Enzymes (lectures 6,7&8)

Question 1

What are enzymes?

Answer 1

proteins
catalysts
show specificity
sensitive to changes in physical & chemical environments

Question 2

Examples of protein function

Answer 2

structure 
contractile 
transport 
defence 
storage 
regulatory 
catalytic

Question 3

What are tamiflu & relenza?

Answer 3

neuraminidase inhibitors
neuraminidase removes neurotic acid residues from the surface of host cells to ease the release stage of the virus lifecycle
a mutation in the active site leads to tamiflu resistance

Question 4

What does penicillin do?

Answer 4

binds to the enzymes that make the bacterial cell wall

Question 5

What is the equilibrium constant (Keq)

Answer 5

at equilibrium the ratio of the concentration of products over reactants
Keq = [C][D] / [A][B]
Keq is dependent on the energy values of reactants & products

Question 6

Gibbs Free Energy

Answer 6

the equilibrium constant of a reaction is correlated with the energy released by the reaction
Keq is proportional to delta G
Free energy = - RT(lnKeq)
for a reaction to be feasible free energy < 0
the bigger the free energy change the further over the equilibrium constant is

Question 7

transition states

Answer 7

is is thermodynamically feasible for a high energy molecule to convert to a low energy molecule so releasing energy
intermediate ‘transition’ states in chemical reactions have a higher energy than reactants
so energy is requires to generate this reaction

Question 8

Activation energy

Answer 8

determines the reaction rate

all catalysts lower AE

Question 9

Enzyme-Substrate complex

Answer 9

enzyme (E) forms a complex with the substrate (S) before catalysing the conversion to product (P)
E + S –> ES –> P + E
formation of the ES complex leads to the alternative transition state

Question 10

catalysis mechanisms

Answer 10

Proximity - closeness of substrate molecules
Orientation - correct relative orientation of substrates
Strain/distortion - binding puts strain on bond making it easier for reaction to occur
Acid-base catalysis - H+ donated/accepted or OH- generated
Covalent catalysis - temporary covalent bond between E & S

Question 11

Specificity

Answer 11

enzymes can only bind to certain substrates
due to interactions between specific structural regions on the E & S
can be explained in 2 ways:
1) Lock & key - binding site has complementary shape
2) Induced fit - contact between part of the binding site & the S induces a change in shape of the active site

Question 12

Kinds of specificity

Answer 12

Absolute - only works on 1 molecule
Bond - works on a group of molecules but targets a specific bond
Group - targets a specific group
Stereo - very stereo specific

Question 13

6 types of enzyme reactions

Answer 13

1) Oxidation/reduction - oxidise or reduce a molecule
2) Transferases - transfer a group
3) Hydrolases - add hydroxyl groups
4) Lyases - elimination reaction
5) Isomerases - change type of isomer
6) Ligases - create covalent bond

Question 14

What is the turnover number (Kcat)?

Answer 14

The no. of substrate molecules that can be converted to product by 1 enzyme in 1 second

Question 15

How can we measure enzyme activity?

Answer 15

Either measure:
1) Disappearance of substrate
2) Appearance of product
if either is coloured we can use a spectrophotometer

Question 16

The effect of varying substrate concentrations

Answer 16

At low [S], the no. of substrate molecules present determines how fast the reaction takes place
As [S] increases there is more chance of it colliding with an enzyme & for catalysis to occur
As [S] increases no. of ES complexes increases

Question 17

How is rate linked to [S] ?

Answer 17

Rate of reaction = activity = velocity = V
V is proportional to [S]
First order kinetics
At high [S] all the enzymes will have bound substrates so will be working at their maximum rate

Question 18

What is the Michaelis-Menten equation?

Answer 18

V = Vmax[S] / (Km + [S])
Where:
V = rate at specified [S]
Vmax = max value of V attainable by the enzyme under given conditions
Km = Michaelis constant = [S] at half Vmax

Question 19

What does the Michaelis-Menten equation show?

Answer 19

Km measures stability of the ES complex - indicates affinity of enzyme for its substrate
High Km = low affinity
Low Km = high affinity
Km is in units of concentration (M)
Vmax is the fastest rate at which the enzyme can work
Vmax only occurs at infinite [S]

Question 20

What are enzyme inhibitors?

Answer 20

reduce the rate of reaction
can either be:
1) Irreversible
2) Reversible - competitive, non-competitive & uncompetitive

Question 21

Irreversible inhibitors

Answer 21

binds irreversibly to the enzyme
usually bind via a covalent bond
bind to an amino acid side chain at or near the active site
commonly bind to either Ser or Cys
binding permanently inactivates the enzyme
usually prevents substrate binding

Question 22

examples of irreversible inhibitors

Answer 22

DFP
Aspirin
Penicillin

Question 23

Competitive inhibitors

Answer 23

compete with S for access to active site
often have similar structure to S
when bound to E stops S binding
can be overcome by increasing [S] until it outcomes inhibitor
Km increases but Vmax remains unchanged

Question 24

Examples of competitive inhibitors

Answer 24

Tamiflu
Acarbose
Statins

Question 25

Non-competitive inhibitors

Answer 25

bind away from the active site
modify the reaction rate
still binds substrate with some affinity
influences capacity to catalyse reaction
Km unaltered but Vmax decreases

Question 26

Uncompetitive inhibitors

Answer 26

occurs with multi substrate reactions
not important for drug therapy
binds only to ES complex
Km decreases & Vmax decreases

Question 27

When will pH decrease enzyme activity?

Answer 27

If overall structure of active site is altered

If group involved in binding to S changes charge