ENZYMES – Nature’s CatalystsDone by: Azizul, Izzah and

Fatin

What Are Enzymes ?• Enzymes are Proteins.

(Tertiary and Quaternary)

• Act as catalyst to accelerate a reaction.

• Not permanently changed in the process.

• Enzymes are specific, will only catalyse one particular reactions.

How Do Enzymes Work ?

Enzymes work by weakening the bonds which lowers the activation energy.

FREEENERGY

} ACTIVATION ENERGY

REACTANTS

PRODUCTS REACTION PATHWAY

How Do Enzymes Work ?Without Enzyme

With Enzyme

A restricted region of an enzyme molecule which binds to the substrate.

The shape and the chemical environment inside the active site permits a chemical reaction to proceed more easily.

The Active Site

© H.PELLETIER, M.R.SAWAYA ProNuC Database

An additional non-protein molecule that is needed by some enzymes to help the reaction.

Tightly bound cofactors are called prosthetic groups

Cofactors that are bound and released easily are called coenzymes

Vitamins are most likely coenzymes.

COFACTORS

Nitrogenase enzyme with Fe, Mo and ADP cofactors

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

THE SUBSTRATE

Fit between the substrate and the active site of the enzyme is exact

Like a key fits into a lock very preciselyThe key will be the enzyme and the substrate

will be the lock. Temporary structure called the enzyme-

substrate complex formed 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

THE LOCK AND KEY HYPOTHESIS

THE LOCK AND KEY HYPOTHESIS

S

E

P

P

Enzyme-substrate complex

Reaction Pathway

Free Energy

1. The enzyme is surrounded by variously shaped substrate molecules.

2. The enzyme is capable of changing the shape of the active site slighty to accomodate for the substrate and ultimately form an Enzyme-Substrate Complex.

3. The enzyme breaks down the substrate into smaller pieces.4. The enzyme then releases the substrate and the active site

then returns to its original shape, ready to bind to more substrate molecules.

The Induced-Fit Hypothesis

The are 6 factors: 1. Temperature 2. pH 3. Enzyme Concentration 4. Substrate Concentration 5. Presence of Inhibitors 6. Presence of Cofactors

What Affects Enzyme Activity ?

Increasing the temperature, - double the reaction - more collisions between particles but only up to optimum temperature. (c) As temperature increases ( >c), more bonds, especially

the weaker Hydrogen and Ionic bonds, will break as a result of this strain. Breaking bonds within the enzyme will cause the Active Site to change shape.

This change in shape means that the Active Site is less Complementary to the shape of the Substrate, so that it is less likely to catalyse the reaction. Eventually, the enzyme will become Denatured and will no longer function.

1. Temperature

40

Temperature En

zym

eA

ctiv

ity

Temperature

OPTIMUM TEMP.

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.

2. pH

pH

3 119751

Trypsin

Pepsin

Optimum pH values

pH

Enzyme activity

5. Presence Of InhibitorsA substrate that can slow down or stop

an enzyme controlled reaction – they combine with the enzyme and stop the substrate attaching to it.

Can be reversible and non-reversibleThere are two types of inhibitors:-

Competitive InhibitorsNon-Competitive Inhibitors

Competitive inhibitorsHas a structure that similar to that of the

substrate and so compete with the substrate for the active site.

As a result, it will reduce the no. of enzyme substrate complexes and so reduce the reaction rate.

This type of inhibition is reversible by an increase in substrate concentration: eventually allow substrate to bind.

So the more substrate available, the less inhibitor will bind its active site.

Non-Competitive InhibitorsDoes not compete for active site.

It attached to another part of enzyme causing the overall shape of the enzyme molecules to change.

Changes the shape of the active site so substrate can no longer bind.

Increasing the amount of substrate will not reduce this inhibitors since there are not in competition for active site.

Removal of inhibitor will restore normal shape of the enzymes.

Irreversible InhibitorsSome inhibitors bind irreversibly to an

enzyme and so destroy its catalytic properties completely.

Usually poisonous to the cells because they close down some part of the metabolism.

Are always non competitive inhibitors.

6. Presence Of Co-FactorsOther small molecules can improved the

action of enzymes, sometimes enzymes will not work without them.

These substances are called Co-factors.Substances derived from vitamin can often

act as Co-factors, in some cases just improving the action of enzymes.

E.g: Carbonic Anhydrase. Has an Zn2+ as an active site. Without them, the enzyme will not work.