Factors Affecting Enzyme Activity (Page 2 of 3) in pdf

Factors Affecting Enzyme Activity

5. Cofactors

3. Enzyme concentration

Many enzymes require cofactors to function properly. There are three

The effect of enzyme concentration on the rate of reaction is shown in Fig

main types of cofactor; co-enzymes, inorganic ions and prosthetic groups.

4. At low enzyme concentrations there are more substrate molecules than

there are available active sites. Increasing the number of active sites by

1. Coenzymes are organic molecules which often contain a vitamin

increasing the concentration of the enzyme, therefore, effectively increases

molecule as part of their structure. Coenzymes become loosely bound

the rate of the reaction. Eventually, at point x, increasing the enzyme

to the enzyme and move away from the enzyme once the reaction is

concentration has no effect on the rate of reaction. This is because it is now

completed. One coenzyme, e.g. NAD

may react with many different

the number of substrate molecules which has become the limiting factor.

enzymes in many different types of reaction. NAD

transfers hydrogen

in reactions involving dehydrogenase enzymes.

Fig 4. Effect of enzyme concentration on enzyme activity

2. Inorganic metal ions are also known as enzyme activators. They

change the charge in the active site, enabling the enzyme substrate

complex to form. Some become intimately bound to the enzyme, e.g.

in catalase. Most others accelerate the binding between the enzyme

and the substrate, e.g. Mg

in phosphotransferases.

3. Prosthetic groups are coenzymes that bind permanently to the enzyme

molecule and remain there even after the reactions are complete, e.g.

FAD (flavin adenine dinucleotide). Like NAD

it carries hydrogen

atoms, this time with oxidase enzymes.

6. Inhibitors

Inhibitors slow down the rate of reaction. As such, they are an essential

Enzyme

form of cellular control, allowing enzyme reaction rate to be slowed when

necessary. Some enzymes are inhibited by the end product of the reaction

they catalyse (see Factsheet 31 Enzyme control of metabolic pathways).

4. Substrate concentration

Fig 5 shows the effect of substrate concentration on the rate of reaction.

(a) Reversible inhibitors

There are two types of reversible inhibitor:

Fig 5. Effect of substrate concentration on enzyme activity

•

competitive reversible inhibitor

•

non-competitive reversible inhibitor

Competitive reversible inhibitors are structurally similar to the normal

substrate and compete with the normal substrate for the active sites (see

Fig 6).

Fig 6. Competitive reversible inhibition

Substrate concentration

normal substrate

glucose

At low substrate concentration the reaction proceeds slowly. This is because

active site

there are not enough substrate molecules to occupy all of the active sites on

the enzyme. As substrate concentration increases, the rate increases because

arabinose

glucose

there are more enzyme substrate complexes formed. At point x, however,

= competitive

oxidase

increasing the substrate concentration will have no further effect on the

inhibitor

= enzyme

rate of reaction. This is because all of the enzyme’s active sites are now

occupied by substrate molecules – increasing the substrate concentration

further will have no effect, because no more enzyme substrate complexes

can form. The rate of reaction now depends on the turnover rate of the

enzyme, i.e. the number of substrate molecules transformed by one molecule

of enzyme per second. Carbonic anhydrase has the highest turnover rate of

any known enzyme (Table 1).

Table 1. Enzyme turnover rates

Typical Exam Questions

Enzyme

Turnover rate

1. Describe and explain the effect of pH, temperature, enzyme

concentration etc. on rate of reaction

Carbonic anhydrase

36 x 10

Catalase

5.6 x 10

2. Explain the induced fit hypothesis

Lysozyme

3. Explain the role of cofactors

Factors Affecting Enzyme Activity Page 2

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