KREB’S CYCLEKREB’S CYCLE

Kreb’s cycle orKreb’s cycle or

Citric Acid Cycle orCitric Acid Cycle or

Tricarboxylic Acid Cycle Tricarboxylic Acid Cycle (TCA cycle) (TCA cycle)

The Aerobic Breakdown of The Aerobic Breakdown of Carbohydrates Carbohydrates

Glycolysis is Glycolysis is inhibitedinhibited by aerobic conditions by aerobic conditions i.e. the i.e. the presence of oxygenpresence of oxygen, mainly due to 2 , mainly due to 2 reasons:reasons:

(1)-(1)- The presence of oxygen The presence of oxygen inhibitsinhibits the the dehydrogenationdehydrogenation of pyruvic acid to lactic of pyruvic acid to lactic

acid.acid. (2)-(2)- The pyruvic acid produced in glycolysis is The pyruvic acid produced in glycolysis is

rapidly oxidizedrapidly oxidized as it is produced. as it is produced. The aerobic breakdown involves the The aerobic breakdown involves the same same

sequencesequence of reactions as in of reactions as in glycolysisglycolysis upto the upto the formation of formation of pyruvic acidpyruvic acid. .

Entry of pyruvic acid into Entry of pyruvic acid into the TCA cyclethe TCA cycle

The The pyruvic acidpyruvic acid is then is then oxidizedoxidized to to COCO22 & H & H22O in the O in the presence of presence of oxygenoxygen through through Kreb’s cycleKreb’s cycle. .

For entry into the TCA cycle, it has For entry into the TCA cycle, it has to convert to to convert to Acetyl CoAAcetyl CoA..

Oxidation of pyruvate to Acetyl CoA Oxidation of pyruvate to Acetyl CoA is catalyzed by the is catalyzed by the Pyruvate Pyruvate dehydrogenasedehydrogenase in the mitochondria. in the mitochondria.

NADNAD NADHNADH22

Pyruvic acid Pyruvic acid acetyl – acetyl – SCo – ASCo – A

SHCoA COSHCoA CO22

CH3-CO-COOH + CoASH + NAD CH3-CO-SCoA + CO2 + NADH

*

Oxaloacetate Acetyl CoA

Citric acid Cis-aconitic acid

Isocitric acidOxalosuccinate α-Ketoglutarate

Succinyl-CoA Succinate Fumarate

Malate Oxaloacetate

Citrate synthase Aconitase , Fe

AconitaseFe

Dehydrogenase

α-ketoglutarte dehydrogenase

Succinate thiokinase Succinate dehydrogenase

Fumarase

Malate dehydrogenase

H2O CoA-SH H2O

++

++

+H2O

NADH+H NAD

Isocitrate dehydrogenase

CO2

NAD

NADH+H

CoA-SH

CO2

ADP+Pi ATP CoA-SH FAD FADH2

H2O

NADH2 NAD

(1) (2)

(3)

(4)(5)

(6)

(7) (8)

(9)

(10)

*

Step-wise reactions of the TCA cycle

1)- Aldol condensation of oxaloacetate with acetyl CoA followed by hydrolysis of CoASH to give citrate. Enzyme: citrate synthase.

2,3)- Dehydration followed by hydration leads to interchange of -H and –OH & isomerization of citrate to isocitrate.

Enzyme aconitase

4,5)Oxidative decarboxylation of isocitrate is catalyzed by isocitrate dehydrogenase to form α-keto-glutarate - NAD is reduced to NADH & one C is removed as CO2. 6) This is the second oxidative decarboxylaion

catalyzed by αα-ketoglutarate dehydrogenase. CO2 is removed, NADH is formed from NAD & a thioester bond is formed with CoASH to form succinyl CoA.

7) Conversion of thioester of Succinyl CoA to Succinate by succinate thiokinase.

8) Succinate is oxidized to fumarate by succinate dehydrogenase. A double bond is introduced between the 2 central carbons & FAD is reduced to FADH2.

9) Fumarate is hydrated to malate by the enzyme fumarase.

10) In the final step, malate is oxidized to oxaloacetate with the coupled reduction of NAD to NADH by the enzyme malate dehydrogenase.

ENERGATICS OF TCA ENERGATICS OF TCA cycle. cycle.

As a result of oxidation by dehydrogenases – 4 molecules of NADH & 1 of FADH2 are produced for each molecule of acetyl-CoA.

Reoxidation of each NADH results in formation of 3 ATP, FADH2 - 2 ATP.

In addition, 1 ATP is formed by substrate-level phosphorylation by succinate thiokinase.

ATP gained = 15 for one molecule of pyruvic acid (one cycle).

Total ATPs for two molecule of pyruvic acid = 15 * 2 = 30

According to old concept ATP from glycolysis = 8 ATP from Kreb’s cycles = 30 Total --------- 38

According to New concept ATP from glycolysis = 6 ATP from Kreb’s cycle = 30 Total --------------- = 36

Significance of TCA cycleSignificance of TCA cycle Several intermediates of this cycle take part

in the formation of substances of great physiological importance.

1) – Citrate is provided to the lens of the eye, bone & seminal fluid. Citrate stimulate fatty acid synthesis by activating acetyl-CoA carboxylase.

2) – Succinyl-CoA is utilized in the synthesis of porphyrins needed for Hb formation.

3) – Acetyl-CoA is used in the synthesis of cholesterol, fatty acids, acetylcholine etc.

4) – αα-Ketoglutarate & Oxaloacetate can undergo transamination forming glutamic acid & aspartic acid (amino acids).

Mitochondria = consist of inner & outer membrane & Mitochondria = consist of inner & outer membrane & matrix, PDH complex & TCA cycle in matrix (except matrix, PDH complex & TCA cycle in matrix (except succinate dehydrogenase). succinate dehydrogenase).

FAD = Flavin Adenine Dinucleotide – oxidized form.FAD = Flavin Adenine Dinucleotide – oxidized form. FADH2 = Flavin Adenine Dinucleotide – reduced form.FADH2 = Flavin Adenine Dinucleotide – reduced form. NAD = Nicotinamide Adenine Dinucleotide – oxidized form.NAD = Nicotinamide Adenine Dinucleotide – oxidized form. NADH + H = Nicotinamid Adenine Dinucleotide – reduced NADH + H = Nicotinamid Adenine Dinucleotide – reduced

form.form. NADP = Nicotinamide Adenine Dinucleotide Phosphate – NADP = Nicotinamide Adenine Dinucleotide Phosphate –

oxidized form.oxidized form. NADPH = Nicotinamide Adenine Dinucleotide Phosphate – NADPH = Nicotinamide Adenine Dinucleotide Phosphate –

reduced form.reduced form.

Harper’s , Mushtaq Ahmed volume I, and Hashmi’s . Harper’s , Mushtaq Ahmed volume I, and Hashmi’s .