Biological Molecules - Factors Affecting Enzyme Activity

Question 1

How does temperature influence enzyme activity?

Answer 1

TEMPERATURE:

Temperature increases
more KE
so molecules move faster
so enzymes more likely to collide with substrate molecules
energy of these collisions also increases
so each collision is more likely to be successful

TEMP TOO HIGH:

Temperature increase
enzyme molecules vibrate more
if temp too high the vibration breaks some bonds that hold the enzymes in shape
the active site changes shape
and the substrate and enzyme can’t fit together
so enzyme is denatured
– it can no longer function as a catalyst

Question 2

How does pH affect enzyme activity?

Answer 2

pH:

Enzymes have an optimum pH value.
Most human enzymes work best at pH7, but there are exceptions
e.g. Pepsin works at pH2
This is useful because it’s found in the stomach.
Above and below optimum pH,
the H+ and OH- ions found in the acids and alkalis
can mess up the ionic and hydrogen bonds
which hold the tertiary structure in place.
This makes the active site change shape,
so the enzyme is denatured.

Question 3

How does enzyme concentration affect the rate of reaction?

Answer 3

ENZYME CONCENTRATION:

More enzyme molecules
more likely a substrate molecule is to collide with one and form an enzyme-substrate complex,
so increasing concentration of enzymes
increases the rate of reaction.

BUT IF SUBSTRATE IS LIMITED:

there comes a point when there’s more than enough enzyme molecules to deal with all the available substrate,
so adding more enzyme has no further effect.

Question 4

How does substrate concentration affect the rate of reaction?

Answer 4

It affects the rate UP TO A POINT:

Higher substrate concentration
means faster reaction –
– more substrate molecules means a collision between substrate and enzyme is more likely and so more active sites will be used.

This is only true up to ‘saturation’ point though.
After that, there are so many substrate molecules that the enzymes have about as much as they can cope with
(all the active sites are full), and adding more makes no difference.

Substrate concentration DECREASES with TIME during a reaction
(unless more substrate is added to the reaction mixture),
so if no other variables are changed, the RATE OF REACTION WILL DECREASE OVER TIME too.
This makes the initial rate of reaction the highest rate of reaction.

Question 5

Explain competitive inhibition.

Answer 5

• Competitive inhibitor molecules have a similar shape to that of the substrate molecules.
• They compete with the substrate molecules to bind to the active site, but no reaction takes place.
• Instead they block the active site, so no substrate molecules can fit in it.
• How much the enzyme is inhibited depends on the relative concentrations of the inhibitor and the substrate.
• If there’s a high concentration of the inhibitor, it’ll take up nearly all the active sites and hardly any of the substrate will get to the enzyme.
• But if there’s a higher concentration of substrate, then the substrate’s chances of getting to an active site before the inhibitor decrease. So increasing the concentration of substrate will increase the rate of reaction (up to a point).

Question 6

Explain non-competitive inhibition.

Answer 6

Non-competitive inhibitor molecules bind to the enzyme away from its active site.

• This causes the active site to change shape so the substrate molecules can no longer bind to it.
• They don’t ‘compete’ with the substrate molecules to bind to the active site because they’re a different shape.
• Increasing the concentration of substrate won’t make any difference to the reaction rate – enzyme activity will still be inhibited.