Enzymes as catalysts Flashcards
In an enzyme kinetics experiment what are you studying?
- initial velocity at varied initial concentrations of substrate
Initial velocity equation
initial change in substrate concentration/ change in time
When does the initial rate depend on substrate concentration?
At low substrate concentrations
At low substrate concentrations what limits how much ES can form and what is the result?
[S] limits
-v (initial rate) is dependent on [S]
At high substrate concentrations, what limits how much ES can form and what is the result?
[E] limits
- v is dependent on enzyme concentration, so v (initial rate) does not change as [S] changes
when is Km approximately equal to Kd?
when the rate of ES dissociation is much greater than the rate of product formation
k2»k3
what is the equation for the dissociation constant (Kd) for the ES complex?
Kd=k2/k1
what is the benefit of having a low Km value?
it would allow an enzyme to show significant levels of catalysis at low concentrations of substrate (would be important for tissues that have very small amounts of substrate)
what is the benefit of having a high Km value?
it would allow sensitive regulation of enzyme activity in response to small changes in [S] in the range of 2-5 mM substrate
this would be critical for enzyme regulation in tissues with high level of [S]
what is another term for k3?
Kcat or turnover number
v=
Vmax [S] / ( [S] + Km)
Vmax =
K3 [E]t
what does a higher ratio of Kcat/Km imply??
higher efficiency
means you are getting more conversions to product for a given amount of ES complex
What does km mean?
The [S] at which v=1/2 Vmax
When is km approximately equal to the dissociation constant - kd?
when k2»k3
-when the substrate binds and dissociates before it goes to form product
When is the catalytic rate most sensitive to [S]?
When [S] «< km
fES will be higher for a low km or a high km?
low km
fES =
v/vmax = [s] /( [s] +km)
what are the units of Kcat?
inverse time s^(-1)
what is the definition of Vmax and what are its units?
maximal velocity achievable for a SPECIFIC concentration of enzyme
units= concentration/time
what does a higher ratio of Kcat/Km imply??
higher efficiency
when would you use a lineweaver-burk plot?
to accurately determine Km and Vmax
definition of an irreversible inhibitor
binds very tightly to an enzyme and inactivates an essential functional group on that enzyme
definition of a reversible inhibitor
binds reversibly to enzyme and gives a temporarily inactivation when it is bound
how can a competitive inhibitor be over come?
by increasing [S]
what is the definition of Ki?
the strength at which an inhibitor binds to an enzyme
it is the disassociation constant for the enzyme-inhibitor complex
what does a higher Ki imply?
weaker inhibition (it disassociated easily)
how will a competitive inhibitor effect the y and x intercepts on a lineweaver-burk plot?
the y intercept will NOT change but the x intercept will change
how will a non-competitive inhibitor effect the y and x intercepts on a lineweaver-burk plot?
the y intercepts will change and the x intercept will NOT
what value will appear to change in the presence of a competitive inhibitor?
Km
what value will appear to change in the presence of a non-competitive inhibitor?
Vmax
What is the equation for Kd- the dissociation constant in terms of concentration of substrates and products?
Kd = koff/kon = [A][B]/[AB]
At equilibrium what two values are equal?
association constant and dissociation constant
