Module 1

Section 1.2

Enzymes

AS Biology

Specification

Enzymes

Enzymes are biological catalysts

This means they speed up reactions without

being chemically changed themselves

They are GLOBULAR proteins

See notes on molecules to recall what these are

Enzyme Definitions

Active site This is the region of the enzyme where catalysis

occurs and substrates fit

Specificity An enzyme may catalyse only one, or else lots of

reactions. Its specificity determines this. An enzyme that catalyses one reaction is a highly specific enzyme.

Affinity This is how strong an enzyme binds its substrates

Active Site

The substrate is held in the active site by a

variety of bonds, such as hydrogen bonds and

electrostatic interactions

Enzyme Definitions

Cofactors

These are non-protein chemical compounds that

are bound to proteins and that are necessary for the

protein to have biological activity. “Helper

molecules” that are usually inorganic molecules.

Common examples include NAD+, FAD, Coenzmye

A, Mg2+

Coenzymes

A loosely bound cofactor, usually an organic

molecule

Prosthetic group

A tightly bound cofactor

Why do reactions take so long?

For a reaction to occur, say A and B turning into C

The rate will depend on how long A and B take to

associate and then turn into C

This depends on how much energy you can

supply to make this happen – the ACTIVATION

ENERGY

Enzymes and Activation energy

Enzymes function by lowering the activation

energy for reactions so that can be performed

easier

Enzyme-Substrate complex

The enzyme binds both substrates, and lowers

the reactions activation energy

This is called the ENZYME-SUBSTRATE

COMPLEX

This allows the reaction to occur faster than if the

reactants were floating about free in solution

Models of enzyme action

How enzymes work is quite an open area of

debate in biology!

The main theories for how enzymes work are;

The lock and key hypothesis

The induced fit hypothesis

The lock-and-key hypothesis

In this model;

The reactants fit exactly into the enzyme active site like a key fits into a lock. Once the catalysis has occurred, the products are a different shape and fit than the reagents and so they fall out of the active site and the enzyme is free to start a new reaction

The induced fit hypothesis

In this model;

The reagents don’t fit exactly into the enzymes

active site. But because enzymes are globular

proteins with flexible shapes, the enzyme

changes its active site shape to accommodate

the reagents

How can we make enzymes work

better?

Because enzymes are proteins they are easily

affected by pH, temperature and concentrations

of starting materials

The effect of heat on enzyme

activity

Enzymes are held together by hydrogen bonds and other weak(ish) bonds

At temperature above body temp (37 oC), these bonds being to break and enzymes will no longer function. But from 0 to 37, the enzyme activity will steadily rise! Not all enzymes however are like this, it depends on each particular enzymes OPTIMUM TEMPERATURE!

As complex, folded proteins, enzymes are susceptible to changes in pH. Like with temperature, there is an OPTIMUM pH for every enzyme

For instance the enzymes in your stomach love being at pH 1 or 2. Yet enzymes that are found in your blood will only work well at pH 7.4-7.6. On either side of the optimum pH the enzyme rate will decrease to 0 as the pH is not optimum.

The effect of pH on enzyme activity

Substrate means that thing that will be reacted with and changed by the enzyme

As you put in more substrate, the enzyme will work and convert it to product! And this will happen in a directly proportional way until ALL the enzyme active sites are filled and the enzymes are working at peak rate.

The effect of substrate concentration

on enzyme activity

Amount of enzyme will also affect a reactions

rate. As you put in more enzyme, the rate will

increase proportionally as there is nothing to slow

down the speed of the enzyme.

Eventually however, the amount of substrate will

begin to limit the reaction and the graph will level

off.

The effect of enzyme concentration on

enzyme activity

Summary

Enzyme Inhibition

Competitive inhibitorsThese bind to the active site of the enzyme and prevent

substrates from binding and reacting.

Non-competitive inhibitorsThese bind anywhere on the enzyme EXCEPT the active

site. This changes the shape of the enzyme and the active site and so stops substrate binding.

Enzyme Immobilisation

Enzymes are a multi-million pound business

because they are cheap to make and can be

used for a multitude of purposes

They can make reactions happen that would

otherwise take hours or days to happen without

the help of an enzyme

One way to increase the efficiency of enzyme

catalysed reactions is to immobilise the enzymes

on solid supports in such a way that their active

site is easy to access

Methods of immobilisation

Why do we bother immobilising?

Immobilising an enzyme allows for cost effective

usage of enzymes

Immobilising can lead to;

Increased enzyme stability

Increased pH tolerance

Increased temperature tolerance

It allows continuous flow operation compared to

time consuming batch operation

Batch vs. Continuous culture

Problems with immobilising

Reduced enzyme

efficiency in some

cases as the steps to

immobilise the

enzymes can damage

them

Sometimes the

enzymes get attached

the wrong way around

which means that the

active site cannot be

accessed by the

reagents

Lactose-free milk

An example of

immobilised enzyme

continuous culture is in

the production of lactose

free milk for people who

suffer lactose intolerance

or for cats

Clinistix

Medical diagnostic

kits use immobilised

enzymes to text for

diabetes and kidney

failure

Pregnancy tests also

use immobilised

enzymes that detect

human gonadotrophin

in urine