lecture 9 enzymes

8/8/2019 Lecture 9 Enzymes

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ENZYMESA protein with catalytic properties due to its

power of specific activation


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Enzymes speed up chemical reactions


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ENZYMES In the test tube, catalysts such as charcoal and

platinum facilitate reactions but usually only

at high temperatures or pressures, at extremesof high or low pH, or in organic solvents.

As the cells protein catalysts, however,

enzymes must function effectively in aqueousenvironment at 37C, 1 atmosphere pressure,

and pH 6.57.5.


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ENZYMES Two striking properties

1. Enormous Catalytic Power

rates of enzymatically catalyzed reactions to be 1061012 times that of thecorresponding uncatalyzed reactions under otherwise similar conditions.

2. Specificityenzyme-catalyzed reactions of L-amino acids take place much more rapidly than

do those of D-amino acids, even though both stereoisomers of a given amino acidare the same size and possess the same R groups

Around 3700 different enzymes in enzyme database.

Some common (like protein, nucleic acid, phospholipid synthesis)

Some specific (like for conversion of tyrosine to dopamine (a neurotransmitter) in

nerve cells)


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Chemical reactions Every chemical reaction between molecules

- involves bond breaking and bond forming

The initial energy needed to start a chemical reaction

-called the free energy of activation (Activation

energy (AE)

During this part of the reaction the molecules are said to

be in a transition state.

Activation energy is often supplied in the form of heat

from the surroundings


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Making reactions go faster Increasing the temperature make molecules move faster

Biological systems are very sensitive to temperature

changes. Enzymes can increase the rate of reactions without

increasing the temperature.

They do this by lowering the activation energy.

They create a new reaction pathway a short cut

Enzymes do not affect the change in free energy (G),

instead they hasten reactions that would occur eventually

(thus would catalyze only energy favourable reactions)


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Reaction Pathway


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As the cells protein catalysts, however, enzymes must function

effectively in aqueous environment at 37C, 1 atmosphere pressure, and

pH 6.57.5.

An enzyme controlled pathway


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Enzyme structure Enzymes are

proteins

They have aglobular shape

A complex 3-Dstructure

Human pancreatic amylase


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The active site The region on the

enzyme where substrate

binds is called the active

site.

The shape and the

chemical environment

inside the active site

permits a chemicalreaction to proceed more

easily


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ACTIVE SITE Certain amino acid side chains of an enzyme are important in determining its

specificity and catalytic power. In the native conformation of an enzyme,

these side chains are brought into proximity, forming the active site.

Active sites thus consist of two functionally important regions:

1. Substrate recognition and binding

2. catalytic the one that catalyzes reaction once substrate is bound.

In some enzymes, the catalytic region is part of the substrate-binding region;

in others, the two regions are structurally as well as functionally distinct.


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Cofactors Non-protein enzyme

helpers

Organic cofactors can be1. Tightly bound cofactors are

called prosthetic groups

2. Cofactors that are bound andreleased easily are called

coenzymes (NADH,NADPH, ATP)

Many vitamins arecoenzymes

Vitamin C (Ascorbic Acid-Scurvy)

Nitrogenase enzyme with Fe, Mo and ADP cofactors)

Cytochroma C with

heme coenzyme

)


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The substrate The substrate of an enzyme are the reactants

that are activated by the enzyme

Enzymes are specific to their substrates

The specificity is determined by the active

site


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The Lock and Key Hypothesis Fit between the substrate and the active site of the enzyme is

exact

Like a key fits into a lock very precisely

The key is analogous to the enzyme and the substrateanalogous to the lock.

Temporary structure called the enzyme-substrate complexformed

Products have a different shape from the substrate

Once formed, they are released from the active site

Leaving it free to become attached to another substrate


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The Lock and Key Hypothesis

Enzyme may

be used again

Enzyme-

substratecomplex

E

S

P

E

E

P

Reaction coordinate


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The Lock and Key Hypothesis This explains enzyme specificity

This explains the loss of activity when

enzymes denature


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The Induced Fit Hypothesis Some proteins can change their shape

(conformation)

When a substrate combines with an enzyme, itinduces a change in the enzymes conformation

The active site is then moulded into a preciseconformation

Making the chemical environment suitable for thereaction

The bonds of the substrate are stretched to make thereaction easier (lowers activation energy)


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The Induced Fit Hypothesis

This explains the enzymes that can react with a

range of substrates of similar types

Hexokinase (a) without (b) with glucose substrate


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Factors affecting Enzymes substrate concentration

pH

temperature

inhibitors


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Substrate concentration: Non-enzymic reactions

The increase in velocity is proportional to the

substrate concentration

Reaction

velocity

Substrate concentration


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Substrate concentration: Enzymic reactions

Faster reaction but it reaches a saturation point when all theenzyme molecules are occupied.

