Chapter 2 - Enzymes Flashcards
Define Enzymes and how they function
Enzymes are protein catalysts that accelerate reactions by reducing the activation energy without being consumes or irreversibly altered by the reaction.
Note that enzymes only affect the rate of reaction and not the amounts of products formed, the overall energy change of the reaction (∆G), or the equilibrium constant (K).
What is the lock and key theory of Enzyme specificity?
The theory of enzyme catalysis stating that the active site’s structure is complementary to the structure of the active site. It suggests that the enzyme’s active site (lock) is already in the appropriate conformation for the substrate (key) to bind, and that no alteration of the tertiary or quaternary structure is necessary upon binding of the substrate.
What is the induced fit hypothesis of Enzyme specificity?
The theory of enzyme catalysis stating that partial binding of a substrate to an enzyme alters the structure of the enzyme so that its active site becomes complimentary to the structure of the substrate, enabling binding. Importantly, a substrate of the wrong type will not cause the appropriate conformational shift in the enzyme, and so the active site will not be adequately expose, the transition state is not preferred, and no reaction occurs.
What are endothermic and exothermic reactions? Describe these using a reaction diagram.
An endothermic reaction is one that requires energy input, whereas an exothermic reaction is one in which energy is given off. Endothermic reactions have a positive Gibbs free energy change (∆G>0) while exothermic reactions have an negative Gibbs free energy change (∆G<0).
What is activation energy and how does it relate to enzymes
Activation energy is the difference in potential energy between the initial state (reactants) and the transition state. Reactants must overcome the activation energy in order to undergo the reaction. Enzymes act as catalysts to decrease the activation energy of a reaction by stabilizing the transition state.
Activation energy is often abbreviated ∆G‡ and is related to the rate constant by the Arrhenius equation:
k = A e^{–Ea/(R T)}
where A is a constant, Ea is the activation energy, k is the rate constant, R is the gas constant, and T is the temperature in Kelvin.
Define free energy change and how it relates to enzymes
Free energy change is the difference in potential energy between the initial state (reactants) and the final state (products). It is often abbreviated ∆G. A negative ∆G indicates a spontaneous (exothermic) reaction, whereas a positive ∆G indicates a nonspontaneous (endothermic) reaction. Enzymes do not alter the free energy change of a reaction.
What are seven key features of enzymes
Enzymes
1) lower the activation energy
2) Increase the rate of the reaction
3) do not alter the equilibrium constant
4) are not changed or consumed in the reaction (and so will appear both in the reactants and products)
5) Are pH and temperature sensitive, with optimal activities at specific pH ranges and temperatures
6) do not affect the overall ∆G of the reaction
7) are specific for a particular reaction or class of reaction
Define Holoenzyme
An enzyme containing all necessary cofactors and so is catalytically active.
Mnemonic – think Whole-o-enzyme
Define Apoenzyme
An enzyme that requires cofactors in order to become catalytically active
Describe two common types of cofactors
Two common types of cofactors are small metal ions (coenzymes) and small organic groups. Cofactors can be attached in a variety of ways, from weak noncovalent interactions to strong ones. Tightly bound cofactors are known as prosthetic groups.
What is an allosteric enzyme?
Allosteric enzymes are enzymes with multiple binding sites, including the active site as well as at least one other site that can regulate the availability of the active site. Allosteric enzymes alternate between more than one conformation, usually an active form or an inactive form. Binding to the allosteric site can result in allosteric activation or inhibition, depending on the conformational shift induced by the molecule bound to the allosteric site (regulator).
Define Regulator
A regulator is a molecule other than the substrate that can bind to the allosteric site of an enzyme and either increase its activity (allosteric activator) or decrease its activity (allosteric inhibitor).
An activator will result in a shift that makes the active site more available for binding to the substrate, while an inhibitor will make it less available.
What is the Michaelis constant? How is it related to enzyme kinetics and reaction velocity?
Michalis constant, Km, is the ratio of the breakdown of an enzyme-substrate complex to its formation in simple Michaelis-Menton reactions of the form:
E + S ES E + P
A low Km indicates a strong affinity between an enzyme and its substrate. When the reaction rate is equal to half the maximum velocity (1/2 Vmax), Km = [S] and can be understood to be the point at which half the enzymes active sites are full. When [S] < Km, changes in [S] will greatly affect the reaction rate; when [S] > Km reaction rate approaches Vmax and changes in [S] have little affect on reaction rate.
Describe how noncompetitive inhibitors function. How do they alter Michaelis-Menton enzyme kinetics?
A noncompetitve inhibitor is a molecule that inhibits the activity of an enzyme by binding to a regulatory site on the enzyme, thereby changing the conformation of the enzyme.
Because these inhibitors do not directly compete with substrate, increasing substrate concentration usually has little effect on the catalytic rate.
Noncompetitve inhibitors decrease Vmax but leave Km unchanged.
Describe how competitive inhibitors function. How do they alter Michaelis-Menton enzyme kinetics?
A competitve inhibitor is a molecule that inhibits the activity of an enzyme by directly binding to the active site of the enzyme.
Because this type of inhibitor competes with substrate for the active site, it is usually reversible and can be overcome by increasing the substrate concentration.
Competitive inhibitors increase Km (decrease substrate affinity) but leave Vmax unchanged.
