Enzymes

Question 1

Function of enzymes:

Answer 1

Enzymes are biological catalysts, meaning they speed up chemical reactions. This happens at a cellular level, as well as affecting the whole organism.

Question 2

What is an intracellular enzyme and example?

Answer 2

An enzyme that acts inside cells, e.g. catalase. catalase works inside cells to catalyse the break down of hydrogen peroxide into oxygen and water.

Question 3

What is an extracellular enzyme and example?

Answer 3

An enzyme that acts outside of cells, e.g. amylase and trypsin. They work outside cells in human digestive system. Amylase hydrolyses starch into maltose in the mouth, and trypsin catalyses the hydrolysis of peptide bonds.

Question 4

Enzyme action:

Answer 4

The active site of an enzyme is a specific shape, depending on the reaction that it catalyses, meaning that other molecules won’t fit into the active site

Question 5

Active site:

Answer 5

The area on an enzyme to which the substrate binds

Question 6

What is the lock and key hypothesis?

Answer 6

The theory of enzyme action in which the enzyme active site is complementary to the substrate molecule, like a lock and key.

Question 7

Why do scientists not use the lock and key hypothesis?

Answer 7

They realised it didn’t give the full story- enzyme and substrate do have to fit together in the first place, but new evidence suggests the enzyme substrate complex changed shape slightly to complete the fit. This locks the substrate even more tightly to the enzyme, leading scientists to come up with the induced fit hypothesis.

Question 8

What is the induced fit hypothesis?

Answer 8

The theory of enzyme action in which the enzyme molecule changes shape to fit the substrate molecule more closely as it binds to it

Question 9

What is an enzyme-product complex?

Answer 9

The intermediate structure in which product molecules are bound to an enzyme molecule, lowering of activation energy

Question 10

What is the effect of pH on enzyme activity levels?

Answer 10

Low pH = lots of H+ ions
H+ ions have a positive charge
Either extreme of H+ ion concentration can interfere with the hydrogen and ionic bonds holding the tertiary structure together.
If the pH change affects the charge on the amino acids at the active site, then the properties of the active site change and the substrate can no longer bind
At high pH values, the –COOH group will dissociate to become a charged –COO- group

Question 11

What is the effect of temperature on enzyme activity levels?

Answer 11

Up to a certain point, increasing temperature will increase the rate of reaction, as there will be more collisions between enzymes and the substrate, and more of these collisions will have the required activation energy for the reaction to proceed.
But heat also makes the molecules vibrate. They have more kinetic energy. This puts strain on the hydrogen bonds and they may break.
As enzymes are proteins there are large numbers of these bonds holding the tertiary structure, and especially the active site, in place.
As the temperature increases, more and more of these bonds are broken and the tertiary structure disintegrates further and further
The rate of reaction decreases
If enough of these bonds are broken, the entire tertiary structure will unravel and the enzyme will stop working
This is not reversible and is known as denaturation

Question 12

what is the effect of substrate concentration on enzyme activity levels?

Answer 12

As the substrate concentration rises, the rate of reaction rises because there are more substrate molecules to react and there are free active sites. At higher concentrations, all of the active sites become filled (the enzyme is saturated) so the rate of reaction remains the same

Question 13

what is the effect of enzyme concentration on enzyme activity levels?

Answer 13

As enzyme concentration increases, the rate of reaction increases as there are more active sites are available, until the substrate concentration becomes a limiting factor and the rate stops increasing

Question 14

What is a cofactor?

Answer 14

Ions (eg Cl-) that increase the rate of enzyme-controlled reactions. They bind to the enzyme and allows enzyme substrate complexes to form more easily.

Question 15

What is a coenzyme?

Answer 15

Small, organic, non-protein molecules that bind for a short period of time to the active site.
They may bind just before, or at the same time, as the substrate binds. In many reactions, coenzymes take part in the reaction, and like substrate, are changed in some way. Unlike the substrate, coenzymes are recycled back to take part in the reaction again. The role of coenzymes is often to carry chemical groups between enzymes so they link together enzyme-controlled reactions that need to take place in sequence.
Some cofactors are permanent parts of the enzymes- prosthetic groups. These contribute to the shape of the enzyme.

Question 16

What is a competitive inhibitor?

Answer 16

Have a similar shape to that of the substrate molecule.
This means that they occupy the active site, forming enzyme-inhibitor complexes.
These complexes do not lead to the formation of products because the inhibitor is not identical to the substrate.
The level of inhibition depends on the concentrations of inhibitor and substrate.
Where the number of substrate molecules is increased, the level of inhibition decreases because a substrate molecule is more likely than an inhibitor molecule to collide with the active site.
Most competitive inhibitors do not bind permanently to the active site. They bind for a short period of time and then leave. Their action is described as reversible, as the removal of the inhibitor form the reaction mixture leaves the enzyme molecule unaffected

Question 17

What is a non competitive inhibitor?

Answer 17

Do not compete with substrate molecules for a place in the active site. Instead, they attach to the enzyme, molecule in a region away from the active site.
The attachment of non competitive inhibitors distorts the tertiary structure of the enzyme molecule, leading to the shape of the active site changing.
This means that they substrate no longer fits into the active site so the enzyme-substrate complexes cannot form and the reaction rate decreases.
The level of inhibition depends on the number of inhibitor molecules present. If there are enough inhibitor molecules to bind to all of the enzyme molecules present, then the enzyme controlled reaction will stop.
Changing the substrate concentration will have no effect on this form of inhibition
Most non-competitive inhibitors bind permanently to the enzyme molecule. The inhibition is irreversible, and any enzyme molecule bound by inhibitor molecules are effectively deactivated.