In this lecture …

- What is Cellular Respiration (Production of Energy).

- The three Stages:- A. Glycolysis.- B. Krebs Cycle (TCA Cycle).- C. Electron Transport Chain.

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Cellular Respiration- Glycolysis, Krebs cycle, Electron Transport [3D Animation]

Cellular Respiration

• Process of respiration

is split into four parts

• By breaking it into four parts we will have less to learn at any one stage

• Site of Glycolysis

• Glycolysis takes place in the cytoplasm (cytosol) of cells.

• Glycolysis is the first stage of respiration.• It is the oxidation of glucose into pyruvate in the presence of oxygen and into lactate in absence of oxygen.

• Glycolysis splits one molecule of glucose 6-C molecule into two smaller molecules of pyruvate 3-C molecules

Glycolysis

- Glycolysis is a catabolic pathway. - Glycolysis is the major pathway for glucose metabolism.- Glucose and oxygen carbon dioxide, water and

energy- Glycolysis occurs in the cytoplasm (cytosol) of all cells.- It can function either aerobically or anaerobically. RBCs,

which lack mitochondria, are completely reliant on glucose as their metabolic fuel and metabolize it by anaerobic glycolysis.

- Glycolysis involves ten individual steps, including three isomerizations and four phosphate transfers. The only redox reaction takes place in step [6].

- There are two stages of Glycolysis: Phosphorylation and Oxidation.

Stage One - Phosphorylation

1. Glucose is phosphorylated by adding 2 phosphates from 2 molecules of ATP to give a hexose phosphate.

2. The hexose phosphate is split using water (hydrolysis)

3. 2 molecules of triose phosphate and 2 molecules of ADP are created.

Glucose Glucose-6-phosphate fructose-6-phosphate

Dihydroxyacetone phosphate fructose 1,6 bisphosphate

Glyceraldehyde -3- phosphate

Glucokinase or hexokinase

Phosphohexose isomerase

ATP ADP ATP

ADP

phosphofructokinase

aldolase

1 2

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Stage One - Phosphorylation

Triose phosphate isomerase

[1] Glucose, is first phosphorylated to glucose 6-phosphate, with ATP being consumed.

[2] Glucose 6-phosphate is isomerized into fructose 6-phosphate.

[3] Using ATP again, another phosphorylation takes place, giving rise to fructose 1,6-bisphosphate. Phosphofructokinase is the most important key enzyme in glycolysis.

[4] Fructose 1,6-bisphosphate is broken down by aldolase into the C3 compounds: glyceraldehyde 3-phosphate and glycerone 3-phosphate (dihydroxyacetone 3-phosphate).

[5] The latter two products are placed in fast equilibrium by triose phosphate isomerase.

Stage Two - Oxidation

1. The triose phosphates are oxidised (lose oxygen), forming two molecules of pyruvate.

2. Coenzyme NAD+ collects the hydrogen ions, forming 2 reduced NAD+ (NADH + H+).

(A coenzyme is a helper molecule that carries chemical groups or ions, e.g. NAD+ removes H+ and carries it to other molecules).

3. 4 ATP are produced, but 2 were used up at the beginning, so there is a net gain of 2 ATP.

Gly-3-phosphate dehydrogenase

NADH+ H+ NAD+

Phosphoglycerate kinase

Glyceraldehyde-3-phosphate 1,3-biphosphoglyceric acid

3 phosphoglyceric acid

2 phosphoglyceric acid

2 phospho enol-pyruvic acid

Pyrovate

ADP

ATP

Phosphoglycerate mutase

ADP

ATP

enolase H2O

Pyruvate kinase

oxidation and phosphorylation

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[6] Glyceraldehyde 3-phosphate is oxidized by glyceraldehyde-3-phosphate dehydrogenase, with NADH + H+ being formed. In this reaction, inorganic phosphate is taken up into the molecule (substrate level phosphorylation; and 1,3-bisphosphoglycerate is produced. [7] Catalyzed by phosphoglycerate kinase, this phosphate residue is transferred to ADP, producing 3-phosphoglycerate and ATP. The ATP balance is thus once again in equilibrium.[8] As a result of shifting of the remaining phosphate residue within the molecule, the isomer 2-phosphoglycerate is formed.[9] Elimination of water from 2-phosphoglycerate produces the phosphate ester of the enol form of pyruvate (Phosphoenolpyruvate). This reaction also raises the second phosphate residue to a high potential.[10] In the last step, pyruvate kinase transfers this residue to ADP. The remaining enol pyruvate is immediately rearranged into pyruvate, which is much more stable.

Glycolysis can function anaerobically by regenerating oxidized NAD+ (required in the glyceraldehyde-3-phosphate dehydrogenase reaction) by reducing pyruvate to lactate.

Lactate is the end product of glycolysis under anaerobic conditions (eg, in exercising muscle).

Glycolysis is regulated mainly by three enzymes catalyzing nonequilibrium reactions: hexokinase, phosphofructokinase, and pyruvate kinase.

Pyruvate is oxidized to acetyl-CoA by a multienzyme complex, pyruvate dehydrogenase, that is dependent on the vitamin cofactor thiamin diphosphate.

