Oxidation, and phosphorylation

A High-Energy Chemical Intermediate Coupling Oxidation and Phosphorylation Proved Elusive... 

Figure 29.10 Mechanism of Step 6 in Figure 29.7, the oxidation and phosphorylation of glyceraldehyde 3-phosphate to give 1,3-bisphosphoglycerate.

The rate of respiration of mitochondria can be controlled by the availability of ADP. This is because oxidation and phosphorylation are tightly coupled ie, oxidation cannot proceed via the respiratory chain without concomitant phosphorylation of ADP. Table 12-1 shows the five conditions controlling the rate of respiration in mitochondria. Most cells in the resting state are in state 4, and respiration is controlled by the availability of ADP. When work is performed, ATP is converted to ADP, allowing more respiration to occur, which in turn replenishes the store of ATP. Under certain conditions, the concentration of inorganic phosphate can also affect the rate of functioning of the respiratory chain. As respiration increases (as in exercise). 

The action of uncouplers is to dissociate oxidation in the respiratory chain from phosphorylation. These compounds are toxic in vivo, causing respiration to become uncontrolled, since the rate is no longer limited by the concentration of ADP or Pj. The uncoupler that has been used most frequently is 2,4-dinitrophenol, but other compounds act in a similar manner. The antibiotic oligomycin completely blocks oxidation and phosphorylation by acting on a step in phosphorylation (Figures 12-7 and 12-8). 

In most tissues, where the primary role of the citric acid cycle is in energy-yielding metabohsm, respiratory control via the respiratory chain and oxidative phosphorylation regulates citric acid cycle activity (Chapter 14). Thus, activity is immediately dependent on the supply of NAD, which in turn, because of the tight couphng between oxidation and phosphorylation, is dependent on the availabihty of ADP and hence, ulti-... 

Substances that functionally separate oxidation and phosphorylation from one another are referred to as uncouplers. They break down the proton gradient by allowing ions to pass from the intermembrane space back into the mitochondrial matrix without the involvement of ATP synthase. Uncoupling effects are produced by mechanical damage... 

A muscle biopsy from a patient with the rare disorder, Luft disease, showed abnormally large mitochondria that contained packed cristae when examined in the electron microscope. Basal ATPase activity of the mitochodria was seven times greater than normal. From these and other data it was concluded that oxidation and phosphorylation were partially uncoupled. Which of the following statements about this patient is correct ... 

Rabbit muscle glyceraldehyde-3-phosphate dehydrogenase (MW = 140,000) catalyzes the oxidation and phosphorylation of glyceraldehyde-... 

Glycolysis proceeds in the presence of arsenate, but the ATP normally formed in the conversion of 1,3-bisphosphoglycerate into 3-phosphoglycerate is lost. Thus, arsenate uncouples oxidation and phosphorylation by forming a highly labile acyl arsenate. 

Such a defect (called Luft syndrome) was found in a 38-year-old woman who was incapable of performing prolonged physical work. Her basal metabolic rate was more than twice normal, but her thyroid function was normal. A muscle biopsy showed that her mitochondria were highly variable and atypical in structure. Biochemical studies then revealed that oxidation and phosphorylation were not tightly coupled in these mitochondria. In this patient, much of the energy of fuel molecules was converted into heat rather than ATP. 

Matsuura and co-workers (756) have reexamined the reactions of the A -oxides of 2,5-dimethylpyrazine and found that 2,5-dimethylpyrazine di-A -oxide (29) when heated with phosphoryl chloride at 160° gave 2,5-dichloro-3,6-dimethylpyrazine (6%) (30), 3-chloro-2,5-dimethylpyrazine 1-oxide (5%) (31), and 5-chloromethyl-2-methylpyrazine 1 -oxide (9%) (32). In addition small amounts of other chlorinated products, 3-chloro-2-chloromethyl-5-methylpyrazine (33) and 2,5-bischloromethyl-pyrazine (34), were identified. These authors also examined the action of p-tosyl chloride, methane sulfonyl chloride, and mixtures of phosphoryl chloride and concentrated sulfuric acid, but state that these did not give good results. Pyrazine 1-oxide and phosphoryl chloride have been shown to give 2reaction conditions it gave 2-chloropyrazine 1-oxide (757). Pyrazine 1,4-dioxide and benzenesulfonyl chloride also gave a low yield of 2-chloropyrazine 1-oxide (758). 

STEP 6 Glyceraldehyde 3-phosphatc is oxidized and phosphorylated to yield 3-phosphoglyceroyl phosphate. 

Presumably by reacting with the sulfhydryl groups, antimony can inhibit oxidative and phosphorylating enzymes like monoamine oxidase, succinoxidase, pyruvate brain oxidase, and phosphoftuctokinase. Inhibition of these enzymes can alter activities such as glucose metabolism and nerve transmission. Ten percent of the trivalent form is excreted by the kidney in 24 h 50-60% of the pentavelent form is found in the urine within 24 h. 

The predominant circulating form of vitamin Bg is pyridoxal phosphate. Absorbed pyridoxine is oxidized and phosphorylated in intestinal mucosal cells, liver, and erythrocytes. Pyridoxine enters hepatocytes and erythrocytes by passive diffusion and is mostly retained by phosphorylation. Pyridoxal phosphate is transported in the blood bound to albumin. The blood-brain barrier has limited permeability to pyridoxal. 

Mitchell s highly innovative hypothesis that oxidation and phosphorylation are coupled by a proton gradient is now supported by a wealth of evidence. Indeed, electron transport does generate a proton gradient across the inner mitochondrial membrane. The pH outside is 1.4 units lower than inside, and the membrane potential is 0.14 V, the outside being positive. As calculated on page 508, this membrane potential corresponds to a free energy of 21.8 kj (5.2 kcal) per mole of protons. 

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