Enzymes Flashcards
What is an enzyme?
A globular protein which acts as a biological catalyst, controlling the rate of biological reactions.
What is a cofactor?
Non protein component required for activity.
What is a prosthetic group?
A non-protein component that is covalently bonded to or very tightly associated with the enzyme.
What is the difference between a cofactor and a prosthetic group?
A prosthetic group IS a cofactor (non- protein component) that is bonded to or tightly associated with the enzyme.
Cofactor may not need to be attached to the enzyme to aid with enzyme activity.
What is an apoenzyme?
The protein constituent of an enzyme to which the prosthetic group attaches.
What is meant by the term holoenzyme?
This is the whole enzyme (Apoenzyme + prosthetic group)
What is the active site of an enzyme?
The area on the enzyme to which the substrate attaches to and is acted upon.
What are the six classes of enzymes?
- Oxioreductases
- Transferases
- Lyases
- Hydrolases
- Ligases
- Isomerases
What type of reactions do oxioreducatases catalyse?
Transfer of electrons
What type of reactions do transferases catalyse?
Group transfers
What type of reactions do hydrolases catalyse?
Hydrolysis
What type of reactions do Lyases catalyse?
Formation of /Addition to double bonds
What type of reactions do Ligases catalyse?
Formation of C-C, C-S, C-N or C-O.
What type of reactions do isomerases catalyse?
Transfers within molecules
What three theories are used to explain enzymes reducing activation energy?
- Induced fit
- Desolvation
- Entropy reduction
What is meant by the induced fir theory of enzyme action?
Substrate binds to active site of enzyme
Enzyme undergoes conformational change so that active site is now specific in shape to substrate.
Reaction proceeds
What is meant by the entropy reduction theory of enzyme action?
Enzyme binds substrate molecules in correct orientation and close enough together to allow the breaking/ making of bonds required for the reaction.
Remember, if the enzyme binds these substrate molecules, they are no longer free in solution and so there has been a reduction in entropy.
What is meant by the desolvation theory of enzyme activity?
Weak bonds between the enzyme and the substrate replace Hydrogen bonds between the substrate and aqueous environment.
What is meant by Vmax?
This is the maximum rate of reaction which can occur for a particular enzyme concentration.
It occurs when all of the reaction sites are saturated with substrate.
Increase in substrate conc. after this point will have no effect.
What is Km?
The ratio of the rate constant of the conversion of ES to E + S to the rate constant for the conversion of E+S to ES.
When does Km occur?
When the initial rate of reaction is exactly half of that of the maximum rate of reaction. Here it gives a value for the substrate conc.
What does a large Km indicate?
Less stable ES complex, low affinity of enzyme to bind to substrate
What are isozymes?
Different proteins which catalyse the same reaction
E.g. Hexokinase and Glucokinase
What s the Michaelis- Menten equation?
Vo= (Vmax.[S])/(Km+[S])
What is an allosteric enzyme?
An enzyme with many different subunits, each containing an active site. Substrate binds by cooperative binding.
E.g. Haemoglobin
What is meant by cooperative binding?
Binding of a substrate to one sub-unit causes a conformational change which increases the ease of which a substrate binds to the next sub-unit and so in.
What shape is the kinetics curve on graph of reaction velocity against substrate concentration for an allosteric enzyme?
Sigmoidal
What are the models of action on which allosteric enzyme activity is based?
- Concerted model
2. Sequential model
Describe the concerted model of allosteric enzyme activity.
Each sub unit can exist in one of two different conformations
One conformation binds substrate molecules well, the other does not.
The subunits continuously flip IN CONCERT between the two different conformations
When a substrate molecule binds to one active site, it locks the other subunits in the binding conformation, making it easier for further subunits to bind.
Allosteric activators will stabilise the binding conformation. Allosteric inhibitors will stabilise the non-binding sub-unit confromation
Describe the sequential model of allosteric enzyme activity.
No flipping between conformations
Sub-units exist in a conformation which may bind activators, inhibitors and substrate molecules.
Substrate binding causes a change in one subunit, which causes a change in the next subunit, making it more likely to bind substrate.
This process continues.
Describe the action of competitive enzyme inhibitors.
These molecules resemble the substrate and bind to the active site (i.e. they compete with the substrate for the active site)
How do competitive inhibitors affect Vmax and Km?
Competitive inhibitors reduce the affinity of the enzyme for the substrate and so increase Km.
Increasing [S] can overcome the problem so Vmax is unchanged.
Describe the action of non-competitive enzyme inhibitors.
Non- competitive inhibitors bind non-covalently, away from the active site. They make the enzyme-substrate complex which forms inactive.
How do non-competitive inhibitors affect Vmax and Km?
ES complexes can still form and so Km is unchanged.
Increasing [S] will have no effect and so Vmax is decreased.
What are proproteins/ proenzymes?
These are inactive precursors to enzymes.
Cleavage by proteases generates the active form of the enzyme.
This is how digestive enzymes operate.
Name three ways in which enzymes can be modulated.
- Allosteric modulation
- Covalent modulation
- Proteolytic cleavage