Insulin induces several enzymes involved in glycolysis [3, 5, 7]. Glucagon represses pyruvate kinase [7], a key enzyme of

glycolysis.

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Net Production of Energy

At the end of the Glycolysis

Some chemical compounds does not start Glycolysis from the beginning.

In glycolysis, two molecules of ATP are initially used for activation ([1], [3]). Later, two ATPs are formed per C3 fragment. Overall, therefore, there is a small net gain of 2 molecules ATP per molecules of glucose.

• Kreb’s Cycle• TCA (Tri Carboxylic

Acid) Cycle• Citric Acid Cycle

Objectives

Overview of TCA

Reaction of the TCA cycle: A) Oxidative decarboxylation of pyruvate B) The eight sequential reactions of TCA Energy produced by the cycle Regulation of the TCA cycle

• Definition:• Citric acid cycle or TCA is a final common

pathway for oxidation of active acetate derived from the catabolism of carbohydrates, fats and proteins.

• It is a cycle of chemical reactions important for complete oxidation of active acetate (acetyl~ SCoA) derived from carbohydrates, fats and proteins.

• During this process two molecules of CO2 are released, three molecules of NADH, one molecule of FADH2 and one molecule of ATP are formed.

• Site

All components are inside the mitochondrial matrix except succinate dehydrogenase which is located in inner mitochondrial membrane.

• 1-Acetyl-CoA condensed with oxaloacetate forming citrate by citrate synthase enzyme.

• 2-Citrate is converted to isocitrate in two steps of dehydration following by hydration in the presence of aconitase enzyme. Glutathione and iron are activators for the enzyme.

• 3-Isocitrate is converted to α-ketoglutarate in two steps of oxidation then decarboxylation in the presence of isocitrate dehydrogenase.

• 4-α-ketoglutarate undergoes oxidative decarboxylation to form succinyl-Co-A in the presence of α-ketoglutarate dehydrogenase complex that requires, TPP, NAD, FAD, CoA-SH, lipoic acid and Mg2+.

• 5- Succinyl-CoA is converted to succinate by succinate thiokinase, with the formation of one ATP at the substrate level.

• 6- Succinate is oxidized into fumarate by succinate dehydrogenase in the presence of FAD as a hydrogen carrier.

7- Fumarate is hydrated into malate by fumarase.

8- Finally malate is oxidized to oxaloacetate by malate dehydrogenase in the presence of NAD as hydrogen carrier.

Importance of citric acid cycle• It is final pathway for complete oxidation of active acetate

(acetyl~ SCoA) derived from carbohydrates, fats and proteins.

• It is important for interconversion between carbohydrates, fats and proteins.

• It produces energy. Oxidation of one molecule of acetyl~ SCoA, gives 12 ATP:

3 NAD → respiratory chain → 3X3 ATP = 9 ATPs

1 FAD → respiratory chain → 2 ATPs

1 ADP +Pi → at substrate level → 1 ATPs

• CO2 released is used in the following reactions:

- Carboxylation reactions

- Formation of Carbamoyl phosphate needed for urea and pyrimidine synthesis.

Regulation of TCA

• Increased acetyl-CoA and oxaloacetate activate citrate synthase.

• Increased succinyl-Co-A inhibits citrate synthase and α-ketoglutarate dehydrogenase.

• Elevated ratio of NADH/NAD and ATP/ADP inhibit citrate synthase, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.

• The cycle is inhibited under anaerobic conditions due to inhibition of respiratory chain and increased NADH/NAD.

• Elevated calcium during muscle contraction activates citrate synthase, isocitrate dehdrogenase and α-ketoglutarate dehydrogenase. To supply contracting muscles with energy.

After emerging from glycolysis, the two pyruvate molecules are transported into the mitochondria. There, the pyruvate undergo a transition stage before entering the actual citric acid cycle. In this phase the pyruvate is transformed into acetyl-coenzyme A (acetyl-CoA), the starting product in the citric acid cycle.

2 Pyruvate + 2 coenzyme A + 2NAD+ -> 2 acetyl-CoA +2CO2 + 2 NADH

Electron Transport Chain

a) It is found in all cells.

b) It is located in a mitochondrion.

c) Includes a chain of proteins integrated in the inner mitochondrial membrane.

d) This chain of proteins use the energy of electrons to create a hydrogen gradient.

e) Needs oxygen (O2) for its function.

Chain of Proteins in Electron Transport Chain

a) Belong to oxidoreductases enzymes.

b) Can transfer either H or electrons

c) Called Complex I, II, III and IV

d) Transfer protons in both directions

The figure is found at http://plaza.ufl.edu/tmullins/BCH3023/cell%20respiration.html (December 2006)

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cellular respiration pt. 5_ oxidative phosphorylation

The function of Electron Transport Chain

a) is to regenerate NAD+ from NADH

b) is to regenerate FAD from FADH2

c) is to finish oxidation of energy substrates and conserve their energy in a form of ATP

d) Oxygen is reduced to H2O

e) Protons (H+) are transfered into an intermembrane space

f) ATP is formed from ADP by addition of one phosphate

g) ATP is transported from a mitochondrion into a cytoplasm by exchange with ADP