ENZYMES

By - Kinjal

ENZYMES

Enzymes are large biological molecules

responsible for the thousands of metabolic processes that sustain life.

Enzymes are proteins that acts as catalyst in biochemical reaction.

catalyst- substance that increase the rate of chemical reaction

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Enzyme Activity

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Common features of enzymes

• They are catalysts so the make reactions easier. This increases productivity and yield.

• As catalysts they are not consumed by the reaction. They may be used over and over again.

• Enzymes show specificity to the reaction they control.

• Enzymes are sensitive to their environment so they can be controlled by adjusting the temperature, the pH or the substrate concentration.

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Specific features of enzymes:

1. Specificity of action

2. Sensitivity to temperature

3. Sensitivity to pH

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Enzymes active sites

•Active site – specific region in the enzyme to which substrate molecule is bound

•Substrate usually is relatively smallmolecule

•Enzyme is large protein molecule

•Therefore substrate binds to specific area on the enzyme

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Properties of Enzymes

Specificity of enzymes

1.Absolute – one enzyme acts only on one substrate(example: urease decomposes only urea; arginase splits only arginine)

2.Relative – one enzyme acts on different substrates which have the same bond type(example: pepsin splits different proteins)

3.Stereospecificity – some enzymes can catalyze the transformation only substrates which are in certain geometrical configuration, cis- or trans-

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Theories of active site-substrate interaction

Fischer theory (lock and key model)

The enzyme active site (lock) is able to accept only a specific type of substrate (key)

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Sensitivity to pH

Each enzyme has maximum activity at a particular pH(optimum pH)

For most enzymes the optimum pH is ~7 (there are exceptions)

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-Enzyme will denature above 45-50oC

-Most enzymes have temperature optimum of 37o

Each enzyme has maximum activity at a particulartemperature (optimum temperature)

Sensitivity to temperature

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ENZYMATIC REACTION PRINCIPLES

• Biochemically, enzymes are highly specific for their substrates and generally catalyze only one type of reaction at rates thousands and millions times higher than non-enzymatic reactions.

• Two main principles to remember about enzymes are

1) they act as CATALYSTS (they are not consumed in a reaction and are regenerated to their starting state).

2) they INCREASE THE RATE of a reaction towards equilibrium (ratio of substrate to product), but they do not determine the overall equilibrium of a reaction.

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Chemical reactions

• Chemical reactions need an initial input of energy = The activation energy

• During this part of the reaction the molecules are said to be in a transition state.

• For a reaction to proceed from starting material to product, the chemical transformations of bond-making and bond-breaking require a minimum threshold amount of energy, termed activation energy.

• The energy maxima at which the reaction has the potential for going in either direction is termed the transition state.

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Reaction pathway

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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”

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An enzyme controlled pathway

• Enzyme controlled reactions proceed 108 to 1011 times faster than corresponding non-enzymic reactions. 15

Inhibitors

• Inhibitors are chemicals that reduce the rate of enzymic reactions.

• The are usually specific and they work at low concentrations.

• 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 inhibition

In a tissue and cell different chemical agents (metabolites, substrate analogs, toxins, drugs, metal complexes etc) can inhibit the enzyme activity.

Inhibitor (I) binds to an enzyme and prevents the formation of ES complex or breakdown it to E + P

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Reversible and irreversible inhibitors

Reversible inhibitors – After combining with enzyme (EI complex is formed) can rapidly dissociate.• Enzyme is inactive only when bound to inhibitor.

• EI complex is held together by weak, noncovalent interaction.

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Irreversible Enzyme Inhibition

• very slow dissociation of EI complex

• Tightly bound through covalent or noncovalent interactions

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