Topic C: ENZYMES Flashcards
What are the common properties of enzymes?
- Enzymes are highly specific. This is due to the specificity of the enzymes active site which recognises specific groups of atoms or a particular type of bond present in the substrate.
- Enzymes are effectve in small amts as they have high turn-over rates.
- Enzymes remain chem unchanged at end of rxn. Enzymes can be reused repeatedly without undergoing permnt chem dmg.
- Enzymes are affected by factors such as [substrate],[enzyme], temp and pH
- Some enzymes req aid of cofactors to perform their functions. Cofactors are non protein components that are required for the functioning of the enzyme. Cofactors may either remain chemically unchanged at the end of a rxn or be regened by a later process. The enzyme-cofactor complex is a HOLOENZYME. Enzyme (no cofactor)- APOENZYME.
- Enzyme activity is tightly regulated. Enhanced by activators or reduced by inhibitors.
What are the 3 types of cofactor?
- Inorganic ions
- Coenzymes are organic moleqs which bind loosely to enzymes.
- Prosthetic groups are organic moleqs which remain tightly bound to enzymes
What is the Active Site?
3D conformation with an active site.
Formed by 3-12 AA from different parts of 1 polypp chain.
Substrate binding site: AA residues that recog and bind to substrate, determining enzyme specificity
Catalytic Site: AA residues that catalyses the reaction when substrate is bound.
What is the enzyme-substrate complex?
When substrate binds to active site of enzyme, ES complex formed.
Substrates are held in active site by weak bonds such as H bonds, ionic bonds and hydrophob interxns
R grps on some AA that make up active site catalyse conversion of substrate to product.
Once products are formed, they are no longer complementary to the active site and will leave the enzyme.
What is the lock-and-key hypothesis?
Enzyme acts as a lock and substrate as a key, which fits precisely. Active site of an enzyme is PERFECTLY COMPLEMENTARY to substrate in terms of shape, size, charge and orientation.
Substrate binds to enzymes active site to form ES complex.
More probable for enzymes that only work on ONE type of substrate.
What is the induced fit hypothesis?
Enzymes work in a more flexible manner.
Active site is not perfectly complementary to the substrate in terms of shape,size and orientation.
Upon forming some bonds with the substrate, enzyme changes its shape, which leads to a precise fit to form ES complex.
More probable for enzymes that work on a group of closely related substrates.
How are enzyme kinetics measured?
Measured by the amt of product formed/unit time or amt of substrate depleted/unit time.
It is important that only one specific factor varies while all other factors are kept constant and at optimum for particular enzyme.
It is important to measure the initial rate of rxn as the concentration of substrate would decrease once the rxn starts.
What is Michaelis constant?
The Michaelis constant (Km) of an enzyme is the substrate concentration at which the rate of rxn catalysed by the enzyme equals to half its maximum rate.
It is the indication of the affinity of the enzyme for its substrate moleqs. Low Km means that there is a high affinity between enzyme and substrate. High Km means that there is a low affinity between enzyme and substrate.
How does substrate concentration affect enzyme activity?
Graph Description: At low [substrate], there is a linear increase in rate of rxn with increase in substrate concentration.
As [substrate] increases, the increase in rate of rxn slows.
Any further increase in [substrate] will not increase the rate of rxn. Rate of rxn plateaus off and max rate of rxn is reached.
Explanation: AT LOW [SUBSTRATE], increase in substrate concentration increases the number of effective collisions btw the enzyme and substrate moleqs. Not all active sites of the enzyme moleqs present will be occupied at any one time. There is an increase in the number of ES complex formed/unit time. Proportional increase in rate of formation of products. [Substrate] lim R.
AT HIGH [SUBSTRATE], further increase in substrate concentration results in all active sites of enzymes being occupied by the substrate at any 1 time. Max no. of ES complex formed/unit time. Other factors like enzyme concentration is now lim rate of rxn.
How does enzyme concentration affect enzyme activity?
At low [enzyme], there is a linear increase in RoRXN with increase in [enzyme]. As [enzyme] increases, increases in RoRXN slows. Any further increase in [enzyme] will not increase RoRXN. RoRXN plateaus off and max RoRXN is reached.
What is reversible inhibition?
If the EI complex can dissociate, the inhibition is reversible. Association of enzyme with inhibitor is a loose one and can be easily removed. Removal of inhibitor restores the activithy of the enzyme to normal. This is reversible only when the inhib and enzyme are held by weak bonds (H bonds). These weak bonds eventually break as neighbouring moleqs collide and the inhibs leave the enzyme moleqs. It can occur as either competitive or non-competitive inhib. They affect the Vmax and Km on the enzyme differently.
What is competitive inhibition?
Comp. inhibitors are structurally similar to the substrate moleqs.
This binds to the active site of the enzyme and thus competes with the substrate for the active site.
This reduces the number of active sites available for the substrates to bind and form ES complexes.
Km of the enzyme will increase in the presence of competitive inhibition. Vmax can be reached eventually at a higher [substrate]. When substrate concentration is low, it is more likely for enzyme to collide w CompInhib moleqs and form EI-complex. Thus, the no. of active sites available for substrate moleqs will be reduced and rate of ES complex formation will decrease. When [substrate] is high, it is more likely for enzyme moleqs to collide with substrate moleqs and form ES complexes.
What is non-competitive inhibition? (km)
Non-comp inhibs are structurally dissimilar to the substrate molecule. Binds at a site away from the active site. This interaction alters specific 3D conformation of the enzyme moleq such that active site is distorted and no longer complementary to substrate, thus unable to bind to the substrate properly.
Km of the enzyme remains UNCHANGED in presence of non-comp inhibs since it does not compete with substrate molecules for active site.
Vmax is LOWERED as noncomp-inhibs reduce number of functional enzymes.
What is irreversible inhibition?
If the EI complex cannot dissociate, the inhib is irreversible. They bind permanently to the enzyme and cause permanent dmg to the enzyme, such that the enzyme is unable to carry out catalytic activity.
Strong covalent bonds are formed between enzyme and inhib.
What is allosteric inhibition?
Allosterically regulated enzymes are constructed from 2 or more subunits.
The entire protein complex alternates between 2 different shapes, one catalytically active and one inactive. In allosteric regulation, an activator or inhib binds to an allosteric site, often located where subunits join. The binding of an activator to a regulatory site stabilizes the shape that has a functional active site. The binding of an inhibitor stabilises the inactive form of the enzyme.
