Many proteins are enzymes

What are enzymes?

Enzymes are globular proteins that act as catalysts by increasing the rate of a reaction without being used up in the reaction itself or undergoing permanent change themselves.

How does an enzyme speed up the rate of reaction?

Enzymes provide an alternate reaction pathway with a lower activation energy

So the reaction can occur at a lower temperature, at body temperature.

What are the 2 types of enzymes?

Intracellular enzymes - These act within the cells that produce them

Extracellular enzymes - These act outside the cells that produce them.

Describe the induced fit model

In the induced fit model, initially the active site is not complementary to the substrate, hence it does not fit perfectly but the substrate is specific to the enzyme.

If it collides with the correct activation energy and in the correct orientation, the enzyme and substrate bind to the specific active site and form an enzyme-substrate complex. This fit isn't perfect, therefore the enzyme contorts into a transitional state and undergoes conformational changes, moulding itself around the substrate.

This allows the enzyme-substrate to fit tightly, putting physical strain or pressure on the chemical bonds of the substrate, which reduces repulsion between molecules and destabilizes bonds in the substrate structure

So less energy is needed to overcome them and form new products, hence producing an alternative reaction pathway.

Why does a mutation affect the active site of an enzyme?

A mutation in the gene changes the primary structure by altering the sequence of amino acids in the polypeptide chain.

This affects the bonding between different amino acids and their R groups (e.g. hydrogen and ionic bonds), changing how the chain folds.

As a result, the tertiary structure and shape of the active site are altered, so the enzyme becomes less specific to its substrate

Why are enzymes specific to a certain substrate only?

Enzymes are specific to the substrate because they have a specific active site

The tertiary shape of the active site is complementary to one substrate or one reaction only

Only the substrate that is complementary to the active site can bind to form an enzyme-substrate complex.

How does temperature affect enzyme action? (At low temperature)

At a lower temperature, enzymes have a low kinetic energy, so they collide less frequently with the substrates and with less energy.

So there are less enzyme-substrate complexes formed

So rate of reaction is slow initially

How does temperature affect enzyme action? (As temperature increases)

As the temperature rises, the enzyme and substrate collide more frequently and more energetically

So more active sites are used up and more enzyme-substrate complexes are formed

This causes the rate of reaction to increase

How does temperature affect enzyme action? (Beyond optimum temperature)

Once the temperature exceeds the optimum, enzyme molecules gain excess kinetic energy and vibrate more vigorously.

This vibration can break or destabalise the hydrogen and ionic bonds that maintain the enzyme's tertiary structure.

As a result, the shape of the active site changes, so the substrate can no longer bind effectively.

Fewer enzyme-substrate complexes form, as active site is now no longer specific to substrate

So rate of reaction is lower as enzyme becomes non functional and denatures

How does pH affect enzyme action?

Above and below the optimum pH, changes in H⁺ and OH⁻ ion concentration disrupt ionic and hydrogen bonds that maintain the enzyme's tertiary structure.

This alters the shape of the active site, reducing enzyme-substrate complex formation

This lowers the rate of reaction

What are competitive inhibitors?

Competitive inhibitors are molecules that have a similar shape to the specific substrate and bind reversibly to the active site, forming temporary bonds.

They compete with the substrate for the active site, reducing enzyme-substrate complex formation and hence decreasing the rate of reaction

How does competitive inhibitors affect enzyme action (Lower concentration of substrates)

At higher inhibitor concentrations, the competitive inhibitors collide more frequently with the enzyme than the substrate

So they bind to the active site and form temporary bonds with the active site (because their shape is similar to the substrate)

As a result more active sites are occupied by the inhibitor which prevents the substrate from binding

So fewer enzyme-substrate complexes form and as a result, the rate of reaction decreases

How does competitive inhibitors affect enzyme action (Higher concentration of substrates)

If the substrate concentration is high, substrate molecules collide more frequently with the enzyme than the inhibitor

So the substrate can out-compete the inhibitor for the active site and bind to the active site

As a result more active sites are occupied by the substrate than inhibitor

So more enzyme-substrate complexes form

As a result, the rate of reaction increases despite the presence of the inhibitor

What are non competitive inhibitors?

Non-competitive inhibitors are molecules that don’t have the same structure as the substrate the enzyme binds to.
They bind to a site on the enzyme other than the active site, known as the allosteric site

How do non competitive inhibitors affect rate of reaction?

This causes a change in the tertiary structure of the enzyme, which as a result changes the shape of the active site
So enzyme’s active site will no longer be complementary to the molecule after undergoing conformational changes, hence enzyme substrate complexes cannot be formed
So rate of reaction drops

What effect does concentration of substrate have on non competitive inhibitors?

No effect
They do not compete for the active site
Alters tertiary shape of enzyme and active site, making enzyme non functional
So increasing or decreasing concentration of reactants doesn’t have an effect

How does substrate concentration effect enzyme action? (Increasing substrate concentration, at start of the graph)

As substrate concentration increases, rate increases
This is because more active sites are being occupied
So more frequent successful collisions occur
So more enzyme substrate complexes are formed which causes rate to increase

How does substrate concentration effect enzyme action? (Increasing substrate concentration, in the middle of the graph)

As substrate concentration further increases, the active site of the enzymes become more occupied
This reduces the amount of active sites available for enzymes to bind to
This reduces the formation of enzyme substrate complexes
So rate of reaction slows down (rate of increase of rate decreases)

How does substrate concentration effect enzyme action? (Increasing substrate concentration, end of the graph)

Eventually beyond this point, all enzymes become saturated as all the active sites are occupied
So no free active sites left
So no more enzyme substrate complexes formed
So enzyme becomes the limiting factor
Reaction will reach its Vmax (so rate will no longer increase regardless of how much more substrate is added)

How does enzyme concentration effect enzyme action? (Increasing enzyme concentration, at start of the graph)

As enzyme concentration increases initially more active sites are available
So the frequency of successful collisions increases
So more enzyme substrate complexes formed
So the rate increases

How does enzyme concentration effect enzyme action? (Increasing enzyme concentration, in the middle of the graph)

Beyond a certain point, the substrate becomes limiting factor and are all already occupying active sites
There are more active sites available then substrate molecules
So less frequent and successful collisions
So less enzyme substrate complexes formed, so rate decreases
Rate continues to decrease until it stops (when the substrates completely run out)