Biochemistry

The Citric Acid Cycle

Aerobic cells use a metabolic wheel – the citric acid cycle – to generate energy by acetyl CoA oxidation

Glucose

Glucose-6-phosphate
Pyruvate
Glycogen
Ribose, NADPH
Pentose phosphate pathway
Synthesis of glycogen
Degradation of glycogen
Glycolysis
Gluconeogenesis
Lactate
Ethanol
Acetyl Co A
Fatty Acids
Amino Acids
The citric acid cycle is the final common pathway for the oxidation of fuel molecules — amino acids, fatty acids, and carbohydrates. Most fuel molecules enter the cycle as acetyl coenzyme A.

Names: The Citric Acid Cycle Tricarboxylic Acid Cycle Krebs Cycle

In eukaryotes the reactions of the citric acid cycle take place inside mitochondria
Hans Adolf Krebs. Biochemist; born in Germany. Worked in Britain. His discovery in 1937 of the ‘Krebs cycle’ of chemical reactions was critical to the understanding of cell metabolism and earned him the 1953 Nobel Prize for Physiology or Medicine.

1. Citrate Synthase

Citrate formed from acetyl CoA and oxaloacetate Only cycle reaction with C-C bond formation Addition of C2 unit (acetyl) to the keto double bond of C4 acid, oxaloacetate, to produce C6 compound, citrate
citrate synthase

2. Aconitase

Elimination of H2O from citrate to form C=C bond of cis-aconitate Stereospecific addition of H2O to cis-aconitate to form isocitrate
aconitase
aconitase

3. Isocitrate Dehydrogenase

Oxidative decarboxylation of isocitrate toa-ketoglutarate (a metabolically irreversible reaction) One of four oxidation-reduction reactions of the cycle Hydride ion from the C-2 of isocitrate is transferred to NAD+ to form NADH Oxalosuccinate is decarboxylated to a-ketoglutarate
isocitrate dehydrogenase
isocitrate dehydrogenase

4. The -Ketoglutarate Dehydrogenase Complex Similar to pyruvate dehydrogenase complexSame coenzymes, identical mechanismsE1 - a-ketoglutarate dehydrogenase (with TPP) E2 – dihydrolipoyl succinyltransferase (with flexible lipoamide prosthetic group) E3 - dihydrolipoyl dehydrogenase (with FAD) a-ketoglutarate dehydrogenase

5. Succinyl-CoA Synthetase

Free energy in thioester bond of succinyl CoA is conserved as GTP or ATP in higher animals (or ATP in plants, some bacteria) Substrate level phosphorylation reaction
HS-
+
GTP + ADP GDP + ATP
Succinyl-CoA Synthetase


Complex of several polypeptides, an FAD prosthetic group and iron-sulfur clusters Embedded in the inner mitochondrial membrane Electrons are transferred from succinate to FAD and then to ubiquinone (Q) in electron transport chain Dehydrogenation is stereospecific; only the trans isomer is formed
6. The Succinate Dehydrogenase Complex
Succinate Dehydrogenase

7. Fumarase

Stereospecific trans addition of water to the double bond of fumarate to form L-malate Only the L isomer of malate is formed
Fumarase

8. Malate Dehydrogenase

Malate Dehydrogenase
Malate is oxidized to form oxaloacetate.


Stoichiometry of the Citric Acid Cycle
Two carbon atoms enter the cycle in the form of acetyl CoA. Two carbon atoms leave the cycle in the form of CO2 . Four pairs of hydrogen atoms leave the cycle in four oxidation reactions (three molecules of NAD+ one molecule of FAD are reduced). One molecule of GTP,is formed. Two molecules of water are consumed.
9 ATP (2.5 ATP per NADH, and 1.5 ATP per FADH2) are produced during oxidative phosphorylation. 1 ATP is directly formed in the citric acid cycle. 1 acetyl CoA generates approximately 10 molecules of ATP.

Integration of metabolism. The citric acid cycle is amphibolic (both catabolic and anabolic).
Functions of the Citric Acid Cycle

The cycle is involved in the aerobic catabolism of carbohydrates, lipids and amino acids.

Intermediates of the cycle are starting points for many anabolic reactions.
Yields energy in the form of GTP (ATP). Yields reducing power in the form of NADH2 and FADH2.

Regulation of the Citric Acid Cycle

Pathway controlled by: (1) Allosteric modulators (2) Covalent modification of cycle enzymes (3) Supply of acetyl CoA (pyruvate dehydrogenase complex)
Three enzymes have regulatory properties citrate synthase (is allosterically inhibited by NADH, ATP, succinyl CoA, citrate – feedback inhibition) isocitrate dehydrogenase (allosteric effectors: (+) ADP; (-) NADH, ATP. Bacterial ICDH can be covalently modified by kinase/phosphatase)-ketoglutarate dehydrogenase complex (inhibition by ATP, succinyl CoA and NADH

NADH, ATP, succinyl CoA, citrate

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Regulation of the citric acid cycle