Enzymes Flashcards
What are enzymes?
Biological catalysts.
What type of protein are enzymes?
Globular.
Are enzymes specific?
Yes.
What conditions are required for enzyme action?
Mild pH/temp.
Can enzyme action be controlled?
Yes- action like dimmer switches.
What is a ribozyme?
Catalytic RNA component with no protein component.
What is a cofactor?
Non-protein component needed for function.
Presence = Function
What is a coenzyme?
Complex organic molecule, usually produced from a vitamin (FAD/NAD etc).
What is a prosthetic group?
Cofactor that is covalently bound to the enzyme.
What is the apoenzyme?
The protein component of an enzyme that contains a cofactor.
What is the holoenzyme?
The ‘whole’ enzyme- enzyme + cofactors.
What is the substrate?
The molecule which the enzyme acts on?
What is the active site?
The site at which the enzyme and substrate bind.
What do enzymes exist to do?
Increase the rate of biochemical reactions.
What reactions do enzymes act on?
Increase the rate of spontaneous reactions.
Don’t make non-spontaneous reactions spontaneous
What effect do enzymes have on activation energy?
Lower the Ea.
What effect do enzymes have on equilibrium?
They do not alter equilibrium but they accelerate movement towards it.
Does the lock/key idea exist?
No.
What is the active site actually complementary to?
The transition state.
How is activation energy reduced? (3 steps)
Entropy reduction
Desolvation
Induced fit
How does entropy reduction reduce Ea?
Molecules in free solution will only react through bumping into each other. Enzymes force the substrate into a correct orientation by binding them in the formation they need to be in for the reaction to occur.
How does desolvation reduce Ea?
Weak bonds between the substrate and enzyme replace the bonds between the substrate and aqueous solution.
How does induced fit reduce Ea?
A conformational change occurs in the protein structure when the substrate fits which allows progression to the transition state.
What are enzymes analysed by?
Enzyme kinetics
Mutagenesis
3D structure
What is the Michaelis-Menten equation?
An equation which accounts for the hyperbolic curve seen within enzyme kinetics.
What is Km?
Km is the substrate concentration when reaction rate is at half its maximum. (Half of all binding sites occupied)
What is Vmax?
Km is the concentration of enzyme required for reaction rate to reach its maximum. (All binding sites occupied).
What are isoenzymes?
Isoenzymes are enzymes that catalyse the same reaction but are structurally different.
What reaction do glucokinase/hexokinase catalyse?
Phosphorylation of Glucose to Glucose-6-Phosphate (G6P).
Where does glucokinase act?
Liver
Where does hexokinase act?
Muscle cells in rest of body (not liver).
What is the Km/Vmax of glucokinase?
Km = High Vmax = High
When does glucokinase increase?
After meal- when blood sugar increases.
What is the Km/Vmax of hexokinase?
Km = Low Vmax = Low
How do glucokinase/hexokinase actions differ?
When blood sugar increases after meals, glucokinase activity increases to maintain healthy levels. However, hexokinase doesn’t respond as it is already working at its Vmax.
Hexokinase acts in muscle cells across the rest of the body. When blood sugar decreases, glucogenesis releases glucose from the liver but glucokinase can’t catalyse glucose back into G-6-P under these conditions so it is used outwith.
What can enzymes in the wrong place indicate?
Pathology.
How is enzyme concentration given clinically?
Arbituary value- e.g. 100% = normal.
How can isoenzymes be studied?
Gel electrophoresis.
How does gel electrophoresis work?
Separates plasma proteins according to pH.
Why is gel electrophoresis useful clinically?
Can demonstrate abundance or lack of particular proteins.
What does catalysis of reactions with multiple substrates usually require?
Transfer of groups from one substrate to another.
How can transfer of groups from one substrate to another occur?
Ternary complex formation (Ordered/Random)
No formation of ternary complex
What are the 2 types of enzyme reaction involving a ternary complex?
Ordered
Random
What happens when no ternary complex is formed?
