Enzyme kinetics Flashcards
Rate constant, Michaelis-Menten kinetics, week 7 practical, KM, Vmax and kcat.
Which property asks ‘does a reaction proceed, how far’?
Thermodynamics.
Which property asks ‘how fast does a reaction proceed’?
Kinetics.
Give 2 reasons as to why the rate of a reaction is important.
- Most cellular chemicals are
thermodynamically unstable but rely on
their rate of breakdown being too slow to
matter (without catalysis). - Some diseases are caused by imbalances
in the rates at which reactions are taking
place.
What is k subscript un?
Rate constant for the unfolding of a protein/biomolecule under certain conditions.
Give the rate equation for the reaction A→C and give the order.
Rate = k[A]
First order
Give the rate equation for the reaction
A + B → C
Rate = k[A][B]
Second order
What is k subscript f?
Rate constant for forwards reaction.
What is k subscript r?
Rate constant for reverse reaction.
What does Michaelis-Menten kinetics assume?
(unsimplified reaction is on the summary sheet)
A simplified reaction scheme:
- Release of product is very fast (compared
to the reaction itself)
- Reverse reaction is sufficiently slow that
we can ignore it/the product is reacting
with something else so that it doesn’t
accumulate.
- The concentration of the enzyme-
substrate complex is at a steady state.
Give the simplified reaction scheme.
E + S –> k1, <–k-1 ES –> kcat E+P
What is the kcat?
The catalytic rate constant = the number of substrate molecules converted to product per enzyme molecule per unit time.
In 1913, the Michaelis-Menten equation was made due to simplified kinetics.
Give this equation.
V = Vmax[S]/(KM+[S])
V = reaction velocity
Vmax = maximal rate
[S] = substrate concentration
KM = Michaelis constant
What does the M-M equation tell you?
How rate varies with substrate concentration.
What is Vmax affected by?
The amount of enzyme but for a given amount of enzyme it is a constant.
How do you get the M-M curve?
(look at summary sheet)
Measuring rate at various substrate concentrations.
And if the reaction is following M-M kinetics.
Why is Vmax not usually reached?
It requires very high substrate concentration (needs to be almost infinitely high).
Therefore, what is the difficulty with M-M kinetics?
Finding Vmax in the first place.
Give an alternative.
(if confused, watch the recap)
Use different [S] and extrapolate to find out what Vmax must be.
How do you find KM?
Read off from Vmax/2 to x-axis.
What is K subscript M?
The concentration of substrate at which half the active sites are filled.
Provides a measure of the substrate concentration required for significant catalysis to occur.
KM provides an approximation of the substrate concentration in…
vivo.
What is Vmax?
The maximum rate for the amount of enzyme used.
If you double the amount of enzyme, Vmax…
doubles.
What is k subscript cat?
The turnover number, the number of times each enzyme molecule goes through a catalytic cycle per second.
