Enzyme kinetics

Question 1

what is the rate of reaction referred to?

Answer 1

Velocity - V = micromole/min

Question 2

what are the reasons for tailing off ‘plateau’ in reaction?

Answer 2

• product inhibition
• reverse reaction
• substrate depletion

Question 3

what is the specific activity of an enzyme expressed in?

Answer 3

micromole of substrate converted per minute per mg of protein - umol/min/mg protein

Question 4

why do enzymes reach maximum velocity?

Answer 4

enzyme running out of substrate
enzyme instability
enzyme containing a limited number of sites for the substrate to bind

Question 5

what is the initial velocity of reaction dependant on?

Answer 5

the [substrate]

Question 6

what is Vmax?

Answer 6

occurs when all of the active sites of an enzyme are saturated and this occur when [S] is much greater than kM

aka maximal rate of an enzyme reaction in micromole/min

Question 7

what is Km?

Answer 7

Michaelis constant - Vmax/2

Question 8

what happens at high [S] and low [S]

Answer 8

high [S] - Velocity is independent of [S]

Low [S] - velocity is linearly proportional to [S]

Question 9

give Michaelis menten equation

Answer 9

V = Vmax x [S] / Km x [S]

Question 10

what is the line weaver burke plot?

Answer 10

1/V = kM/Vmax x 1/[S] + 1/Vmax

Question 11

what does Vmax reveal?

Answer 11

the turnover number of an enzyme - how many enzyme substrates can turn into product in 1 second

Question 12

Define turnover number

Answer 12

The number of molecules converted to product per enzyme molecule in unit time when enzyme is saturated

Question 13

Equation for the turnover number of an enzyme (Kcat / K2)

Answer 13

Vmax / [ET]

(thus = Vmax = kCat x [ET]

Question 14

what is catalytic efficiency / equation

Answer 14

how fast an enzyme can go -

Kcat / Km = conc per second

Question 15

what does a higher value mean in catalytic efficiency?

Answer 15

the higher the value the better the enzyme works on that particular substrate

Question 16

what is the physiological significance of Km?

Answer 16

Isozymes - isoforms of an enzyme which perform identical reactions. e.g Hexokinase isozymes.

Question 17

Km values for hexokinase I, II, III and IV

Answer 17

I, II, III - 0.04 mM

Iv - 10 mM

Question 18

what is the role of hexokianse IV?

Answer 18

when glucose is abundant, hexokinase IV is active. The liver contains hexokinase IV and more glucose-6-phosphate is produced which is used in synthesis of glycogen

Question 19

how does pH affect enzymes?

Answer 19

enzymes are dependant on pH due to ionisation of side chains of amino acids. These form the active site and bind substrates - if they change chemically then they won’t bind to substrate and will stop working.
Some have optimal pH 7 e.g cytoplasm, mitochondria, cell surface enzymes

Some have more acidic optimal pH e.g in stomach - compartmentalised enzymes = lysosomes, endoscopes

Question 20

how does temperature affect enzymes?

Answer 20

rate increases as temperature increases. BUT, after the optimal temperature if reached (37) then the rate will decrease as the enzyme has denatured - bonds are irreversibly broken or changed.