No intermediate complex formation.
What are allosteric enzymes?
Allosteric enzymes are multi sub-unit complexes.
What do the multiple sub-units of allosteric enzymes usually have?
Multiple binding sites.
What can now substrate binding to one sub-unit cause?
Can cause changes in binding at other sub-units.
What is co-operativity?
Co-operativity is when the the shape of one subunit of an enzyme (consisting of several subunits) is altered by the substrate or some other molecule so as to change the shape of a neighbouring subunit.
In what molecule is cooperatively often seen?
Haemoglobin- oxygen binding.
What can affect enzyme function?
Temperature
pH
Enzyme inhibition
How does temperature affect enzyme function?
Increasing the temperature increases the energy that particles have, and leads to increased movement so they are more likely to collide with each other. However, increased temperature can eventually denature enzymes.
How does pH affect enzyme function?
Increasing the pH changes the charge of amino acids- if they are changed in a way that does not aid binding they will cease enzyme function and cause denaturation.
What are the 3 types of enzyme inhibition?
Competitive inhibition
Uncompetitive inhibition
Non-competitive inhibition
How do competitive inhibitors work?
Block action by MIMIC of the substrate action on the active site. Bind non-covalently.
What do competitive inhibitors usually resemble?
Substrate- complementary to active site.
How do competitive inhibitors affect affinity?
Lead to a decreased affinity between the genuine substrate and the active site, so the Km substrate-complex increases.
How can competitive inhibitors be overcome?
Increasing substrate concentration, so the same Vmax can be achieved.
How do competitive inhibitors relate in terms of Km and Vmax?
Exhibit increased Km values but can obtain the same Vmax.
How do uncompetitive inhibitors work?
Uncompetitive inhibitors work by binding to the enzyme when the substrate is already locked into the active site.
How do non-competitive inhibitors work?
Non-competitive inhibitors work by binding non-covalently to the enzyme and usually attach at an area that is not the active site.
How do non-competitive inhibitors affect Km and Vmax?
Km will remain the same because the substrate can still bind to the active site. Vmax increases.
What type of inhibition is usually irreversible?
Covalent.
How does feedback inhibition work in enzymes?
Feedback inhibition can work to control and is triggered by a build-up/lack of substances which can allow inhibition to occur on a regulatory molecular basis.
What is the major attraction in enzyme/substrates?
Transition state.
Why do inhibitors try to mimic the transition state rather than the substrate?
Transition state is the strongest attraction- allows most effective pathway of molecular inhibition.
What are allosteric effectors?
Usually cell metabolites that bind non-covalently to a site on the enzyme that is not the active site.
What are allosteric effectors an example of?
Non-competitive inhibition.
What do allosteric effectors result in?
Alteration of enzyme structure, subsequently having an effect on the efficacy of substrate binding and reaction. Some are activators and some are inhibitors.
What are the 2 models of allosteric enzyme kinetics?
Concerted model
Sequential model
Describe the concerted model of allosteric enzyme kinetics.
– Each sub-unit can exist in two different conformations
– One binds substrate well the other doesn’t
– with no substrate the enzyme flips between the two conformations
– All sub-units must be in the same conformation (so they flip in concert)
Describe the sequential model of allosteric enzyme kinetics.
- Substrate binding causes a change in one sub-unit
- This causes a change in another sub-unit allowing it to bind S more readily
- Like the first model, binding of some substrate sensitises the enzyme to bind more
What does the sequential model assume?
– No flipping between different conformation states
– Sub-units exist in a conformation that can bind S, activators, inhibitors
– It is the binding that causes a conformational change
What is covalent modification?
The modification of an enzyme through covalent addition of a molecule.
Give an example of a common covalent modification.
Phosphorylation.
What is protoleatic cleavage?
Protoleatic cleavage occurs when an enzyme exists in a precursor non-active form (proenzyme/proprotein) and is activated through cleavage.
Give an example of a common protoleatic cleavage.
Insulin from precursor proinsulin.
