Enzymes 2

Question 1

What reaction do glucokinase and hexokinase catalyse?

Answer 1

conversion of glucose to glucose-6-phosphate

Question 2

What properties do glucokinase have? (Km and Vmax)

Answer 2

high Km and High Vmax

Question 3

Where do you find glucokinase activity?

Answer 3

the liver

Question 4

What are isoenzymes?

Answer 4

enzymes that catalyse the same reaction

Question 5

What technique is a useful way to separate plasma proteins?

Answer 5

electrophoresis

Question 6

In reference to the action of enzymes with 2 or more substrates, what is meant by an ordered sequential mechanism?

Answer 6

pyruvate to lactate by enzyme lactate dehydrogenase is an example of this mechanism
- the enzyme exists in a ternary complex first with the substrates of the reaction and then after with the products of the reaction
so in this example,coenzyme NADH always binds first and lactate is always released first

Question 7

What is a random sequential mechanism?

Answer 7

the order of binding and release of substrate and product is random but a ternary complex still forms first with substrates and then the products of the reaction

Question 8

What is a double displacement or ‘ping-pong’ reaction?

Answer 8

example is transamination process

no ternary complex formed

Question 9

What are allosteric enzymes?

Answer 9

enzymes that change their conformation on the binding of an effector which in turn causes an apparent change in binding affinity at a different ligand binding site
haemoglobin is an example of this

Question 10

What is an uncompetitive enzyme inhibitor?

Answer 10

one that binds to the enzyme at an area other than the active site

Question 11

What is a competitive inhibitor?

Answer 11

one that binds to the active site, preventing the normal substrate from binding

Question 12

What does a competitive inhibitor do?

Answer 12

will compete with substrate molecule for active site

Question 13

What does the presence of a competitive inhibitor do to the Km and Vmax values?

Answer 13

Km increases - decrease in affinity of enzyme for substrate

Vmax remains unchanged

Question 14

Why does the Vmax not change in the presence of a competitive inhibitor?

Answer 14

increasing the substrate concentration can overcome the inhibition

Question 15

What is a ‘real world’ example of a competitive inhibitor?

Answer 15

the drug AZT working to inhibit the reverse transcriptase used by HIV to produce the dsDNA

Question 16

What is an example of a drug that works on the transition state rather than the substrate?

Answer 16

oseltamavir

Question 17

What does a non-competitive inhibitor do?

Answer 17

bind to enzymes non covalently and will usually attach to a site other than the active site of the enzyme

Question 18

What effect does a non-competitive inhibitor have on the Km and Vmax?

Answer 18

Km remains unchanged

Vmax will decrease

Question 19

Why does the Km remain unchanged in the presence of a non-competitive inhibitor?

Answer 19

the substrate is usually still able to bind to the active site

Question 20

How does an irreversible inhibitor bind to the enzyme?

Answer 20

covalently

Question 21

What are the 2 ways regulatory enzymes modulate reactions?

Answer 21

• allosteric enzymes

- covalently modified enzymes

Question 22

What is meant by feedback inhibition in reference to regulating enzymes?

Answer 22

example would be build up of product inhibiting enzyme by binding to a part other than the active site resulting in a conformational change

Question 23

What are allosteric effectors?

Answer 23

usually cell metabolites and will either increase or decrease the efficiency of substrate binding
are examples of non-competitive inhibitors

Question 24

How can enzymes be regulated by covalent modification?

Answer 24

example is by phosphorylation

Question 25

How can enzymes be regulated by proteolytic cleavage?

Answer 25

enzymes can exist as an inactive precursor protein called a proprotein or a proenzyme that can be cleaved to produce active enzyme