MULTIPLE ENZYME COMPLEXES

Dr. Tijani A. S. Learning objectives

This topic exposes the students to:

 Multienzyme complexes and where they are found

 Give examples of multienzyme complexes

 Composition of some multienzyme complexes

 Their mechanisms of actions Multienzyme Complex In a number of metabolic pathways, several enzymes which catalyze different stages of the process have been found to be associated non-covalently, giving a multienzyme complex.

 The proximity of the different types of enzymes increases the efficiency of the pathway; The overall reaction rate is increased with respect to catalysis by unassociated units, and Side reactions are minimized.

 In some cases molecular mechanisms have been identified for the transfer of metabolites from one enzyme to the next within the complex.

 Multienzyme complex is the structural and functional entity that is formed by the association of several different enzymes which catalyze a sequence of closely related reactions.

 A multi enzyme complex is a protein possessing more than one catalytic domain contributed by distinct parts of a polypeptide chain or by distinct subunits.

 The regulation of this enzyme complex illustrates how a combination of covalent modification and allosteric regulation results in specific regulated flux through a metabolic step. Multienzyme Complex Examples include:

(1) Pyruvate dehydrogenase complex (PDHC)

(2) Pyruvate carboxylase

(3) Fatty acid synthase Pyruvate Dehydrogenase Complex (PDHC)

This multienzyme complex contains:  3 enzyme subunits and 5 coenzymes and other proteins.

 The pyruvate dehydrogenase complex catalyzes the oxidative decarboxylation of pyruvate to acetyl CoA. It is an organized assembly of 3 different catalytic subunits of this complex enzyme. The reaction catalyzed is summarized thus,

Pyruvate + CoASH + NAD+ → CO₂ + Acetyl CoA + NADH + H+

 It has coenzymes namely, CoA, lipoamide, NAD, thiamine PPO₄ and FAD. Composition of Pyruvate Dehydrogenase Complex (PDHC)

Table showing structural composition of PDHC from E. coli ENZYME ABBREVIATION NO OF COENZYME REACTION CHAINS CATALYSED

Pyruvate Dehydrogenase E1 24 Thiamine pyrophosphate (TPP) Decarboxylation of pyruvate (provides a carbanion for Component nucleophilic attack on the substrate)

Dihydrolipoyl Transacetylase E2 24 Lipoamide, Coenzyme A Oxidation of carbon 2 units (CoASH) (transfers substrate to coenzyme A, retains

hydrogen)

Dihydrolipoyl Dehydrogenase E3 12 Flavin adenine dinucleotide Regeneration of the oxidized (FAD), Nicotinamide adenine form of Lipoamide + dinucleotide (NAD ) (transfers H2 from lipoamide to NAD+) Mechanism of Action of Pyruvate Dehydrogenase Complex

There are four steps in the conversion of pyruvate to acetyl CoA, which are catalyzed by three different catalytic subunits of the enzymes:

1. Decarboxylation of pyruvate after its combination with TPP

(thiamine pyrophosphate) in a reaction catalyzed by E1 (pyruvate dehydrogenase)

2. Two steps of the conversion are catalyzed by E2 (dihydrolipoyl transacetylase) enzyme:

 oxidation of the 2-carbon (acetyl) unit, which is transferred to the lipoamide prosthetic group of the enzyme, giving an acetyllipoamide group  transfer of the acetyl group from the lipoamide to CoA, giving acetyl CoA

3. Finally, E3 (dihydrolipoyl dehydrogenase) regenerates the oxidized form of lipoamide. This involves the FAD prosthetic group. That is dihydrolipoyl + dehydrogenase transfers the H2 that was retained in the preceding steps to NAD

Mechanism of Action of Pyruvate Dehydrogenase Complex

Fatty Acid Synthase Mechanism of Action of Fatty Acid Synthase Complex References

 Biochemical pathways. An atlas of biochemistry and molecular biology. 2nd Edition. John Wiley J. & Sons, Inc. 2012 by Michal G. and Schomburg D.

 Principles of Biochemistry. 5th Edition. Pearson, 2012 by Moran L.A., Horton H.R., Scrimgeour K.G., Perry M.D.

 Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012 by Nelson D.L., Cox M.M. Lehninger.

 . Harper’s illustrated biochemistry. 30th Edition. McGraw-Hill Education, 2015 by Rodwell V.W., Bender D.A., Botham K.M., Kennelly P.J., Weil P.A

 Biochemistry. 4th Edition. John Wiley J. & Sons, Inc. 2011 by Voet D. and Voet J.D.

 Molecular and cellular enzymology. Springer, 2009 by Yon-Kahn J., Hervé G.

 Notes on multienzyme complex by Ashwathi P

 Pyruvate dehydrogenase complex by Christine Hrycyna

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