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])