OLIGOMERIC ENZYMES

What are enzymes?

Enzymes are proteins which act as catalysts

Catalyst :

A catalyst is something which by its very

nature increases the rate of a reaction and

remain uncharged at the end of reaction.

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catalyst

enzyme

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Enzymes control and regulate

the various metabolic activities

inside living cells.

The definition of enzymes?

Enzymes are powerful and highly effectual

biocatalyst produced by living tissues which

increase the rate of reactions that occur in the

tissue.

What are enzymes made up of ?

Almost all enzymes are make up of proteins

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monomeric sarcosineoxidase

Monomeric enzymes

Monomeric enzymes only contains tertiary structure

trypsin

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Chymotypsin and trypsin

14Oligomeric enzyme

Oligomeric enzymes

contains two or more polypeptide chains associated

by noncovalent forces.

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Zymogen Several enzymes are produced and stored as

inactive precursors called zymogens

IsozymesEnzymes which catalyze the same reaction but have different primary and quaternary structure.

The most common isozymes are polymeric enzymes.May have similar but not identical amino acid sequences.May have common evolutionary origin.

Oligomeric enzymes consist of two or more polypeptide chains which are usually linked to each other by non covalent interactions and never by peptide bonds. The component polypeptide chains are termed sub-units and may be identical ,they are some times called protomers. Dimeric proteins consist of two,trimeric proteins of three and tetrameric proteins of four sub-units. The molecular weight is usually in excess of 35000.

The vast majority of known enzymes are oligomeric for example ,all of the enzymes involved in glycolysis process either two or four sub-units. It is therefore, reasonable to assume that the sub-units of oligomeric proteins in association that they do not have in isolation. such enzymes are not synthesized as inactive zymogens, but their activities may be regulated in a far more precise way by feed-back inhibition. This is possible because many oligomeric proteins exhibit allostery their different binding sites interact.

LACTATE DEHYDROGENSE

Vertebrate LDH is an ex of oligomeric enzyme where each subunit has the same function ,in this

case to catalise the reaction

CH3.CH.CO2־ + NAD+ CH3.C=O.CO2־

| || PYRUVATE

OH O

(S)-LACTATE + NADH + H+

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e.g. LDH(lactate dehydrogenase)

LDH 1,2,3,4,5 are HHHH, HHHM,

HHMM, HMMM and MMMM . i.e. LDH

isozymes are tetramers formed by 2

sets of subunits.

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Significance :

In the heart, LDH1 (4 H) H=heart, catalyzes Lactate convert to pyruvate for energy supply

In muscles, LDH5 ( 4M ) M=muscle, convert pyr to Lact. For energy storage

Clinical diagnosis using isozyme. E.g. when heart attack(infarction) happens, enzymes release from injured cells to the blood showed different enzyme(isozyme ) pattern.

Isozyme pattern: different isozymes appear as a peak sooner or later followed by the progress of the disease.

LACTSOSE SYNTHASE

• Mammary gland lactose synthase is an example of oligomeric enzyme where a non functional sub unit modifies the behavior of a functional sub unit. This enzyme as isolated from milk , consist of two subunits: one of these catalytically inactive protein , α-lactalbumin , found only in mammary gland; the other is N-acetyl lactosaminesynthase, an enzyme present in most tissues .

• is N-acetyl lactosaminesynthase, in the absence of α-lactalbumin, catalyses the reaction

This is important in the synthesis of the carbohydrate components of glycoprotein's, the enzyme is also produced and stored in the mammary gland during pregnancy ,when levels of α-lactalbumin are low. After the birth of the baby, reduced synthesis of the hormone progesterone in the mother leads to increased synthesis of the luteotrophic hormone (prolactin), stimulating the production of alpha lactalbumin in the mammary gland. This combines with the stored is N-acetyl lactosamine synthase to form lactose synthase an enzyme which speculates production of the lactose components lactose synthase, an enzyme with facilitates the production of the lactose components of the milk required for the newborn baby. Lactose synthase catalyses the reaction:

UDP-galactose + glucose UDP + lactose

TRYPTOPHAN SYNTHASE• Tryptophan synthase of E.coli is an ex of oligomeric

enzyme which contains two different functional sub- units the enzyme catalyses the reaction

• Indole-3-glyserolphasphate+L-serine

• L-tryptophan+glyceraldehyde-3-phosphate• It can be dissociated into two or further units, each of

molecular weight 29000 and a beta-2 subunit, of molecular weight 90000, the beta-2 subunit further dissociates the presence of 4 M urea to give two beta subunits, each of which has a binding site for the co-enzyme pyrodoxal phosphate.

• The isolated alpha-subunit will catalyze the reaction:• Indole-3-glyserolphasphate indole + glyceraldehyde-3-

phosphate.• The isolated beta-subunit also has catalytic activity but

for the reaction:• Indole + L-serine L- tryptophan.

Pyridoxal phosphate

Pyridoxal phosphate

PYRUVATE DEHYDROGENASE

• The same type of organization as tryptophan synthase, but on an even larger scale. The Enzyme Commission recommended that such a complex should be regarded as a system of separate enzymes rather than as single enzyme.

• Pyruvate dehydrogenase enables pyruvate to enter the TCA Cycle by, catalyzing its overall conversion to acetyl-coA:

• Pyruvate + Co ASH + NAD+ acetyl– CoA +

• CO2 + NADH.

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several factors contribute to enzyme catalysis

2. Electrostatic effects

3. Acid-base catalysis

4. Covalent Catalysis

1. Proximity effects and orientation arrange:

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