy phosphates per mole of Respiratory Control Ensures glucose are captured in the citric acid cycle during the a Constant Supply of ATP conversion of succinyl CoA to succinate. All of these phosphorylations occur at the substrate level. When The rate of respiration of mitochondria can be con- substrates are oxidized via an NAD-linked dehydrogen- trolled by the availability of ADP. This is because oxi- ase and the respiratory chain, approximately 3 mol of dation and phosphorylation are tightly coupled; ie, oxi- inorganic phosphate are incorporated into 3 mol of dation cannot proceed via the respiratory chain without ADP to form 3 mol of ATP per half mol of O2 con- concomitant phosphorylation of ADP. Table 12–1 sumed; ie, the P:O ratio = 3 (Figure 12–7). On the shows the five conditions controlling the rate of respira- other hand, when a substrate is oxidized via a flavopro- tion in mitochondria. Most cells in the resting state are tein-linked dehydrogenase, only 2 mol of ATP are in state 4, and respiration is controlled by the availabil- formed; ie, P:O = 2. These reactions are known as ox- ity of ADP. When work is performed, ATP is con- idative phosphorylation at the respiratory chain verted to ADP, allowing more respiration to occur, level. Such dehydrogenations plus phosphorylations at which in turn replenishes the store of ATP. Under cer- the substrate level can now account for 68% of the free tain conditions, the concentration of inorganic phos- energy resulting from the combustion of glucose, cap- phate can also affect the rate of functioning of the respi- tured in the form of high-energy phosphate. It is evi- ratory chain. As respiration increases (as in exercise), Succinate Choline Proline 3-Hydroxyacyl-CoA 3-Hydroxybutyrate Glutamate Fp Malate (FAD) Pyruvate Isocitrate FeS Fp Lipoate Fp NAD (FMN) Q Cyt b Cyt c1 Cyt c Cyt aa3 O2 (FAD) FeS FeS Cu α -Ketoglutarate Fp FeS (FAD) ETF FeS (FAD) Fp FeS: Iron-sulfur protein (FAD) ETF: Electron-transferring flavoprotein Fp: Flavoprotein Q: Ubiquinone Glycerol 3-phosphate Acyl-CoA Cyt: Cytochrome Sarcosine Dimethylglycine Figure 12–4. Components of the respiratory chain in mitochondria, showing the collecting points for reduc- ing equivalents from important substrates. FeS occurs in the sequences on the O2 side of Fp or Cyt b. ch12.qxd 2/13/2003 2:46 PM Page 95 THE RESPIRATORY CHAIN & OXIDATIVE PHOSPHORYLATION /95 O H OH H OH (H+ + e–) (H+ + e–) CH3O CH3 CH3 CH3O [CH2CH CCH2]nH O •O OH Fully oxidized or Semiquinone form Reduced or quinol form quinone form (free radical) (hydroquinone) Figure 12–5.