Enzymes 2

Question 1

Effect of substrate concentration

At low substrate concentrations [S]

If the number of enzyme molecules [E] remains constant

Answer 1

The number of substrate molecules present [S] determines how fast the reaction takes place

straight line graph (plotting v against S)
V proportional to [S] (First order kinetics)

Question 2

Rate of reaction =

Answer 2

activity = IRV = v

Question 3

Effect of substrate concentration

At high substrate concentrations [S]

If the number of enzyme molecules [E] remains constant

Answer 3

The number of enzyme molecules present [E] determines how fast the reaction takes place

V proportional to [S]. First order kinetics = straight line graph

V proportional to [S] (Zero order kinetics) = plateu

Question 4

michaelis-mentin kinetics

equation (dont need to memorise)

Answer 4

v= Vmax [S] / Km + [S]

1. v= IRV at a specified [S]
2. vmax= maximum IRV attainable by the enzyme under given concentrations
3. [S] = substrate concentration
4. Km = michaelis constant

Question 5

michaelis-mentin kinetics
equation
Km
3 points

Answer 5

1. Km = [S] at half V max
   (half maximum rate or 1/2 v max)
2. indicator of affinity of enzyme for its substrate
3. high Km - low affinity
   low Km high affinity

Question 6

Michaelis-Menten kinetics

3 points

Answer 6

1. allows to compare reactions
2. Plateau on Michaelis-Menten graph only truly reached at infinitely high [S]
3. Cannot carry out experiments at those high concentrations in lab
   - > alternative

Question 7

alternative / derivation of michaelis menten

2 points

Answer 7

1. inverse of the Michaelis-Menten equation

2. Lineweaver-Burk equation

Question 8

Lineweaver - Burke

4 points

Answer 8

1. 1/v plotted against 1/[S]
2. y intercept = 1/Vmax
3. X intercept = -1/Km
4. Gradient = Km/Vmax

not expected to repember equation

Question 9

Key features of Michaelis-Menten kinetics

6 points

Answer 9

1. Km measures the stability of the ES complex
2. Km is in units of concentration (M)
3. The lower the ES stability the higher the Km i.e. the more substrate you need to stabilise the ES complex.
4. Km = the value of [S] that causes V= ½ Vmax
5. Vmax is the fastest rate at which the enzyme can work
6. Vmax only occurs at infinite [S] (difficult to truly measure

Question 10

2 Types of enzyme inhibitors

Answer 10

1. irreversible

2. reversible

Question 11

3 types of reversible enzyme inhibitors

Answer 11

competitive *
non-competitive *
uncompetitive

Question 12

Irreversible Inhibitors

6 points

Answer 12

1. Bind irreversibly to enzyme
2. Usually bind via a covalent bond
3. Bind to an amino acid side chain at or near the active site
4. Commonly bind to either Ser (-CH2-OH) or Cys (-CH2-SH) side chains
5. Binding permanently inactivates the enzyme
6. Usually prevents substrate binding

Question 13

irreversible inhibitor e.g. DFP (nerve posion)

2 points

Answer 13

1. Covalently binds to a Ser residue in acetylcholine esterase
2. Prevents breakdown of the neurotransmitter acetylcholine -> stimulates nerves

Question 14

irreversible inhibitor e.g. penicillin (antibiotic)

2 points

Answer 14

1. Covalently binds to a Ser residue in glycopeptide transpeptidase
2. Prevents synthesis of bacterial cell wall peptidoglycan

Question 15

Competitive Inhibitors

5 points

Answer 15

1. Reversible inhibitor
2. Compete with substrate for access to active site
3. Often have structure similar to substrate
4. When bound to enzyme prevents binding of substrate
5. Can be overcome by increasing [S] until it out-competes inhibitor

Question 16

see desktop. need to be able to reproduce competitive inhibitor and non competitive graphs in exam

Answer 16

.

Question 17

Non-Competitive Inhibitors

7 points

Answer 17

1. Reversible inhibitor
2. Bind at a site other than the active site
3. Bind before or after the substrate binds
4. Do not prevent the substrate from binding
5. Prevent catalytic act from taking place
6. Cannot be overcome by increasing [S]
7. Can be removed by repeated dialysis

Question 18

Allosteric enzymes

5 points

Answer 18

1. Composed of two or more subunits
2. Binding of substrate to one sub-unit initiates a change of form
3. The substrate binds to the second and subsequent sub-units more readily
4. Results in sigmoidal kinetics when v plotted against [S]
5. The subunits can exist in one of two forms
   
   • One form has a low affinity for the substrate
   • One form has a high affinity for the substrate

Question 19

see desktop for allosteric graphs

Answer 19

.

Question 20

Activation and Inhibition of Allosteric Enzymes

4 points

Answer 20

1. Each sub-unit has one or more effector binding sites
2. Specific non-substrate molecules bind to specific effector binding sites
3. Positive effector, allosteric activator- Increase the overall enzyme activity
4. Negative effector, allosteric inhibitor- Decrease overall enzyme activity

Question 21

Enzyme Classification

Oxidation/reduction reactions

Answer 21

Oxireductases

Question 22

Enzyme Classification

Transfer of functional groups from one compound to another

Answer 22

Transferases

Question 23

Enzyme Classification

Split bonds by addition of water

Answer 23

Hydrolases

Question 24

Enzyme Classification

Add groups across a double bond

Answer 24

Lyases

Question 25

Enzyme Classification

Catalyse isomerisation reactions

Answer 25

Isomerases

Question 26

Enzyme Classification

Form new bonds with energy provided by cleavage of ATP

Answer 26

Ligases