Enzymes & Coenzymes

Question 1

Define the importance of enzymes.

Answer 1

• ‘nature’s catalysts’, carry out all reactions that occur in cell
• many inherited diseases cause by mutations in key enzymes
• measurements of certain enzymes in blood used to diagnose diseases
• used commercially in food products & detergents
• used in drug synthesis.

Question 2

Give the 6 main groups of enzymes.

Answer 2

• oxidoreductase
• transferase
• hydrolase
• lyase
• isomerase
• ligase

Question 3

What is the deltaG for a favourable reaction?

Answer 3

negative (products have lower energy than reactants)

Question 4

The speed of the reaction is determined by the…

Answer 4

activation energy.

Question 5

What does the Arrhenius eqn define?

Answer 5

relationship between Ea & the reaction rate constant.

Question 6

Define transition state.

Answer 6

transient high-energy intermediate species in the conversion of substrate to product.

Question 7

Catalysts _ Ea but _ deltaG

Answer 7

Catalysts reduce Ea but do not change deltaG

Question 8

Define the role of an enzyme as a catalyst.

Answer 8

• provides a specific environment of the substrate where the reaction is more favourable.
• the pocket on the enzyme where this occurs is called the active site.
• the enzyme acts by lowering the Ea.
• the enzyme is not sued up in the process, acts to accelerate conversion of S>P (and P>S)
• ES & EP are reaction intermediates
• E + S <> ES <> EST <> EP <> E + P
• if the enzyme binds tightly to substrate the free energy of that complex decreases, meaning an even bigger Ea is required to get that complex to the transition state.

Question 9

How is the enzyme pocket designed to the compliment the reaction transition state?

Answer 9

• the same forces that stabilize protein folding also stabilize EST complex.
• in some enzyme mechanisms, a transient covalent bond is formed during the reaction.
• the energy derived from the complex formation is called the binding energy.
• enzyme is complimentary to the transition state.

Question 10

Define co-enzymes.

Answer 10

non-protein organic molecules required for catalysis

Question 11

Give examples of common co-enzymes.

Answer 11

Coenzyme A, ATP< TPP, S-adenyl methionine (SAM), PLP, lipoate, GSH, biotin.

Question 12

Define co-factors.

Answer 12

inorganic substances that are required for catalysis

Question 13

Define holoenzyme.

Answer 13

combination of protein & co-enzyme/co-factor

Question 14

Define apoenzyme

Answer 14

protein without co-enzyme/co-factor

Question 15

Define the purpose of co-enzymes/co-factors.

Answer 15

allow enzymes to access chemistries that amino acid side chains cant access.

Question 16

Describe general acid-base catalysts.

Answer 16

-stabilise unstable charged transition states by transferring protons to & from them, decreasing their free energy
-amino acid side chains act as weak proton donors or acceptors:
Glu, Asp, Lys, Arg, His, Cys, Tyr.

Question 17

Describe metal ion catalysts.

Answer 17

• interactions between enzyme bound metal & substrate can orient a substrate for reaction; stabilise charged reaction states; mediate oxidation0reduction reactions.
• 1/3 of all known enzymes require a metal for catalytic activity.

Question 18

Describe covalent catalysis.

Answer 18

-formation of transient covalent bond between enzyme & substrate
-covalent complex undergoes a reaction to regenerate the free enzyme
-in comparison with the uncatalyzed reaction, the formation of the covalent bond offers an alternative route to the same products that has a lower Ea.
-detailed case study: serine proteases
Endoproteases that hydrolyse the peptide bond, have a catalytic triad, aspartate, histidine & serine in their active sites.

Question 19

Give the protease mechanism.

Answer 19

1. ES complex- Michaelis Complex
2. First transition state- Tetrahedral intermediate
3. Acyl enzyme intermediate
4. Acyl enzyme water complex
5. Second transition state- Tetrahedral intermediate
6. Free enzyme

Question 20

Where does specificity originate from & why?

Answer 20

from the binding pocket by the active site that only accommodate certain residues depending on the pockets conformation & residues. After these residues is where the enzyme will cleave the chain.