Enzyme Kinetic parameters Flashcards
Substrate concentration effect on rate of reaction explained
Increase in concentration increases the rate of reaction. Increases the number of successful collisions.
Saturation effect explained
At a constant enzyme concentration the reaction rate increases with increasing substrate concentration until it reaches a limiting maximum value.
Maximum ES complex formed so rate of formation of product can no longer be increased even with an increase in substrate.
Evidence for ES complex formation
What may the ES complexes go through in a catalytic cycle?
reaction intermediates.
Enzyme concentration effect on rate of reaction
For an unlimited substrate concentration, an increase in enzyme concentration increases the rate of reaction.
Rate equation definition
Gives experimentally observed dependence of rate on the concentration or reactants.
General scheme for one substrate one product reaction
E+S ⇌ ES –> E + P
What is the rate represented by for each arrow?
Top arrow- K1
Bottom arrow K-1
final arrow K2
Equilibrium Assumption
Formation of ES complex from E and S is at equilibrium. Therefore the value of K2 is considered to be much smaller in magnitude compared to K1 and K-1
Steady State definition
State variables are constant in spite of ongoing processes that strive to change them.
Entire system must contain flow through a system.
Steady State Assumption
Not always true that K2 is negligible, concentration of ES is unchanged because its formation and disappearance rates are equal.
Michaelis- Menten equation
v= Vmax[S]
Km + [S]
What is Vmax?
The maximum rate of reaction, when the enzyme is fully saturated with substrate.
What is Km?
The concentration of substrate at which 1/2 Vmax.
Units for Vmax
Micromole product formed per minute per mg
Unit for Km
concentration - micro mole
When is Vmax constant?
amount of enzyme catalyst [Et] is fixed
What should [S] in a reaction?
Around the Michaelis Menten constant Km, so if there are changes in enzyme the substrates can accommodate it. If a lot more, they are under-utilised; if not enough may not be able to control changes.
Turnover number definition
The number of catalytic cycles the enzyme can undergo in a unit of time when the enzyme is fully saturated with substrate. The number of substrate molecule each enzyme site converts to product per unit time.
Kcat definition
A fundamental property of the enzyme, the turnover number.
Specificity constant definition
The ratio of Kcat/kM- describes the relative velocities of two substrates competing for a single enzyme. Expresses the ability of an enzyme to discrimination favour of any one substate over another.
The higher the specificity constant the higher the affinity for the substrate
KI definition
The inhibitor complex, showing how potent an inhibitor is. The concentration required to produce half maximum inhibition.
Irreversible inhibitor definition
Permanent inactivation of an enzyme, often as a result of covalent modifications of the enzyme protein. Net result reduces Vmax, while the KM of the remaining active enzyme is unaltered.
Example of irreversible inhibitor
Suicide substrates- Unreactive molecules recognised as substrates by the enzyme however during catalytic turnover the target enzymes convert them into a reactive entity. Such as penicillin .
Lineweaver Burk Plot derivation
V= Vmax[S] / Km + [S]. find inverse
1/V = Km + [S] / Vmax[S]
1/V = 1/[S] ( Km/Vmax) + 1/Vmax
lineweaver Burk plot axis and intersection meanings
Y axis= 1/V X axis = 1/[S] Gradient= Km/Vmax x intersection = -1/Km Y intersection = 1/Vmax
How can Lineweaver Burk plot distinguish between inhibitors?
Competitive inhibitors have the same y intercept as uninhibited enzymes, as the maximum rate is still the same
Noncompetitive inhibitors produce plots with the sam x intercept as inhibited enzymes.
Uncompetitive inhibition definition
Inhibitor reacts exclusively with ES complex, not free enzyme.
