Enzyme Properties

Question 1

Enzymes

Answer 1

Biological catalysts that speed up the rate of reaction without altering the final equilibrium between products and reactants

Question 2

How do enzymes work?

Answer 2

Lower the activation energy required for a reaction, so it is quicker

Question 3

Lock and key theory

Answer 3

Emil Fisher proposed that enzymes are complementary to their substrate - like lock and key - in 1884

Question 4

Induced fit theory

Answer 4

Daniel Koshland - 1958 - suggested that enzymes undergo changes when the substrate binds, which changes shape of the active site

Question 5

Transition state

Answer 5

Unstable high energy intermediate in a chemical reaction - stabilising the transition state is one way that enzymes speed up rate of reaction

Question 6

Alcohol dehydrogenase

Answer 6

Convert primary alcohols to aldehydes

Question 7

Can enzymes act on stereoisomers?

Answer 7

No, enzymes are only complementary to one isomer

Question 8

6 types of enzymes

Answer 8

1) Oxidoreductases
2) Transferases
3) Hydrolases
4) Lyases
5) Isomerases
6) Ligases

Question 9

Oxidoreductases

Answer 9

Catalyse oxidation or reduction reactions
Transfer of H/O atoms from one substance to another

Question 10

Transferases

Answer 10

Catalyse transfer of functional groups from one substance to another

Question 11

Hydrolases

Answer 11

Catalyse formation of two products from a substrate by hydrolysis (splitting using water)

Question 12

Lyases

Answer 12

Catalyse non hydrolytic addition or removal of groups from substrates

Question 13

Isomerases

Answer 13

Catalyse isomerisation changes within a single molecule

Question 14

Ligases

Answer 14

Join together two molecules by synthesis of new bonds (C-O C-S C-N C-C) along with ATP breakdown

Question 15

Effect of temperature on enzymes

Answer 15

When proteins are heated, weak bonds are easily broken, causing tangled structure, and enzyme is denatured

Question 16

Effect of pH on enzymes

Answer 16

Causes unfolding of enzyme causing inactivation

Question 17

Reaction rate equations

Answer 17

Change in product/time
Change in substrate/time

Question 18

What is hyperbolic kinetics enzymes

Answer 18

At low substrate conc, reaction rate is directly proportional to substrate concentration
At high substrate conc, reaction rate is independent of substrate concentration

Question 19

Michael Menten reaction model

Answer 19

E + S >< ES >< E + P

Question 20

Assumptions in Michael Menten reaction model

Answer 20

[S] > [E] so that amount of substrate bound by enzymes at any one time is small
Initial velocities used, so back reaction of products to substrate can be ignored
[ES] does not change with time

Question 21

Michaelis-Menten equation

Answer 21

V0 = Vmax [S] / Km + [S]
V0 - initial reaction velocity
V max - maximum velocity of an enzyme catalysed reaction
Km = Michaelis constant (k-1 + k2)/k1

Question 22

Special relationship between Km and [S] when V0 = 0.5Vmax

Answer 22

Km = [S]

Question 23

Km and enzyme substrate affinity

Answer 23

Where k2 < k-1
Km = k-1/k1

Question 24

Kcat

Answer 24

Number of substrate molecules converted to product in a unit of time on a single enzyme molecule when the enzyme is saturated with substrate

Question 25

Best way to compare catalytic efficiency

Answer 25

Kcat/Km

Question 26

Lineweaver-Burk plot

Answer 26

Turn curve into straight line using the reciprocal
1/v = (km/Vmax) (1/[S]) + 1/Vmax

Question 27

Effect of competitive inhibitors

Answer 27

Their effect can be overcome by increasing substrate concentration
Increase Km
V max does not change

Question 28

Effect of non competitive inhibitors

Answer 28

Decrease V max
Do not change Km

Question 29

Allosteric enzyme

Answer 29

Cooperative substrate binding
When the binding of one substrate affects the binding of another substrate

Question 30

Regulation of enzyme activity

Answer 30

Substrate availability - immediate
Product inhibition - immediate
Allosteric control - immediate
Covalent modification - immediate to minutes
Synthesis/degradation - hours to days

Question 31

Covalent modification

Answer 31

Reversible addition of Ser, Thr, Tyr and His residues by kinase and phosphatase regulatory enzymes

Question 32

Blood glucose control

Answer 32

If blood glucose levels increase, insulin production increases within the pancreas
This increases rate of synthesis of key enzymes involved in glucose metabolism - glucokinase, phosphofructokinase, pyruvate kinase