Citric acid cycle

Krebs cycle, tricarboxylic acid cycle TCAThe central function is the oxidation of acetyl CoA to CO2- It is the final common pathway for oxidation of fuel molecules- Acetyl Co is derived from the metabolism of fuel molecules as amino acids, fatty acids, and carbohydrates. - Citric acid cycle is also an important source of precursors

•Some intermediates are precursors of amino acid• One of the intermediates is used in the synthesis of porphorins• Another is used in the synthesis of fatty acids and sterols.

- Citric Acid Cycle located in the mitochondrial matrix

Citric acid cycle is also an important source of precursors for biosynthetic

reactions

Citric acid cycle intermediates are always in flux

3NAD+ + FAD + GDP + Pi + 2H2O + acetyl-CoA

3NADH + FADH2 + GTP + CoA + 2CO2 + 3H+

Overall reaction

- Citric acid cycle contains a series of oxidation-reduction reactions

- Carbon entering the cycle, leaves fully oxidized as CO2.- “High energy” electrons leave the cycle with high energy electron carriers as NADH and FADH2.- Very little ATP is made directly in the cycle.- No oxygen is used in the cycle.

The Citric acid cycle

The citric acid cycle oxidizes two carbon units. These enter the cycle as Acetyl-CoA

Oxidative Decarboxylation of Pyruvate

- Pyruvate Dehydrogenase complex is a large multi-subunit complex located in the mitochondria - Irreversible reaction; Acetyl CoA cannot converted into pyruvate - Pyruvate dehydrogenase is not a part of citric acid cycle but it a major source of fuel for citric acid cycle which is Acetyl CoA - Pyruvate Dehydrogenase complex is aggregate of three enzymes:

1- Pyruvate dehydrogenase component called (pyruvate decarboxylase)

2- Dihydrolipoyl transacetylase3- Dihydrolipoyl dehydrogenase

Each subunit of this large complex catalyzes a part of the overall reactions.

Cofactors for the Pyruvate Dehydrogenase includeCoA-SH, NAD+, TPP, FAD, Lipolate

Oxidative Decarboxylation of Pyruvate occurs in multi-step reactions E1: Pyruvate dehydrogenase

E2: Dihydrolipoyl transacetylaseE3: Dihydrolipoyl dehydrogenase

Regulation Pyruvate Dehydrogenase complex-Product inhibition

-The enzyme complex is inhibited by Acetyl CoA when it is accumulated; the production rate is higher than the cell capacity of oxidation with citric acid cycle- High NADH/NAD+ ratio inhibits this enzyme complex

Covalent modification-Two forms of the enzyme complex;

- Active non- phosphorylated form - Inactive phosphorylated

-The two forms can be interconverted by the action of two enzymes phosphatase and kinase- The kinase is activated by an increase in the ratio of acetyl CoA/ CoA ratio or NADH/ NAD+. - elevated ADP\ATP ratio demand for energy inhibits the kinase and activate the phosphatase to produce more of the active non-phosphorylated enzymes

Regulation Pyruvate Dehydrogenase complex

Pyruvate Dehydrogenase is regulated both allosterically and by reversible phosphorylation

Pyruvate Dehydrogenase is regulated both allosterically and by reversible phosphorylation

Regulation of Pyruvate Dehydrogenase

4C

6C

6C

5C

4C

The Citric acid cycle

The Citric acid cycle

The Citric acid cycle

The Citric acid cycle

Control Points in the Citric Acid Cycle

Citric acid cycle is controlled at two points

Control Points in the Citric Acid Cycle

Single molecule of glucose can potentially yield ~38 molecules of ATP