Factors Affecting Enzyme Activity printable pdf download

April 1999

Number 43

Factors Affecting Enzyme Activity

Enzymes are globular proteins which act as biological catalysts. This means

The effect of temperature on the rate of a chemical reaction is described by

that they speed up the rate of reaction by lowering the activation energy,

the term “temperature coefficient” (Q

that is the energy required to break bonds. Enzymes are a complex tertiary

and sometimes quaternary shape and catalyse reactions by forming a

= rate of reaction at T + 10

complex (known as the enzyme substrate complex) at a specific region of

rate of reaction at T

the enzyme called the active site.

Many enzymes have a Q

of between 2 and 3. In other words, provided

Enzyme + substrate

enzyme substrate complex

product

that the temperature is not so high that it causes denaturation, an increase

in temperature of 10

C will speed up the reaction by a factor of 2-3, that is

Enzymes are specific; any individual enzyme can usually only catalyse

it will double or treble it (Fig 2).

one particular reaction. The induced fit hypothesis has been put forward

to explain how enzymes work. The key points of the induced fit hypothesis

Fig 2. Effect of temperature on enzyme activity

are as follows (Fig1):

Fig 1. Induced fit hypothesis

Temperature

Enzyme

Substrate

ES complex

2. pH

The effect of a change in pH on enzyme activity is shown in Fig 3. As with

temperature, each enzyme has an optimum pH. If pH increases or decreases

much beyond this optimum, the ionisation of groups at the active site and

1. Substrate approaches the active site of the enzyme.

on the substrate may change, effectively slowing or preventing the formation

2. The shape of the active site then changes to fit precisely around the

of the enzyme substrate complex. At extreme pH, the bonds which maintain

substrate – in other words, the substrate induces the active site to

the tertiary structure – hence the active site – are disrupted and the enzyme

change shape.

is irreversibly denatured.

3. The reaction is catalysed and products form.

Since most human enzymes are intracellular, most have a pH optimum of

7.3-7.4. However, pepsin, which works in the acidic environment of the

4. The products are a different shape from the substrate and therefore

stomach, has an optimum of 2.4 (Fig 3).

diffuse away from the active site. As they do, the active site reverts to

its original shape.

Fig 3. Effect of pH on enzyme activity

Factors affecting enzyme activity

1. Temperature

Enzymes have an optimum temperature – this is the temperature at which

they work most rapidly. Below the optimum temperature, increasing

temperature will increase the rate of the reaction. This is because

temperature increases the kinetic energy of the system, effectively increasing

the number of collisions between the substrate and the enzyme’s active

site.

Temperatures above the optimum will lead to denaturation. This occurs

because the hydrogen bonds and disulphide bridges which maintain the

shape of the active site are broken. Thus, enzyme substrate complexes can

p H

no longer be formed.

Factors Affecting Enzyme Activity

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