Biology

1.6 - Enzymes

13 min

Why enzymes are needed

Catalysts and enzymes

Active site and specificity

Models of enzyme action

Rate rising with temperature

Denaturation and optimum

Effect of pH

note 1 of 3

What an enzyme is

Living cells carry out thousands of chemical reactions that must happen quickly and be carefully controlled.
Raising the temperature would speed reactions up, but inside a living thing it can't be raised far without damaging the cells, and it would speed up unwanted reactions too.
Instead, cells make enzymes - biological catalysts that speed up the rate of a particular reaction without being changed or used up, so they can be used again and again.

Enzymes are proteins with an active site

Enzymes are large proteins: long chains of amino acids.
The order of the amino acids makes the chain fold into a particular three-dimensional shape.
Part of that shape is a pocket called the active site, where the reaction happens.

How an enzyme works

The substance an enzyme acts on is its substrate.
The substrate binds to the active site to form an enzyme-substrate complex.
During the reaction the substrate is either split apart or joined together, and the products are released - leaving the enzyme unchanged and ready to be used again.
Enzymes can both break large molecules down (degradation reactions) and build small molecules up (synthesis reactions).

Specificity and the lock-and-key model

Each enzyme usually catalyses only one reaction - a property called specificity.
This is because the active site has a particular shape, so only a substrate whose shape is complementary to the active site can bind to it - fitting like a key fits a lock. This is the lock-and-key model.
If the substrate's shape doesn't match the active site, the reaction is not catalysed - which is why one enzyme works on one substrate and not on others.

A grey blob labelled enzyme with a green notch labelled active site, and a blue shape labelled substrate dropping into it to give the pair joined and labelled enzyme-substrate complex.

note 2 of 3

How temperature affects enzyme rate

Temperature has a strong effect on how fast an enzyme works.
As the temperature is raised from cold, the rate of reaction increases at first - the enzyme and substrate molecules move about more, so they meet and react more often.
Every enzyme has an optimum temperature, the temperature at which it works fastest. For many human enzymes this is around body temperature.

Above the optimum - Denaturation

Above the optimum the rate does not keep rising - it falls, and quite quickly.
If the enzyme gets too hot, some of the bonds holding its folded shape together break.
This changes the shape of the active site, so the substrate no longer fits, and the enzyme can no longer catalyse the reaction.
An enzyme damaged in this way is said to be denatured.
Denaturation is permanent - the enzyme does not return to its working shape if it is cooled again, and boiling denatures most enzymes.

A substrate sitting in the green active site of an enzyme, with an arrow marked above the optimum temperature leading to a denatured enzyme whose active site changes shape and no longer fits.

The shape of the graph

A graph of rate against temperature rises to a peak at the optimum temperature.
It then drops away steeply as the enzymes denature.

A curve of rate of reaction against temperature in degrees Celsius, rising slowly from 0, peaking at 37 where a dashed line marks optimum temperature, then dropping steeply to zero near 45.

note 3 of 3

How pH affects the rate

Each enzyme has a pH at which it works fastest, called its optimum pH.
For many enzymes the optimum is around neutral (pH 7), but it isn't the same for every enzyme:
- Some work best in acidic conditions.
- Others work best in alkaline conditions.
The optimum matches the part of the body where the enzyme is found.

Moving away from the optimum

If the pH becomes too acidic or too alkaline, the rate of reaction falls.
The wrong pH interferes with the bonds that hold the enzyme's folded shape together.
This changes the shape of the active site, so the substrate no longer fits properly.
If the pH is far enough from the optimum, the active site is changed so much that the enzyme is denatured and stops working.

A graph of rate against pH therefore peaks at the optimum pH and falls away on either side of it.

Rate of reaction against pH from 0 to 11 shows two peaks: one near pH 2 labelled optimum pH for enzyme A (acidic), one near pH 9 labelled optimum pH for enzyme B (alkaline).

key terms

TermDefinition

EnzymeA biological catalyst that speeds up a specific chemical reaction without being changed or used up.

Active siteThe pocket in an enzyme where the substrate binds and the reaction happens.

SubstrateThe substance an enzyme acts on.

Enzyme-substrate complexThe temporary structure formed when a substrate binds to the active site of an enzyme.

SpecificityThe property of an enzyme that allows it to catalyse only one reaction.

Lock-and-key modelThe idea that an enzyme's active site fits only one substrate, like a key fitting a lock (their shapes are complementary).

Optimum temperatureThe temperature at which an enzyme works fastest.

DenaturedDescribes an enzyme whose active site has permanently changed shape, so the substrate no longer fits and the enzyme no longer works.

Optimum pHThe pH at which an enzyme works fastest.

end of notes

Key points to remember

What an enzyme is

Living cells carry out thousands of chemical reactions that must happen quickly and be carefully controlled.

Raising the temperature would speed reactions up, but inside a living thing it can't be raised far without damaging the cells, and it would speed up unwanted reactions too.

Instead, cells make enzymes - biological catalysts that speed up the rate of a particular reaction without being changed or used up, so they can be used again and again.

Enzymes are proteins with an active site

Enzymes are large proteins: long chains of amino acids.

The order of the amino acids makes the chain fold into a particular three-dimensional shape.

Part of that shape is a pocket called the active site, where the reaction happens.

How an enzyme works

The substance an enzyme acts on is its substrate.

The substrate binds to the active site to form an enzyme-substrate complex.

During the reaction the substrate is either split apart or joined together, and the products are released - leaving the enzyme unchanged and ready to be used again.

Enzymes can both break large molecules down (degradation reactions) and build small molecules up (synthesis reactions).

Specificity and the lock-and-key model

Each enzyme usually catalyses only one reaction - a property called specificity.

This is because the active site has a particular shape, so only a substrate whose shape is complementary to the active site can bind to it - fitting like a key fits a lock. This is the lock-and-key model.

If the substrate's shape doesn't match the active site, the reaction is not catalysed - which is why one enzyme works on one substrate and not on others.

How temperature affects enzyme rate

Temperature has a strong effect on how fast an enzyme works.

As the temperature is raised from cold, the rate of reaction increases at first - the enzyme and substrate molecules move about more, so they meet and react more often.

Every enzyme has an optimum temperature, the temperature at which it works fastest. For many human enzymes this is around body temperature.

Above the optimum - Denaturation

Above the optimum the rate does not keep rising - it falls, and quite quickly.

If the enzyme gets too hot, some of the bonds holding its folded shape together break.

This changes the shape of the active site, so the substrate no longer fits, and the enzyme can no longer catalyse the reaction.

An enzyme damaged in this way is said to be denatured.

Denaturation is permanent - the enzyme does not return to its working shape if it is cooled again, and boiling denatures most enzymes.

How pH affects the rate

Each enzyme has a pH at which it works fastest, called its optimum pH.

For many enzymes the optimum is around neutral (pH 7), but it isn't the same for every enzyme:

Some work best in acidic conditions.
Others work best in alkaline conditions.

The optimum matches the part of the body where the enzyme is found.

Moving away from the optimum

If the pH becomes too acidic or too alkaline, the rate of reaction falls.

The wrong pH interferes with the bonds that hold the enzyme's folded shape together.

This changes the shape of the active site, so the substrate no longer fits properly.

If the pH is far enough from the optimum, the active site is changed so much that the enzyme is denatured and stops working.

A graph of rate against pH therefore peaks at the optimum pH and falls away on either side of it.