Proteins - Lecture Twelve

Question 1

Significance of kM

Answer 1

Characterises one enzyme-substrate pair (if an enzyme can act on different substrates, it will have different KM values for each)

Question 2

kM

Answer 2

Is the substrate concentration needed to reach half Vmax

Question 3

Units of kM

Answer 3

Units of concentration

Question 4

Low kM

Answer 4

High affinity

Question 5

High kM

Answer 5

Low affinity

Question 6

Physiological significance of kM

Answer 6

In the cell, for a particular enzyme-substrate interaction, [S] is often below the kM, this means rate will rise to accomodate more substrate, tending to maintain steady state

Question 7

kM specificity

Answer 7

Km is specific for the interaction with the enzyme and particular substrate

Question 8

Isozyme glucoksinase

Answer 8

Glucoksinase found in the liver differs from the hexokinase found in many other tissue

Question 9

Turnover number, kcat

Answer 9

The number of substrate molecules converted to product, per enzyme, per unit of time, when E is saturated with substrate, this helps define the activity of one enzyme molecules

Question 10

If Michaelisp-Menten Model fits, Kcat =

Answer 10

k2, therefore describes the ‘rate-limiting’ step

Question 11

The most effective enzymes should have

Answer 11

A high Kcat and a low kM

Question 12

Kcat

Answer 12

Ability to turnover a lot of substrate into product, per second

Question 13

kM

Answer 13

To achieve near Vmax

Question 14

Viscosity of water

Answer 14

Sets an absolute upper limit at ~109 s-1M-1

Question 15

Enzymes with Kcat/kM above 108s-1M-1

Answer 15

Perfect catalysts

Question 16

Inhibitor

Answer 16

A compound that binds to an enzyme and reduces its activity

Question 17

Inhibitors important because

Answer 17

Natural inhibitors regulate metabolism
Many drugs, poisons & toxins are enzyme inhibitors
Used to study enzyme mechanisms
Used to study metabolic pathways

Question 18

Irreversible inhibitor

Answer 18

Binds covalently to the enzyme. Once the enzymes dead, it won’t come back to ‘life’

Question 19

Reversible inhibitor

Answer 19

Nnot covalently bound to the enzyme. The enzyme can be reactived later

Question 20

Types of reversible inhibitor

Answer 20

Competitive and non-competitive (pure or mixed)

Question 21

Irreversible inihibitor

Answer 21

Binds to the enzyme and permanently inactivates it and reacts with a specific amino acid side chain, usually in the active site, and forms a covalent bond

Question 22

Addition of the bulky tosyl-L-phenylalanine methylketone to the histidine

Answer 22

Disables the catalytic triad and fills the active site, blocking substrate binding

Question 23

Reversible inhibitors

Answer 23

Binds to the enzyme but can subsequently be released, leaving the enzyme in its original active condition

Question 24

Competitive inhibitors

Answer 24

Competes with the substrate for binding in the active site as inhibitor and substrate cannot bind at the same time

Question 25

Enalapril and Aliskiren

Answer 25

Lowers blood pressure

Question 26

Statins

Answer 26

Lowers serum cholesterol

Question 27

Protease inhibitors

Answer 27

AIDS drug

Question 28

Juvenile hormone esterase

Answer 28

Pesticide target

Question 29

Tamiflu

Answer 29

Inhibitor of influenza neuraminidase

Question 30

Non-competitive inhibition

Answer 30

Inhibitors bind at a different site than the substrate, when they bind to the enzyme it alters the shape of the active site
In pure non-competitive inhibition, the binding of the inhibitor has no effect on the binding of the substrate

Question 31

Non-competitive inhibitors

Answer 31

Are allosteric effectors

Question 32

Pure non-competitive inhibition

Answer 32

Binding inhibitors changes the structure of the active site such as that the substrate still binds but the transition state stabilisation is no longer optimal, this results in the Vmax decreasing but the kM staying the same

Question 33

Mixed non-competitive inhibition

Answer 33

Binding of the inhibitor does affect the binding of the substrate resulting in Vmax decreasing and kM increasing
“Binds substrate worse and is worse at turning over”