Enzymes 2

Question 1

How are enzyme reaction rates no longer limited by the chemical steps?

Answer 1

Evolution has acted on it

Question 2

What is now the limiting factor of enzyme reaction rates?

Answer 2

Limited by diffusion rates

Question 3

What is k1?

Answer 3

The binding step

Question 4

What is k3?

Answer 4

The conversion of the substrate to its product and the release from the active site

Question 5

Give the method for proof on the limiting factor reaction of enzymes

Answer 5

Put enzyme and a small amount of substrate and follow the ROR to make the enzyme and product

Question 6

Why cant you do the enzyme reaction with big amounts of substrate?

Answer 6

The substrate will be bound up in E-S complexes so you’d be measuring the K3 not the K1

Question 7

What would affect the ROR if the binding was the limiting factor?

Answer 7

Viscosity of the solution

Question 8

How could you change the viscosity of a solution?

Answer 8

Add glycerol

Question 9

What should the intrinsic rate constant be for a diffusion-limited reaction?

Answer 9

10^8

Question 10

How else can you find out whether an enzyme reaction is diffusion limited (not practical)?

Answer 10

Through theoretical calculations

Question 11

What do you need to take into account when working out the intrinsic rate constant of an enzyme recation?

Answer 11

The size of the substrate and the enzyme

Question 12

What is one of the best understood examples of a perfect enzyme?

Answer 12

Triose phosphate isomerise

Question 13

In glycolysis, what is the conversion of the C-6 molecule to 2x C-3 molecules catalysed by?

Answer 13

Aldenase

Question 14

What does triose phosphate isomerase do in glycolysis?

Answer 14

Converts the dihydroxyacetone phosphate and the glyceraldehyde 3-phosphate meaning that both can be fed down the glycolytic pathway

Question 15

Which proteases hydrolyse peptide bonds?

Answer 15

Serine, cysteine, aspartyl and metallo-proteinases

Question 16

How do serine proteases break peptide bonds?

Answer 16

The very reactive serine attacks the peptide bond to form an acyl-enzyme

Question 17

Which residue in chymotrypsin is a serine?

Answer 17

195

Question 18

How does residue 195 on chymotrypsin react with the peptide bond?

Answer 18

Forms an acyl bond intermediate which can be hydrolysed very easily. This results in the cleavage of the peptide bond

Question 19

What is a catalytic triad?

Answer 19

Constellation of active side groups which come together to make the serine very reactive

Question 20

Why don’t serine proteases cleave all of the peptide bonds?

Answer 20

They have a sequence specificity that is determined by the nature of the residue on the N-terminal side of the peptide bond that will be cleaved

Question 21

When will chymotrypsin cleave peptide bonds?

Answer 21

When the asterisked residue is a phenylalanine, tryptophan or tyrosine

Question 22

What is the same about all of the chymotrypsin asterisked residues?

Answer 22

Hydrophobic