Respiratory chain and oxidative phosphorylation

+Glycogenesis and

Glycogenolysis

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Respiratory chain (RCH)

a) is located in a mitochondrion

b) needs oxygen (O2) for its function

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Impermeable to ions and most other compounds

In inner membrane knobs

the mitochondrion contained the enzymes responsible for electron transport and oxidative phosphorylation

Electron Transport (Respiratory) ChainElectron transport chain (ETC) or respiratory chain (RC)

functions to oxidize NADH and FADH that arise from different metabolic pathways carried out on nutrients, leading to release of energy(exergonic).

Part of the released E is used for the synthesis of ATP

(oxidative phosphorylation).

 

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Components Of The Electron Transport Chain:

a) can transfer either H or electrons

b) are called Complex I, II, III and IV

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The Four Electron Transport Complexes in the Inner Mitochondrial Membrane Respiratory Chain

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(a) Complex I NADH-CoQ reductase (ATP)

(b) Complex II Succinate CoQ reductase (no ATP)

(c) Complex III CoQ- Cytochrome C reductase, (ATP)

(d) Complex IV Cytochrome C oxidase (ATP)

Two small electron carriers are also needed to link these large complexes, they are:

1.Coenzyme Q (CoQ or ubiquinone)

the only electron carrier not bound to a protein.

2.Cytochrome cCytochromes are electron carriers containing

hemes .

Cytochrome c is a small, water-soluble protein.

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The figure is adopted from the book: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th ed. Wiley‑Liss, Inc., New York, 1997. ISBN 0‑471‑15451‑2

proton= H+

electron= e-

Operation of the Electron Transport (Respiratory) Chain

The transfer of electrons is not directly to oxygen but through coenzymes.a)Complex I transfers H+ into an intermembrane space

b)Coenzyme Q accepts e- from both Complex I and Complex II

c)Complex IV transfers electrones to oxygen

d)oxygen is reduced to H2O

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The figure is adopted from the book: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th ed. Wiley‑Liss, Inc., New York, 1997. ISBN 0‑471‑15451‑2

proton= H+

electron= e-

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The function of the RCH

1) is to regenerate NAD+ from NADH

2) is to regenerate FAD from FADH2

3) oxygen is reduced to H2O

4) is to finish oxidation of energy substrates

and conserve their energy in a form of ATP

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Oxidative Phosphorylation

Coupling of the electron transport in RCH

with phosphorylation of ADP to form ATP.

a)needs proton gradient on the inner mitochondrial membrane by RCH

b)is catalyzed by ATP synthase

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Mechanism of Oxidative Phosphorylation

1- During the ETC (located at the inner mitochondrial membrane) , H+ are released into the intermembrane space (outside inner mitochondrial membrane).

2- The inner mitochondrial membrane is impermeable to H+, which accumulate outside the membrane.

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3- An electrochemical gradient is creating across the membrane. An electrical gradient (with more positive charges on the outside of the membrane than on the inside ) and chemical gradient or pH gradient (the outside of the membrane is at lower pH than the inside).

4- This H+ gradient is used by the ATP synthase complex to make ATP via oxidative phosphorylation.

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On the the Inner Mitochondrial Membrane, the ATP Synthase Uses the Proton Gradient Generated by Electron

Transport of the Respiratory Chain to Synthesize ATP

Electron transport chain

ATP yield from oxidative phosphorylation accounts for approximately 85% of the maximal 38 ATP generated in complete oxidation of glucose to CO2.

a) 1 NADH 3ATPb) 1 FADH 2ATP

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Glycogenesis and Glycogenolysis

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GlycogenGlycogen is the main storage form of

carbohydrates in animals. It is present mainly in liver and muscle.

The total quantity of muscle glycogen is more than liver glycogen, because of the large muscle mass.

The glycogen is present in the cytoplasm of the cell, because all the enzymes related to glycogen metabolism are cytoplasmic.

Function of glycogen:The function of liver glycogen is to maintain the

blood glucose concentration (acts as a glucose buffer).

The function of muscle glycogen is to serve as a fuel reserve for synthesis of ATP during muscle contraction (as energy store).

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Glycogen is a polymer of glucose residues linked by

α-1:4 glycosidic bonds, mainly α-1:6 glycosidic bonds, at branch points.

- Glycogen branches contain about 8 -12 glucose

residues.

Glycogen synthesis (glycogenesis)

Steps of Glycogen synthesis (glycogenesis):1-Activation of glucose: UDP-glucose (the active form of

glucose) is formed from glucose 1-phosphate and UTP (Uridine triphosphate) by UDP-glucose pyrophosphorylase.

UDP-glucose is the source of all glucosyl residues that are added to the growing glycogen molecule.

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3- Glycogen synthase:

Activated glucose units, UDP-glucose, are sequentially added by glycogen synthase to glycogen primer molecule(short linear glucose polymer).

The glycogen synthase can add glucose units in only 1,4-linkages.

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Glycogen primer (n) + UDP-glucose ---- Glycogen (n+1) + UDP.

Glycogen synthesis4- Branching enzyme: When the chain has been

lengthened, the branching enzyme transfers a part of the α1:4 chain to a neighbouring chain to form an α l :6 glucosidic link. Thus establishing the branching points in the molecule. The branches grow by further addition of 1:4 glucosyl units.

Degradation of glycogen (glycogenolysis)It is the breakdown of glycogen into glucose in liver and lactic

acid in muscles.

1- Glycogen Phosphorylase catalyzes phosphorolytic cleavage of

the α(14) glycosidic linkages of glycogen, releasing glucose-

1-phosphate as reaction product.

Glycogen(n) + Pi Glycogen (n–1) + glucose-1-phosphate

glucose-1-phosphate

H O

OH

H

OHH

OH

CH 2OH

H

OPO32

H

Glycogenolysis2)- Debranching enzyme is hydrolytic enzyme acts on

α1 - 6 glycosidic link giving free glucose.

3)- G-1-P is converted into G-6-P by the action of

phosphoglucomutase.

In liver, G-6-P is hydrolysed by the action of G-6-

phosphatase free glucose (diffuse from liver

cell to blood stream)

In muscle, G-6-P by glycolysis lactic acid because

absence of G-6-phosphatase

The rate limiting enzymes controlling glycogen metabolism are:

1. Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis.

2. Glycogen synthase catalyzes the rate-limiting step in glycogenesis.

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Glycogenolysis

Muscle glycogen functions to provide muscle with glucose during contraction. .

Liver glycogen functions to maintain blood glucose between meals.

After 12 -18 hr fasting, liver glycogen whereas muscle glycogen is after prolonged exercise.

Difference between muscle and liver glycogen

Liver glycogen Muscle glycogen

Amount - More

Source Glucose and other precursors

Glucose only

Hydrolysis

Give blood glucose Give lactic acid

Starvation

Converted into blood glucose

Not affected

Muscular exercise

Depleted later on Depleted first