Factors that affect enzyme activity

Collision Theory

Helps explain how reactions between particles begin and why rates of reactions can change.

It states that “moving particles are constantly colliding with each other, which can result in reactions, but not all collisions result in a reaction”

First thing that must happen for a successful collision to occur

The reactant particles must collide

Second thing that must happen for a successful collision to occur

The particles must collide with enough energy to break forces holding particles together

The minimum amount of energy required for an effective collision to occur is called the Activation Energy

Third thing that must happen for a successful collision to occur

The particles must collide in the correct orientation(position)

Effect of temperature

For most plant and animal enzymes, there is little activity at low temperatures

• Enzyme activity increases with temperature - maximum activity occurs at the enzyme's optimum temperature

• when the temperature passes through the optimum temperature, enzyme denaturation occurs.

Why does the rate of reaction increase at the optimum temperature?

• Increasing temperature, increases the kinetic energy of the enzyme and substrate

• Molecules move faster, more collisions between the substrate and the enzymes active site

- More enzyme-substrate complexes (ESCs) will be formed

• When the rate of reaction increases, the product is formed faster

What is denaturation

When the temperature is above optimum:

• Molecules within the enzyme vibrate more, putting strain on bonds holding it together

-Hydrogen and ionic bonds break and the structure of protein unravels

• Therefore, the active site changes shape and:

- Substrate fit in less easily so fewer ESCs can from

- Substrate doesn’t fit in all - enzyme is denatured. This is irreversible.

This means that ROR will decrease

Effect of pH

Each enzyme has its own optimum pH where the rate of reaction is maximum. Below optimum and above optimum=denaturation

Changes in pH can affect the ionic and hydrogen bonds responsible for the specific shape of enzymes. Extremes of pH break these bonds and denatured the enzyme.

• Effect of Substrate Concentration

• As substrate concentration increases, there are more collisions between enzyme and substrate.

- More ESCs (enzyme substrate conplexes)form

- More product is formed

- Rate of reaction increases

• If substrate concentration increases further, a maximum rate is reached. All the enzyme molecules are forming ESCs as fast as possible.

- All of the active sites are occupied

• So increasing substrate concentration no longer has an effect on the rate of reaction

Effect of Enzyme Concentration

• When there is a higher enzyme concentration there are more active sites available for the substrate to bind to.

- More ESCs form

- Rate of reaction increases

• If enzyme concentration is increased further, a point will be reached where all substrate molecules are occupying active sites.

- A maximum reaction rate will be reached

Enzyme inhibitors - site competitive

A site competitive inhibitor attaches to the enzyme active site

This means that the substrate is unable to attach to the active site while the inhibitor is in place.

Thus, the substrate and the inhibitor are in direct competition

This is not a permanent effect as once the inhibitor leaves the active site, then the enzyme is functional again

Enzyme inhibitors - non-site competitive inhibitors

These inhibitors bind to another part of the enzyme (not the active site). However, this causes a permanent change in the shape of the active site, so that it is no longer functional, nor will it be again.

Heavy metals e.g mercury, cadmium often will acts as non competitive inhibitors

Enzyme Co-factors

• Sometimes, having other molecules present is a good thing.

• Co-enzymes or co-factors act as a bridge between the enzyme and the substrate, locking the molecules tightly together. They often ‘activate’ an enzyme, allowing them to work.

• A co-enzyme could be metal (e.g. iron or magnesium) or a vitamin.