Proteins - Enzymes L4 Flashcards
What is an Enzyme?
- Protein (ribozymes are RNA)
- Polypeptide chain is folded to form a 3-D structure which has a pocket where substrates bind
and reactions occur: THE ACTIVE SITE - Amino acids from the enzyme and metal ions
at the active site interact with specific parts of
the substrate to bind it in a specific orientation
(this gives rise to substrate specificity)
What is the active site of an enzyme, and how does it contribute to enzyme specificity?
The active site of an enzyme is a pocket where substrates bind and reactions occur.
Substrates form non-covalent bonds (e.g., hydrogen bonds, electrostatic interactions, hydrophobic interactions) with specific amino acid side chains, peptide bonds, and metal ions in the active site.
These interactions position the substrate correctly for the reaction, giving the enzyme its specificity.
How can you write a rate equation for a simple reaction, and what does it represent?
A rate equation for a simple reaction represents the rate of formation of product or loss of substrate with time.
It is written as the product of the rate constant and substrate concentration(s).
For reversible reactions, it includes the forward rate constant minus the reverse rate constant times the concentrations.
Explain the concept of equilibrium in a reversible reaction and the role of the equilibrium constant.
In a reversible reaction at equilibrium, there is no net formation of products.
The equilibrium constant is the ratio of product concentrations to substrate concentrations or the ratio of the forward rate constant to the reverse rate constant.
DG°, the standard free energy change, is related to the equilibrium constant.
What is the significance of ΔG° and its relationship with the equilibrium constant?
ΔG° is the standard free energy change for a reaction at equilibrium.
It is related to the equilibrium constant:
ΔG° = -RT ln(K),
where K is the equilibrium constant.
If ΔG° is negative, the reaction will proceed spontaneously.
What is the difference between ΔG and ΔG‡, and which one determines the rate of a reaction?
ΔG‡ is the free energy of activation, which is always positive and determines the rate constant for a reaction.
ΔG can be positive or negative and indicates whether the reaction is spontaneous or not.
Describe the concept of catalytic power and its significance.
Catalytic power is the ratio of the rate constant of a catalyzed reaction to an uncatalyzed reaction.
It shows how much faster a reaction occurs in the presence of a catalyst.
Compare the lock and key model with the induced fit model of enzyme action.
The lock and key model explains substrate specificity but doesn’t explain catalysis.
The induced fit model explains both substrate specificity and catalysis.
Enzymes exist in two conformational states, with the high-energy state enabling the enzyme to put strain on the substrate bonds, stabilizing the enzyme-bound transition state.
Why do some enzymes require cofactors?
Coenzymes bound in enzyme active sites provide chemical groups for reactions that amino acid side chains cannot perform, enhancing the enzyme’s catalytic capabilities.
Explain the temperature profiles of enzyme-catalyzed reactions.
Enzyme-catalyzed reactions have bell-shaped temperature profiles.
Initially, as temperature increases, the reaction rate increases due to increased collision frequency.
However, above about 45°C, most enzymes denature, leading to a decline in the rate.
Why do many enzymes exhibit a bell-shaped pH profile, and what does pH optimum represent?
Many enzymes have a pH optimum because of amino acid side chains in their active sites that require specific protonation or deprotonation states for correct function.
The pH optimum is the compromise pH where the necessary side chain states are achieved, maximizing enzyme activity.
N.B. In enzyme-catalysed reactions, rate is called the velocity and reactant is called a substrate
N.B. In enzyme-catalysed reactions, rate is called the velocity and reactant is called a substrate
(a) Irreversible - 1st order reaction
e.g. A -> B where k is the rate constant for the reaction.
The rate equation is:
- d[A] = d[B] = k[A]
dt dt
- d[A] and d[B] are the rates of the reaction in terms of reactant depletion or product formation.
dt dt
(b) Irreversible - 2nd order reaction
e.g. A + B -> C
- d[A] = - d[B] = d[C] = k[A] [B]
dt dt dt
e.g. 2A -> C
- d[A] = 2 d[C] = k[A]2
dt dt
Effects of Temperature on Enzyme Activity
Enzyme with optimum
temperature ≈ 37°C
(i) Up to about 45°C - 60°C (for most enzymes) reaction rate increases due to:
- (a) more collisions between E and S
- (b) dependency of rate constants on temp.
For most enzymatic reactions, rate approx. doubles for each 10°
C rise in temp. Q10 = 2.
(ii) Above about 45°C, the decrease in reaction rate (enzyme activity) is due to denaturation of the enzyme (unfolding of the polypeptide chain).
(iii) Some enzymes are more thermally stable:-they come from organisms that live in hot environments e.g. Thermus aquaticus
