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Multi Enzyme Complex

Sudhanshu Shekhar

M.Tech (Biotech) IIIrd Sem

A7110709009

Introduction: A multienzyme complex is a protein possessing more than one catalytic domain contributed by distinct parts of a polypeptide chain ('domains'), or by distinct subunits, or both.

i.e. a single enzyme is made up of one polypeptide chain, but when these polypeptide chain acts as a distinct part, and all act as a different enzyme having distinct catalytic domain, it is called multienzyme complex.

Examples• Eukaryotes:

1. Pyruvate Dehydrogenase Complex

2. Cytochrome p450 enzyme: metabolic intermediates such as lipids and steroidal hormones, as well as xenobiotic substances such as drugs and other toxic chemicals. CYPs are the major enzymes involved in drug metabolism and bioactivation, accounting for 75% of the total metabolism.∼

3. Fatty acid synthase: Fatty acid synthase (FAS) is a multi-enzyme that plays a key role in fatty acid synthesis.

• Prokaryotes:

The Tryptophan Synthase: Tryptophan synthase is commonly found in Eubacteria, Archaebacteria, Protista, Fungi, and Plantae. Ex-Salmonella typhimurium

Tryptophan synthase was the first enzyme identified that had two catalytic capabilities.

• The bifunctional tryptophan synthase α2β2 complex that catalyzes

the final two reactions in tryptophan biosynthesis is a classic example of a multienzyme complex that "channels" a metabolic intermediate (indole) between two active sites.

Pyruvate Dehydrogenase Complex

Glycolysis occurs in the cytosol of cells. Here, one molecule of glucose (6c) is oxidized into 2x3c pyruvate. So, Pyruvate enters the mitochondrion to be metabolized further.

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

Location: Mitochondrial matrix of eukaryotic cells and in the Cytosol of prokaryotes.

Pyruvate formed in the cytosol is transported into the mitochondria by a proton symporter.

The inner core of mammalian Pyruvate Dehydrogenase is an icosahedral structure consisting of 60 copies of E2.

At the periphery of the complex are:

• 30 copies of E1 (a tetramer with subunits a2b2).

• 12 copies of E3 (a homodimer), plus 12 copies of an E3 binding protein that links E3 to E2.

Pyruvate Dehydrogenase: a large complex containing many copies of each of 3 enzymes, E1, E2, & E3.

A regular  polyhedron with 20 identical equilateral triangular faces, 30 edges and 12 vertices.

The icosahedral structure of pyruvate dehydrogenase complex

Pyruvate dehydrogenase multienzyme complex is the gateway to the TCA cycle, producing acetyl CoA, by a process called oxidative decarboxylation. It is a key regulatory enzyme that directly modulates the rate of glucose oxidation.

It is an irreversible oxidation process in which carboxyl group is removed from pyruvate as a molecule of CO2 and the two remaining carbons becomes the acetyl group of acetyl CoA.

Pyruvate dehydrogenase complex (E1+E2+E3)

Metabolic sources and fates of Acetyl Co A

Pyruvate dehydrogenase complex is a noncovalent assembly of three enzymes and five coenzymes.

E1 pyruvate dehydrogenase (24)

Es E2 dihydrolipoyl transacetylase (24)

E3 dihydrolipoyl dehydrogenase (12)

Thiamine pyrophosphate, TPP

HSCoA (pantothenic acid)

Cofactors Lipoic Acid

NAD+

FAD

Table 1: Cofactors for each component of the PDH complex with the cofactor category and vitamin source

Sub-unit

Cofactor Type Vitamin Source

E1 Thiamine pyrophosphate

Prosthetic Thiamine

E2 Coenzyme A

Lipoamide (lipoic acid + lys residue)

Co-substrate (Co-enzyme)

Prosthetic

Pantothenic acid

None - dietary

E3 FAD

NAD+

Prosthetic

Co-substrate

Riboflavin

Niacin

HSCoA

NAD+

Pyruvate dehydrogenase complex:

CO2

CoASHNAD+

NADH+H+

Hydroxyethyl TPPPyruvate

1

Acetyl lipoamide

3

Dihydrolipoamide

Acetyl Co A

2

2

3

1. Pyruvate dehydrogenase 2. Dihydrolipoyl transacetylase3. Dihydrolipoyl dehydrogenase

Figure: The reaction mechanism of Pyruvate Dehydrogenase Complex

Regulation:Pyruvate dehydrogenase complex

• This Irreversible reaction is tightly controlled by three ways:

• Allosteric Inhibition Inhibited by products: acetyl-CoA and NADH Inhibited by high ATP

• Allosteric activation by AMP Ratio ATP/AMP important

• Covalent modification (hormonal regulation) Through Phosphorylation/dephosphorylation of E1 subunit PDH exists in two forms:

Phosphorylated (inactive): Protein kinase enzyme converts

active into inactive enzyme active into inactive enzyme.

Dephosphorylated (active):Phosphatase enzyme converts

inactive into active.

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