Proteins - Lecture Eleven Flashcards
How does an enzyme catalyse a reaction?
Enzyme and catalysis
Enzymes catalyse thermodynamically favourable reactions by lowering the activation energy
Relative speeds of k1 and k-1
Define how tightly substrate binds
The rate of catalysis, k2
Relates to energy of activation for the transition state
The complex ES
Necessary for reaction so [ES] at any time will govern the rate
Steady state
Refers to time during which [ES] does not change (shaded)
Progress curve
Measures the appearance of product (or disappearance of substrate) with time at steady state
The effect of substrate concentration on reaction rate
As [S] increases, the initial rate, V0, increases in a linear way at first, but as all the enzymes active sites become occupied, the rate of reaction stops increasing and creates a curve to represent maximum velocity
Michaelis constant
The substrate concentration at which the enzyme is running at half of its speed
The Michaelis-Menten Model assumptions
Product is not converted back to substrate
Haldane’s steady state assumption: the rate of ES formation equals the rate of its breakdown
Measuring initial rate insures [S] does not change significantly (and [S] is much greater than [E])
First order kinetics
Reactions that occur with a set probability of occurring
ES ➝ E + P
First order kinetics because each ES complex has an equal chance of getting over the energy hump
Assumptions when the Michaelis-Menten Model fits
All ES complexes have same rate of reaction
[S] is in vast excess to [E]
Haldane’s steady state assumption: the rate of ES formation equals the rate of its breakdown.
Initial rate is measured which is early enough that [S] does not change significantly.
The reverse reaction does not occur
Cooperative enzymes
Don’t follow the Michaelis-Menton equation
Vobs VS [S] plot
Sigmoidal (due to coopertavity) and responds more steeply to intermediate changes in [S]
Allosteric enzymes
Respond to effectors binding away from the active site
