learning outcomes

Question 1

describe the way in which enzymes catalyse reactions.

Answer 1

They lower the activation energy by providing alternative pathways- non covalent bonds formed with substrate called ‘the binding energy’- along which the reaction can take place through intermediates.
It allows spontaneous reactions to take place faster and equilibrium to be reached faster.
It does not make reactions spontaneous or alter the position of equilibrium.
The order in which the reaction takes place can either be ordered, random or a ping-pong reaction.
they catalyse reactions by Desolution, Induced fit or reducing entropy.

Question 2

describe the way in which the rate of reaction may vary as a result of changes in enzyme/substrate concentration.

Answer 2

• initially when Substrate concentration low, addition of small amounts of substrate will increase V0 linearly
• at high concentrations the line will plateau
• when enzyme completely saturated- there will be no further changes

Question 3

De ne the term ‘Michaelis constant’, describe how it may be experimentally estimated and describe what a knowledge of the constant may allow to be deduced about the nature of the catalysed reaction.

Answer 3

Michaelis constant= Km
Km=vmax/2. 
Can be estimated from curve. 
When Km high- low affinity
When Km low-high affinity.

Question 4

Describe the molecular nature of competitive and non-competitive inhibition of enzyme-catalysed reactions.

Answer 4

competitive- competes with substrate for active site.
can be overcome increasing the concentration of substrate. Km decreases. Vmax stays the same.
Non-competitve- binds to site on enzyme which is not active site. Km stays the same because active site is still free. Vmax decreases.

Question 5

Discuss the operation of allosterically-regulated and covalently-modifed enzymes.

Answer 5

metabolites bind to allosteric sites to act as inhibitors or activators. It is an example of non-competitive inhibition.
Explained by 2 models:
1. concerted model:
-when no substrate not bound, the subunits flip between conformations.
- all subunits must be in same configuration
-when substrate binds to one sub-unit it causes other sub-units to change their conformation making it more likely for other substrates to bind.
-increases affinity so explains sigmoidal shape of lineweaver-burk
2. sequential
- when no substrate bound, complex does not flip
-conformations different
-when one substrate binds alters shape in one other sub-unit making it more likely for one other substrate to bind.

Covalent modification revolves around phosphorylation of enzymes. alters the enzymes depending on their needs and function it needs to carry out.

Proteolytic cleavage- enzymes exist as pro proteins. can be cleaved by proteases to form active enzymes when required. e.g. digestive enzymes which would otherwise digest the gut wall.