Mod. 2 Enzymes

Question 1

What is the role of enzymes?

Answer 1

enzymes catalyse reactions and enable them to happen at a lower energetic level than they otherwise would. Enzymes also control the reactions, since they can be inhibited by surroundings, competition and inhibiting molecules.

Question 2

What is the mechanism of enzyme action?

Answer 2

Enzymes are in solution, so move around and collide with their substrate in the active site. This must be at the correct orientation. This is affected by temperature and pressures, which will increase the rate of successful collisions.

Question 3

What is the benefit of using an enzyme in cell reactions?

Answer 3

Enzymes lower the activation energy needed for a reaction to take place.
Enzymes are specific.
Enzymes are reusable, they can catalyse many reactions.

Question 4

Explain the enzyme lock and key hypothesis.

Answer 4

The tertiary structure of the active site is complementary to the substrate, and no other molecule. When the substrate collides, an enzyme-substrate complex is made. The substrate(s) then react and the product(s) is/are released and can take part in subsequent reactions. The substrate is held in a way that the correct atom groups are close enough to react. The R-groups of the amino acids also interact by forming temporary bonds.

Question 5

Explain the enzyme induced fit hypothesis.

Answer 5

The active site and enzyme tertiary structure change once the substrate enters the active site. The initial weak interaction with the substrate causes the bonds (hydrogen, ionic, polar) to change, strengthening the binding and putting pressure on the substrate to cause a reaction.

Question 6

What is the role of intracellular enzymes?

Answer 6

Enzymes within cells. They catalyse reactions within the cell, e.g. making polysaccharides from glucose, catalase breaks down hydrogen peroxide into oxygen and water.

Question 7

What is the role of extracellular enzymes?

Answer 7

Extracellular enzymes catalyse reactions outside of the cell, e.g. digestive enzymes. They break down larger molecules that cannot fit through the cell surface membrane into smaller molecules. The broken down molecules can be absorbed into the cells to provide substrates for intracellular enzyme reactions.

Question 8

How do enzymes work in the digestion of starch?

Answer 8

Amylase breaks down starch polymers to maltose, a disaccharide. Amylase is produced from the salivary gland.
Maltase then breaks down maltose into glucose in the small intestine. Glucose can be absorbed into the cells lining the intestine.

Question 9

How do enzymes work in the digestion of protein?

Answer 9

Proteases are enzymes that break down proteins into peptides. Peptides are then broken into amino acids. Trypsin is an example. It is produced in the pancreas and released in the pancreatic juice. The amino acids produced as a result are absorbed into the bloodstream through the cells lining the digestive system.

Question 10

What factors can affect enzyme action?

Answer 10

temperature
pH
concentration of enzyme
concentration of substrate

Question 11

How does temperature affect the rate of enzyme action?

Answer 11

As heat increases, the kinetic energy of molecules also increases, so there are more successful collisions. Q10 the temperature coefficient states that with every rise of 10°C the rate of reaction doubles. Once the temperature surpasses the optimum temperature (usually 40°C), the rate of reaction decreases because the heat energy is breaking the hydrogen bonds maintaining the tertiary structure of the protein, denaturing its precise complementary structure.

Question 12

How have organisms living in extreme temperatures evolved to maintain enzyme action?

Answer 12

In extreme cold, enzymes have much more flexible structures, particularly the active site. The enzymes will be denatured in warmer temperatures.
Thermophiles have specialised enzymes with increased numbers of bonds, particularly hydrogen and sulphur bridges to maintain the precise tertiary structure. They are the most stable of all, and are more resistant in temperature rises.

Question 13

How does pH affect enzyme action?

Answer 13

H bonds and ionic bonds between amino acid R-groups rely on polar and charged atoms in the R-groups of the primary structure. H+ ions are charged and can disrupt these bonds and denature the precise tertiary structure of the enzyme. There is an optimum pH for each enzyme, most around 7, and this is a very narrow margin. Too low and the H+ ions will disrupt the bonds by interacting with the polar bonds, and the opposite happens when the pH is too high. The denaturing of the precised tertiary structure reduces the number of successful collisions, and the rate of reaction will decrease.

Question 14

Which enzymes have specialised within the human to withstand extreme pH levels?

Answer 14

pepsin - breaks down proteins to polypeptides in the stomach (pH 1-2)
in the small intestine (pH 8):
trypsin - proteins to polypeptides
lipase - triglycerides to glycerol and fatty acids
amylase - starch to maltose
maltase - maltose to glucose

Question 15

How does concentration of enzyme affect enzyme action?

Answer 15

If there is a low concentration of the enzyme, the limiting factor on the rate of reaction is the enzyme, because it reduces the collision rate of substrate to the enzyme active site.

Question 16

How does concentration of substrate affect enzyme action?

Answer 16

If there is a low concentration of substrate to enzyme, the substrate is the limiting factor, because the rate of reaction cannot increase until there are more substrates to react.

Question 17

Name the types of enzyme inhibition

Answer 17

competitive
non-competitive
end-product

Question 18

What is the benefit of enzyme inhibition?

Answer 18

rates of reaction can be controlled. The cell can control the rate at which a reaction is happening by controlling the number of active enzymes.

Question 19

How does competitive inhibition work?

Answer 19

a molecule or part of a molecule has a similar shape to the intended substrate, so can bind to the active site, and block the substrate from entering the active site. The enzyme cannot carry out its function. They slow down the rate of reaction and are mostly temporary.
e.g. statins are competitive inhibitors to an enzyme in cholestrol synthesis, aspirin inhibits active site of COX enzymes.

Question 20

How does non-competitive inhibition work?

Answer 20

The inhibitor binds to an enzyme’s allosteric site instead of the active site, so is not competing. The binding causes changes in the bonds, so alters the tertiary structure and therefore the precise active site shape, preventing the substrate from binding to the protein.
e.g. organophosphates used as insecticides inhibits an enzyme needed for neurotransmission.

Question 21

How does end-product inhibition work?

Answer 21

The end product works as the inhibitor to the enzyme that produced it. It works by negative feedback, and controls the rate of reaction. It is non-competitive and reversible.
e.g. in the process of ATP production, two phosphate groups are added to glucose catalysed by PFK. PFK is competitively inhibited by ATP, so stops the process of ATP production by using ATP.

Question 22

What is an enzyme cofactor?

Answer 22

Cofactors transfer atoms or groups from one reaction to another, and sometimes form part of the active site. They are inorganic molecules such as iron, calcium, chloride, zinc.

Question 23

What is a coenzyme?

Answer 23

Coenzymes transfer atoms or groups from one reaction to another and sometimes form part of the active site. They are organic molecules (unlike cofactors), such as vitamin B3 used to synthesise NAD

Question 24

What is the role of a prosthetic group in enzyme function?

Answer 24

Specific cofactors that are required by an enzyme to complete its catalytic function. They are permanently bound to the protein. Zn2+ ions are a prosthetic group of carbonic anhydrase.

Question 25

What is precursor activation in enzymes?

Answer 25

A cofactor needs to be attached to the enzyme to activate it. The enzyme may cause damage to the cell, so having to add a cofactor decreases damage risk, it is an apoenzyme. The cofactor changes the tertiary structure of the enzyme, now a holoenzyme.