Factors affecting enzyme activity

In a nutshell

Enzyme activity is affected by a variety of factors including temperature, pH, substrate and enzyme concentration and inhibitors. Sometimes enzymes will only work if they have a non-protein substance attached: these are known as cofactors.

Temperature

The rate of a reaction controlled by enzymes is increased when the temperature is increased. This is because molecules have more energy and therefore move faster. This means enzymes are more likely to collide with substrates and form enzyme-substrate complexes.

Substrates and enzymes will collide with more energy, so it is more likely for bonds to be formed or broken meaning the reaction will occur. The opposite is true at low temperatures: molecules move slowly and are less likely to collide so the reaction is less likely to occur.

However, enzymes work best at their optimum temperature. If the temperature continues to increase beyond this, the rate of reaction will drop dramatically as the enzyme denatures. This means bonds in the enzyme will break and the tertiary structure of the enzyme will change. The active site is now permanently damaged and the substrate can no longer bind.

Biology; Biological molecules; KS5 Year 12; Factors affecting enzyme activity — 1. The low temperature is too cold for the enzyme to function, 2. High temperatures cause the enzyme to denature.

Curiosity: Most human enzymes have an optimum temperature of $37\degree C$ and will denature in temperatures exceeding $40\degree C$ .

pH

As with temperature, all enzymes have an optimum pH value.

Example

Pepsin is an enzyme found in the stomach that functions best in acidic conditions around pH $2$ .

At extreme pHs, enzymes can become denatured. This is because there is an excess of $H^+$ ions in acidic solutions and $OH^-$ in alkaline solutions. These ions cause hydrogen bonds and ionic bonds that hold the tertiary structures of enzymes together to break. This changes the active site of the enzyme, meaning enzyme-substrate complexes will not form.

Biology; Biological molecules; KS5 Year 12; Factors affecting enzyme activity — A graph showing how different enzymes have different optimum pHs. 1. Pepsin, 2. Urease, 3. Trypsin

Enzyme and substrate concentration

Enzyme concentration

The concentration of enzyme affects the rate of reaction. The more enzyme molecules that are available, the more likely a substrate will collide with one and form an enzyme-substrate complex. Therefore increasing the concentration of enzyme increases the rate of reaction.

However, if the amount of substrate is limited there will be more enzymes than the amount of substrate so adding more enzymes will have no impact on the rate of reaction.

Biology; Biological molecules; KS5 Year 12; Factors affecting enzyme activity — A. Excess substrate, B. All the active sites are occupied, C. Excess enzyme

Biology; Biological molecules; KS5 Year 12; Factors affecting enzyme activity — A graph showing the effect of enzyme concentration on rate of reaction. There is a steady increase as more sites are available and if the substrate amount is limited, the increase in enzyme concentration will have no effect and the graph plateaus.

Substrate concentration

This is also true for substrate concentration. More substrate molecules mean collisions between enzymes and substrates are more likely, and the formation of enzyme-substrate complexes is more likely. This is true up until the 'saturation' point. This is where all of the active sites are occupied, so the addition of more substrates will have no impact on the rate of reaction.

Biology; Biological molecules; KS5 Year 12; Factors affecting enzyme activity — A. Excess enzyme, B. All the active sites are filled, C. Excess substrate

Biology; Biological molecules; KS5 Year 12; Factors affecting enzyme activity — A graph showing the effect of substrate concentration on rate of reaction. There is a steady increase as more substrate molecules are available (1.) and then as all the active sites become occupied the increase in substrate concentration has no effect on the rate (2.).

Substrate concentration decreases over the course of the reaction as it is converted into the product by the enzyme. This means that if no more substrate is added, the rate of reaction will decrease over time. This explains why the initial rate of the reaction is the highest rate.

Inhibitors

Enzymes can be stopped from working by inhibitors. These are molecules that bind to the enzyme and can be competitive or non-competitive.

Competitive inhibitors

Competitive inhibitors have a similar shape to the substrate so they therefore compete with the substrate for the active site. If they bind to the active site of the enzyme, the substrate is blocked from binding and no reaction can take place.

Biology; Biological molecules; KS5 Year 12; Factors affecting enzyme activity — 1. The substrate, 2. The enzyme, 3. The substrate is blocked from binding to the active site, 4 . The competitive inhibitor

Biology; Biological molecules; KS5 Year 12; Factors affecting enzyme activity — 1. Enzyme controlled reaction without an inhibitor, 2. Enzyme controlled reaction with a competitive inhibitor (rate increases as substrate concentration increases).

Non-competitive inhibitors

Non-competitive inhibitors bind to the enzyme at a point different to the active site. This causes the active site to change shape so the substrate can no longer bind to it.

Biology; Biological molecules; KS5 Year 12; Factors affecting enzyme activity — 1. The substrate, 2. The enzyme, 3. The enzyme's active site has changed shape, 4. The substrate no longer fits into the active site, 5. The non-competitive inhibitor

Biology; Biological molecules; KS5 Year 12; Factors affecting enzyme activity — 1. Enzyme controlled reaction without an inhibitor, 2. Enzyme controlled reaction with a non-competitive inhibitor (increasing the substrate levels has little effect on the rate)

Cofactors and coenzymes

Definition

Cofactors are non-protein substances that enzymes need to work. They can be inorganic molecules or ions that work by helping the enzyme and substrate bind together. Cofactors do not directly participate in the reaction so are not used up or changed.

Example

Chloride ions are cofactors for the enzyme amylase. Amylase can only digest starch into maltose when chloride ions are present.

Biology; Biological molecules; KS5 Year 12; Factors affecting enzyme activity — 1. Cofactor, 2. Substrate, 3. Enzyme

When a cofactor is tightly bound to an enzyme, it is called a prosthetic group.

Example

Zinc ions are a prosthetic group for the enzyme carbonic anhydrase, as they are a permanent part of its active site. This enzyme is found in red blood cells and catalyses the production of carbonic acid from water and carbon dioxide.

Coenzymes

When cofactors are organic molecules, they are called coenzymes. Coenzymes do participate in the reaction and are changed. They are often carriers which allows them to transport chemical groups between enzymes so they are recycled between reactions.

Vitamins are an important source of coenzymes.

Examples

Pantothenic acid is a key component of coenzyme A which is needed for the link reaction in respiration.
Nicotinic acid produces coenzymes NAD and NADP which are involved in processes such as photosynthesis and respiration.
Vitamin B produces coenzyme FAD which is required during the Krebs cycle.