If you alter the concentration of the enzyme then Vmax willchange too.

Reaction

velocity

Substrate concentration

Vmax


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The effect of pH Extreme pH levels will produce denaturation

The structure of the enzyme is changed

The active site is distorted and the substratemolecules will no longer fit in it

At pH values slightly different from the enzymes

optimum value, small changes in the charges of the

enzyme and its substrate molecules will occur

This change in ionisation will affect the binding of

the substrate with the active site.


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The effect of pH


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The effect of temperature For most enzymes the optimum temperature is about

30C

Many are a lot lower, cold water fish will die at30C because their enzymes denature

A few bacteria have enzymes that can withstand very

high temperatures up to 100C

Most enzymes however are fully denatured at 70C


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The effect of temperature


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Enzymes in a Common Pathway constitute

multienzyme complexesStructure and function of pyruvate

dehydrogenase, a large multimeric enzyme complex

that converts pyruvate into acetyl CoA.

(a) The complex consists of 24 copies of pyruvatedecarboxylase (E1), 24 copies of lipoamide

transacetylase (E2), and 12 copies of dihydrolipoyl

dehydrogenase (E3). The E1 and E3 subunits are bound to

the outside of the core formed by the E2 subunits.

(b) The reactions catalyzed by the complex include

several enzyme-bound intermediates (not shown). Thetight structural integration of the three enzymes increases

the rate of the overall reaction and minimizes possible

side reactions.


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COMPARTMENTALIZATION

A large increase in the concentration of interacting molecules can be achieved by

confining them to the same membrane-bounded compartment as in a eucaryotic cell.

Cells have another way of increasing the rate of metabolic reactions.

If, for example, the compartment occupies a total of 10% of the volume of the cell,

the concentration of reactants in the compartment can be 10 times greater than in a

similar cell with no compartmentalization


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Inhibitors Inhibitors are chemicals that reduce the rate of

enzymic reactions.

The are usually specific and they work at lowconcentrations.

They block the enzyme but they do not

usually destroy it. Many drugs and poisons are inhibitors of

enzymes in the nervous system.


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Enzyme inhibitors


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Allosteric InhibitionAllosteric activation results when the

binding of an activator molecule to an

allosteric site causes a change in the active

site that makes it capable of binding

substrate.

Allosteric (noncompetitive) inhibition results

from a change in the shape of the active site

when an inhibitor binds to an allosteric site.

When this occurs the substrate cannont bindto it's active site due to the fact that the

active site has changed shape and the

substrate no longer fits.


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Feedback Inhibition


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Uses of inhibitors

Since inhibitors modulate the function of enzymes they areoften used as drugs. An common example of an inhibitor that

is used as a drug is aspirin, which inhibits the COX-1 andCOX-2 enzymes that produce the inflammation messenger

prostaglandin, thus suppressing pain and inflammation.

However, other enzyme inhibitors are poisons. For example,the poison cyanide is an irreversible enzyme inhibitor that

combines with the copper and iron in the active site of theenzyme cytochrome c oxidase and blocks cellularrespiration.[69


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Laundry Industry-

Proteases, lipasesBaking Industry-

Amylase Brewing Industry-

Amylase, glucanase, proteaseSplit polysaccharide and proteins in malt

Dairy Industry-

Rennin

Manufacture of cheese

INDUSTRIAL APPLICATIONS


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Paper Industry-

Amylases, Xylanases,

Cellulases and

ligninases

Molecular Biology

Ligases, Restriction enzymes PCR

Meat Tenderizer

Papain



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Up until now, the jeans have gone through the sameprocedure, and it is during finishing that jeans arebetrothed their own colour and personality. Formerly,jeans were sanded roughly with pumice stones (stonewashing), placing the stones and fabric in a

rotating washing machine to achieve the faded look.However, since pumice stones are volcanic rock, theyare strip-mined and therefore not very environmentallyfriendly. In addition, a small amount of enzyme can dothe same job as several kilograms of pumice stones,plus laundry machines can then contain fewer stonesand more garments, increasing productivity.

Textile Industry-

amylase, pectinase, catalase and cellulase


lecture 9 enzymes

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