CITRIC ACID CYCLE

IntroductionSitePathwayEnergeticsRegulationClinical importance

Glycolysis

Link reaction

Kreb’s cycle

Oxidative Phosphorylation (ETC)

INTRODUCTION

Citric acid cycle

Krebs cycle (1937; NP-1953)

Tricarboxylic acid cycle

Reduced co-enzymes are produced – act as substrate for for ETC

Most important ATP producing pathway

About 65-70% ATP synthesized Sir Hans Krebs [1900-

1981] – on his bike!

Takes place in the matrix of the mitochondria.

It happens once for every pyruvate molecule in glycolysis….

Purpose

• Conversion of Acetyl-CoA to

CO2

• Generates reducing equivalents (NADH + H+, FADH2) & GTP to be oxidized in the respiratory chain to generate ATP

OVERVIEW

STEPS INVOLVED IN TCA CYCLE

Succinate thiokinase

Let’s have a short break

TIPS TO LEARN BIOCHEMISTRY

• Do not rely on passive reading and highlighting/underlining of the textbook.

• Do not sit and stare at the handouts

• Do not try to read 50 review books. (Make your own review book instead!)

• Do focus on identifying key concepts

• Do actively draw and redraw pathways and connections

• Do learn to identify relevant information

Do prioritize:

• What is the purpose of a pathway?

• What are the starting and ending molecules?

• Where is the pathway (in the cell, in a tissue, in an organ system)?

• How does the pathway connect to other pathways?

• What metabolic conditions turn the pathway on and off?

• What are the control points for regulating the pathway?

• reactants, products and enzyme name of each regulatory step

• additional regulatory molecules involved (vitamins, cofactors)

• make sure you know every step that makes or uses ATP

• What specific drugs or diseases associated with the pathway?

TCA cycle is an open cycle

Operates only under aerobic conditions

This is the Final common pathway of oxidative metabolism

Two carbon dioxide molecules are released as a waste product of respiration

GTP+ H+

Energetics : 2 Acetyl CoA from 2 Pyruvate

1NADH+H+ = 3/2.5 ATP

1FADH2 = 2/1.5 ATP

1GTP = 1 ATP

Acetyl-CoA + 3 NAD+ + [FAD] + GDP + Pi + 2 H2O CoASH + 3 NADH+3 H + +[FADH2] + GTP + 2 CO2

×2=24

ATP generation during oxidation of Glucose

Net ATP production depends on shuttle used for the transfer of

reducing equivalents from cytosol to mitochondria.

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SIGNIFICANCE OF TCA CYCLE:

1. Complete oxidation of Acetyl CoA

2. As provider of energy

3. Final common oxidative pathway

4. Integration of major metabolic pathways

5. Fat is burned on the wick of carbohydrates

6. Excess carbohydrates are converted to Neutral fat

7. No net synthesis of carbohydrates from fat

8. Amino acids enters TCA cycle

9. Amphibolic pathway

10. Anaplerotic role

1.Complete oxidation of Acetyl CoA

2. ATP generation

12/10 ATP are generated via substrate level phosphorylation and ETC

Oxaloacetate

Citrate

Isocitrate

α-ketogluterate

3. Final common oxidative pathway

4. Integration of major metabolic pathways

5.Fat is burned on the wick of carbohydrates

For complete oxidation of acetyl CoA , it needs the help of oxaloacetate

The major source of oxaloacetate is pyruvate, derived from carbohydrate

6. Excess carbohydrates are converted to Neutral fat

Glucose Pyruvate Acetyl CoA Fatty Acid

PDH is irreversible – fat can not be converted to glucose.

Participation of TCA cycle in fatty acid synthesis from glucose

7.No net synthesis of carbohydrates from fat

• Acetyl CoA entering in the cycle is completely oxidized to

CO2 by the time the cycle reaches Succinyl CoA.

• So acetyl CoA can not be used for the synthesis of

carbohydrates.

8. Amino acids finally enters the TCA cycle

9. Amphibolic Pathway

• Both anabolic and catabolic

• Metabolic traffic cycle

10. Anaplerotic role of TCA cycle

“Filling up” reactions or “influx” reactions or

“replenishing” reactions

• Pyruvate Oxaloacetate

• Glutamate α-KG

• Aspartate Oxaloacetate

• Pyruvate Malate

Regulation of TCA cycle

Indicator molecules of higher energy state i.e. ATP, NADH, citrate, Acetyl CoA – inhibit TCA cycle

Indicator molecules of low energy state i.e. ADP, AMP, NAD+ – stimulate TCA cycle

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INHIBITORS OF TCA CYCLE

1. Fluoroacetate

Condensation FluoroacetylCoA with Oxaloacetate

Fluorocitrate inhibit Aconitase enzyme accumulation

of citrate

Fluoroacetate pesticide

2. Malonate Succinate dehydrogenase enzyme

3. Arsenite α-ketoglutarate dehydrogenase enzyme

METABOLIC DEFECTS

• Extremely rare1. Defect in PDH - Lactic acidosis - Neurologycal dosorders

2. Defect In Pyruvate carboxylase - Oxaloacetate - Hyperammonemia - Lactic acidosis - Hyperalaninemia.

SUMMARY

• Pyruvate is converted to acetyl-CoA by the action of p yruvate dehydrogenase complex, a huge enzyme complex.

• Acetyl-CoA is converted to 2 CO2 via the eight-step citric acid cycle, generating three NADH, one FADH2, and one ATP (by

substrate-level phophorylation).

• Intermediates of citric acid cycle are also used as biosynthetic precursors for many other biomolecules, including fatty acids, steroids, amino acids, heme, pyrimidines, and glucose.

• Oxaloacetate can get replenished from pyruvate, via a carboxylation reaction catalyzed by the biotin-containing pyruvate carboxylase.

QUESTIONS?

• TCA cycle is the final common metabolic pathway for the oxidation of food stuffs. – Justify

• Conversion of Pyruvate to Acetyl CoA

• Energetics of citric acid cycle

THANK YOU