Chapter 7 (Exam 2) Flashcards
When do enzymes change shape?
Upon substrate binding, which is a phenomenon called induced fit.
Active site forms a shape complementary to the substrate only after the substrate has been bound.
Substrate binding transiently changes the enzyme.
What is the reaction rate?
Change in the concentration of a reactant or a product with time (M/s)
What is initial velocity?
(V0). Determined by measuring product formation as a function of time and then determining the velocity soon after the reaction has started.
What is the velocity of the reaction?
Enzymatic reaction reached the maximum velocity Vmax when the enzyme is saturated with substrate.
Vmax (maximal velocity) varies with the amount of enzyme used in the reaction.
What are the 3 parameters to characterize kinetics of enzyme-catalyzed reaction?
Vmax, maximum velocity.
Kcat, turnover number,
Km, the Michaelis constant.
When is Vmax reached?
When the enzyme is saturated with substrate.
Vmax depends on the amount of enzyme.
What is the Kcat equation?
Kcat = Vmax / [Enzyme]
What is Kcat?
A turnover number.
A constant for an enzyme under given condition.
It is a number of substrate molecules converted to product on a single enzyme molecule per second.
Who were Leonor Michaelis and Maud Menten?
Derived an equation to describe the initial reaction velocity as a function of substrate concentration.
V0 = Vmax ([S] / ([S] + KM))
What is Km?
A constant for the enzyme (Michaelis constant).
Substrate concentration required to get Vmax / 2.
A measure of enzyme’s affinity for its substrate.
Inversely related to affinity (the higher Km, the lower affinity of the enzyme to its substrate; the lower Km, the higher affinity)
What is the Lineweaver-Burk Plot and what does it show?
Linearized, double-reciprocal.
Km is a negative reciprocal of the x-intercept.
Vmax is an inverse of the Y intercept.
Do Km values for enzymes vary?
Yes, widely.
Km value is about equal the substrate concentration of the enzyme in vivo.
How do cells control enzymes?
- Allosteric regulation (binding of non-substrate molecule).
- Covalent modification.
- Access to substrate.
- Control of enzyme synthesis/breakdown.
Combinations of methods can be used.
What are allosteric regulators?
Allosteric effectors or modulators are generally small chemicals.
Allosteric effectors can be positive (improve enzymatic catalysis) or negative (reduce enzymatic catalysis).
The kinetics of allosteric regulators differ from Michaelis-Menten kinetics.
The reaction velocity of allosteric enzymes displays a sigmoidal relationship to substrate concentration.
