The phosphorylation of ADP to ATP

Experimentally, mitochondrial metabolism is measured using the oxygen electrode, in which the percentage saturation of the buffer with oxygen is measured electrochemically as the mitochondria oxidize substrates and reduce oxygen to water. 

2 succinate, which reduces a flavin coenzyme, then ubiquinone. 

The stepwise oxidation of NADH and reduction of oxygen to water is obligatorily linked to the phosphorylation of ADP to ATP. Approximately 3 mol of ATP is formed for each mole of NADH that is oxidized. Flavoproteins reduce ubiquinone, which is an intermediate coenzyme in the chain, and approximately 2 mol of ADP is phosphorylated to ATP for each mole of reduced flavoprotein that is oxidized. 

This means that 2 mol of ADP is required for the oxidation of a substrate such as succinate, but 3 mol of ADP is required for the oxidation of malate. Therefore, the oxidation of succinate will consume more oxygen when ADP is limiting than does the oxidation of malate. This is usually expressed as the ratio of phosphate to oxygen consumed in the reaction the P/O ratio is approximately 3 for malate and approximately 2 for succinate. 

Mitochondria are intracellular organelles with a double-membrane structure. Both the number and size of mitochondria vary in different cells - for example, a liver cell contains some 800 mitochondria, a renal tubule cell some 300 and a sperm about 20. The outer mitochondrial membrane is permeable to a great many substrates, while the inner membrane provides a barrier to regulate the uptake of substrates and output of products (see, for example, the regulation of palmitoyl CoA uptake into the mitochondrion for oxidation in section 5.5.1). 

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Complex V catalyzes the phosphorylation of ADP to ATP. Deficiencies of complex V are rare and clinical presentation is usually nonprogressive muscle weakness. One subject included growth retardation, sensorineural deafness, and involvement of basal ganglia, which are occasional features of various other respiratory chain abnormalities. 

Several enzymes, known collectively as fatty acid oxidase, are found in the mitochondrial matrix or inner membrane adjacent to the respiratory chain. These catalyze the oxidation of acyl-CoA to acetyl-CoA, the system being coupled with the phosphorylation of ADP to ATP (Figure 22-3). 

With these electron transport components, path can be created for the transfer of electrons from substrate to oxygen. Complexes I and IV obviously have enough energy change to support the phosphorylation of ADP to ATP, while complex III is marginal and complex II obviously does not have sufficient energy change. 

A. allow electron transport to continue but prevent the phosphorylation of ADP to ATP... 

The electron transport chain (ETC) or electron transport system (ETS) shown in Figure 16-1 is located on the inner membrane of the mitochondrion and is responsible for the harnessing of free energy released as electrons travel from more reduced (more negative reduction potential, E to more oxidized (more positive carriers to drive the phosphorylation of ADP to ATP. Complex 1 accepts a pair of electrons from NADH ( = -0.32 V)... 

Energy is released as electrons travel from more reduced (more negative reduction potential, E ) to more oxidized (more positive E carriers to drive the phosphorylation of ADP to ATP. 

ADP (Adenosine Diphosphate) An important cellular metabolite involved with energy exchange within the cell. Chemical energy is conserved in a cell by the phosphorylation of ADP to ATP primarily in the mitochondria, as a high energy phosphate bond. ADP combined with CP forms ATP, the usable fuel for muscular contractions. 

The electron transport chain consists of four multisubunit membrane-bound complexes and two mobile electron carriers (coenzyme Q and cytochrome c). The reactions that take place in three of these complexes generate enough energy to drive the phosphorylation of ADP to ATP. Many proteins of the electron transport chain contain iron, either as part of a heme or combined with sulfur. 

In the light reactions of photosynthesis, water is converted to oxygen hy oxidation and NADP+ is reduced to NADPH. The series of redox reactions is coupled to the phosphorylation of ADP to ATP in a process called photophosphorylation. 

Oxidative phosphorylation (1) Process in which the energy of electrons is captured in high-energy bonds as phosphate groups combine with ADP to form ATP. (2) The phosphorylation of ADP to ATP that occurs in conjunction with the transit of electrons down the electron transport chain in the inner mitochondrial membrane. 

Thus, by combining the NADH-ubiquinone reductase complex with a heterogeneous fraction solubilized from bovine heart submitochondrial fractions (referred to as hydrophobic protein ) and phospholipids (phosphatidylethanolamine, phosphatidylcholine, cardiolipid), Ragan and Racker and his associates [146] were able to reconstitute vesicles. These vesicles, which can be isolated on sucrose gradients, carry on the phosphorylation of ADP to ATP, and the oxidative phosphorylation is sensitive to uncouplers. 

The membrane of the cristae contains the coenzymes associated with electron transport, the oxidation of reduced coenzymes, and the reduction of oxygen to water (section 3.3.1.2). The primary particles on the matrix surface of the cristae contain the enzyme that catalyses the phosphorylation of ADP to ATP (section 3.3.1.3). 

The processes of oxidation of reduced coenzymes and the phosphorylation of ADP to ATP are normally tightly coupled ... 

Metabolic fuels can only be oxidized when NAD and oxidized flavoproteins are available. Therefore, if there is little or no ADP available in the mitochondria (i.e. it has all been phosphorylated to ATP), there will be an accumulation of reduced coenzymes, and hence a slowing of the rate of oxidation of metabolic fuels. This means that substrates are only oxidized when there is a need for the phosphorylation of ADP to ATP and ADP is available. The availability of ADP is dependent on the utilization of ATP in performing physical and chemical work, as shown in Figure 3.2. 

Catabolism Metabolic reactions resulting in the breakdown of complex molecules to simpler products, commonly oxidation to carbon dioxide and water, linked to the phosphorylation of ADP to ATP... 

We saw in Case study 4.2 that the phosphorylation of ADP to ATP can be coupled to the exergonic dephosphorylation of other molecules. Indeed, this is... 

FIGURE 2 Some important reactions in metabolism. Shown are the phosphorylation of ADP to ATP, NAD+, NADH, FAD, FADH2 acetate, CoA, and acetyl CoA. For clarity, just the parts of the larger molecules that undergo reaction are shown. NAD+, nicotinamide adenine dinucleotide NADH, nicotinamide adenine dinucleotide (reduced form) FAD, flavin adenine dinucleotide FADH2, flavin adenine dinucleotide (reduced form) CoA, coenzyme A AMP, adenosine monophosphate. 

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