The matrix contains Pyruvate Dehydrogenase, enzymes of ...

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The matrix contains Pyruvate Dehydrogenase, enzymes of Krebs Cycle, and other pathways, e.g., fatty acid oxidation & amino acid metabolism.

matrix

inner membrane

outer membrane

inter- membrane

space

mitochondrion

cristae

Glycolysis occurs in

the cytosol of cells.

Pyruvate enters the

mitochondrion to be

metabolized further.

Mitochondrial

Compartments:

CH 5. Citric acid cycle

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Citric acid cycle

Krebs cycle, tricarboxylic acid cycle TCA

The central function is the oxidation of acetyl CoA to CO2

- It is the final common pathway for oxidation of fuel

molecules

- Acetyl CoA 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

CH 5. Citric acid cycle

3 CH 5. Citric acid cycle

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

CH 5. Citric acid cycle

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Citric acid cycle intermediates are always in flux

CH 5. Citric acid cycle

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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

CH 5. Citric acid cycle

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The citric acid cycle oxidizes two carbon

units. These enter the cycle as Acetyl-CoA

CH 5. Citric acid cycle

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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 transacetylase

3- Dihydrolipoyl dehydrogenase

Each subunit of this large complex catalyzes a part of the overall reactions. CH 5. Citric acid cycle

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Pyruvate

Dehydrogenase: a large

complex containing many

copies of each of 3

enzymes, E1, E2, & E3.

Enzyme Abbreviated Prosthetic Group

Pyruvate

Dehydrogenase E1

Thiamine

pyrophosphate (TPP)

Dihydrolipoyl

Transacetylase E2

Lipoamide

Dihydrolipoyl

Dehydrogenase E3

FAD

Pyruvate Dehydrogenase Subunits

CH 5. Citric acid cycle

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Cofactors for the

Pyruvate

Dehydrogenase include

CoA-SH, NAD+, TPP,

FAD, Lipolate

CH 5. Citric acid cycle

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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 form

-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 CH 5. Citric acid cycle

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Regulation Pyruvate Dehydrogenase (PDH) complex

Pyruvate Dehydrogenase is regulated both allosterically and by reversible phosphorylation

CH 5. Citric acid cycle

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Pyruvate Dehydrogenase is regulated both allosterically and by reversible phosphorylation

Regulation of Pyruvate Dehydrogenase CH 5. Citric acid cycle

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CH 5. Citric acid cycle

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CH 5. Citric acid cycle

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4C

6C

6C

5C

4C

CH 5. Citric acid cycle

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The Citric acid cycle

CH 5. Citric acid cycle

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The Citric acid cycle

CH 5. Citric acid cycle

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The Citric acid cycle

CH 5. Citric acid cycle

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The Citric acid cycle

CH 5. Citric acid cycle

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Control Points in the Citric Acid Cycle

Citric acid cycle is controlled at two points

CH 5. Citric acid cycle

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Control Points in the Citric Acid Cycle

CH 5. Citric acid cycle

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

CH 5. Citric acid cycle