Enzymes

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What is an enzyme? A biological catalyst that promotes and

speeds up a chemical reaction without itself being altered in the process.

Lowers the activation energies of a substance

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Properties of enzymes1) Enzymes are proteins, thus activity easily affected by

temperature & pH.2) Enzymes increases the rate of reaction by lowering the

activation energy barrier, thus allowing reactions to proceed without an input of energy.

3) Mechanism: forms enzyme-substrate complex to facilitate their interaction and reaction.


4) An enzyme only changes the rate at which equilibrium is reached; it does not affect the position of the equilibrium.

5) A small quantity of the enzyme is needed for the reaction because they are unchanged at the end of the reaction.

6) Enzymes catalyze reactions which are specify to themselves.


Structure of Enzymes Enzymes are Proteins that:

Increase the rate of reaction by lowering the energy of activation.

Catalyze nearly all the chemical reaction taking place in the cells of the body.

Have a unique three dimensional shapes that fit the shapes of reactants.


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”


An enzyme controlled pathway

Enzyme controlled reactions proceed 108 to 1011 times faster than corresponding non- enzymic reactions.



Specificity All enzymes operate only on specific substrate: some enzymes will act only on one particular substrate; others act on similar molecules; many will break a particular linkage, e.g. hydrogen

bonds

Properties of enzymes


Reversibility Enzymes do not alter the equilibrium of a reaction

but the speed at which it is reached, e.g. carbonic anhydrase

CO2 + H2O ———— H2CO3

————

carbonic anhydras

e


Factors affecting Enzymes

Substrate concentration pH Température Inhibitors



Substrate concentration: Enzymic reactions

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

If you alter the concentration of the enzyme then Vmax will change too.

Reaction velocity

Substrate concentration

Vmax



The effect of pH Optimum pH values

Enzyme activity Trypsin

Pepsin

pH

1 3 5 7 9 11



The effect of pH Extreme pH levels will produce denaturation.

The structure of the enzyme is changed

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

At pH values slightly different from the enzyme’s optimum value, small changes in the charges of the enzyme and it’s substrate molecules will occur

This change in ionisation will affect the binding of the substrate with the active site.



The effect of temperature For most enzymes the

optimum temperature is about 30°C

Many are a lot lower; cold water fish will die at 30°C because their enzymes denature

A few bacteria have enzymes that can withstand very high temperatures up to 100°C

Most enzymes however are fully denatured at 70°C


Temperature / °C

Enzyme activity

0 10 20 30 40 50

Temperature coefficient

Denaturation


Irreversible inhibitors: Combine with the functional groups of the amino acids in the active site, irreversibly.

Examples: nerve gases and pesticides, containing organ phosphorus, combine with serine residues in the enzyme acetylcholine esterase.


The effect of enzyme inhibition

Reversible inhibitors: These can be washed out of the solution of enzyme by dialysis.

There are two categories.Competitive:

These compete with the substrate molecules for the active site.

The inhibitor’s action is proportional to its concentration.

Resembles the substrate’s structure closely.


Succinate Fumarate + 2H++ 2e-

Succinate dehydrogenase

CH2COOH

CH2COOH CHCOOH

CHCOOH

COOH

COOH

CH2

Malonate


Non-competitive: These are not influenced by the concentration of the substrate. It inhibits by binding irreversibly to the enzyme but not at the active site.

Examples : Cyanide combines with the Iron in the enzymes cytochrome

oxidase. Heavy metals, Ag or Hg, combine with –SH groups. These can be removed by using a chelating agent such as

EDTA.


Enzymatic ReactionsEnzyme combines with a specific substrate to a form an enzyme-substrate complex in a lock and key concept before forming new products.


enzyme

substrateproducts


Applications in food industry Carbohydrases: Production of corn syrups from starch

(glucoamylase); conversion of cereal starches into fermentable sugars in malting, brewing, distillery, baking industry (amylase).

Proteases: Meat tenderizers (bromelin, papain, ficin) Lipases: Flavor production in chocolate and cheese Glucose Oxidase: Desugaring of eggs, flour and potatoes;

preparation of salad dressings. Pectinases: Clarification of fruit juices; increase of yield of juice

from grapes and other products; removal of excess pectin from juices before concentration.

Lipoxygenase: Bleaching of flours. Phosphatase: Quality testing of food products. Phenol oxidase: Imparts the characteristic. dark hue to tea,

cocoa, coffee and raisins. Renin (chymosin): Cheese production.



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Date post:	15-Nov-2014
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Enzymes

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