The Respiratory Chain Provides Most Of The Energy Captured During Catabolism

ADP captures, in the form of high-energy phosphate, a significant proportion of the free energy released by catabolic processes. The resulting ATP has been called the energy "currency" of the cell because it passes on this free energy to drive those processes requiring energy (Figure 10-6).

There is a net direct capture of two high-energy phosphate groups in the glycolytic reactions (Table 17-1), equivalent to approximately 103.2 kJ/mol of glucose. (In vivo, AG for the synthesis of ATP from ADP has been calculated as approximately 51.6 kJ/mol. (It is greater than AG0 for the hydrolysis of ATP as given in Table 10-1, which is obtained under standard

FOOD

Fat-

Carbohydrate -

Fatty acids +

Glycerol -Glucose, etc

Amino acids

Fatty acids +

Glycerol -Glucose, etc

Amino acids

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.

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.

Substrate

NAD+

NADH

NADH

FpH2 Flavoprotein Fp fph2 2Fe

' ein Cytochromes

2Fe'

2Fe'

Figure 12-3. Transport of reducing equivalents through the respiratory chain.

Figure 12-3. Transport of reducing equivalents through the respiratory chain.

concentrations of 1.0 mol/L.) Since 1 mol of glucose yields approximately 2870 kJ on complete combustion, the energy captured by phosphorylation in glycolysis is small. Two more high-energy phosphates per mole of glucose are captured in the citric acid cycle during the conversion of succinyl CoA to succinate. All of these phosphorylations occur at the substrate level. When substrates are oxidized via an NAD-linked dehydrogenase and the respiratory chain, approximately 3 mol of inorganic phosphate are incorporated into 3 mol of ADP to form 3 mol of ATP per half mol of O2 consumed; ie, the P:O ratio = 3 (Figure 12-7). On the other hand, when a substrate is oxidized via a flavopro-tein-linked dehydrogenase, only 2 mol of ATP are formed; ie, P:O = 2. These reactions are known as oxidative phosphorylation at the respiratory chain level. Such dehydrogenations plus phosphorylations at the substrate level can now account for 68% of the free energy resulting from the combustion of glucose, captured in the form of high-energy phosphate. It is evi dent that the respiratory chain is responsible for a large proportion of total ATP formation.

Respiratory Control Ensures a Constant Supply of ATP

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),

Pyruvate \

Lipoate

Proline 3-Hydroxyacyl-CoA 3-Hydroxybutyrate Glutamate Malate Isocitrate

Succinate Choline

a-Ketoglutarate

Cyt c1

Glycerol 3-phosphate

Acyl-CoA Sarcosine Dimethylglycine

FeS ETF Fp Q Cyt

Iron-sulfur protein

Electron-transferring flavoprotein

Flavoprotein

Ubiquinone

Cytochrome

Figure 12-4. Components of the respiratory chain in mitochondria, showing the collecting points for reducing equivalents from important substrates. FeS occurs in the sequences on the O2 side of Fp or Cyt b.

Semiquinone form (free radical)

Fully oxidized or quinone form

Semiquinone form (free radical)

CH3O

ch3o

CH3 I

Reduced or quinol form (hydroquinone)

Figure 12-5. Structure of ubiquinone (Q). n = Number of isoprenoid units, which is 10 in higher animals, ie, Q10.

the cell approaches state 3 or state 5 when either the capacity of the respiratory chain becomes saturated or the Po2 decreases below the Km for cytochrome ¿3. There is also the possibility that the ADP/ATP transporter (Figure 12-9), which facilitates entry of cytosolic ADP into and ATP out of the mitochondrion, becomes rate-limiting.

Thus, the manner in which biologic oxidative processes allow the free energy resulting from the oxidation of foodstuffs to become available and to be captured is stepwise, efficient (approximately 68%), and controlled—rather than explosive, inefficient, and uncontrolled, as in many nonbiologic processes. The remaining free energy that is not captured as high-energy phosphate is liberated as heat. This need not be considered "wasted," since it ensures that the respiratory system as a whole is sufficiently exergonic to be removed from equilibrium, allowing continuous unidirectional flow and constant provision of ATP. It also contributes to maintenance of body temperature.

Figure 12-6. Iron-sulfur-protein complex (Fe4S4). (I acid-labile sulfur; Pr, apoprotein; Cys, cysteine residue. Some iron-sulfur proteins contain two iron atoms and two sulfur atoms (Fe2S2).

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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