Chapter 7.4 Enzyme Kinetics Flashcards
what does kinetics generally relate to?
movement
what is enzyme kinetics?
study of rates of enzymatic reactions
what do enzyme kinetics relate?
relate rates to activation energy (delta G double dagger) and equilibrium
what do enzymes do for reactions? how?
increase the rate that a reaction reaches equilibrium by lowering activation energy
do enzymes change overall reaction energy (delta G)?
nope! they can’t change equilibrium
what are the 2 ways to describe rate?
- apprearance of products over time (dP)
- disappearance of reactants/substrate over time (dS)
what are the units and standard units of rate?
concentration over time, or Molarity/second
what is the general reaction rate law?
v= k[S]^n
what is k in the general reaction rate law?
the rate constant
does k, the rate constant, depend on concentration?
no
can k, the rate constant, be altered by catalysts or temperature?
yes
how is k, the rate constant, determined?
experimentally
what is n in the general reaction rate law?
the order of the reactant, or the effect of substrate concentration on rate
what does the overall order of a reaction tell you? how is it found?
tells how the concentration of all reactants affects the rate of the reaction; is the sum of the exponents in the rate law
what does the order with respect to a reactant tell you? how is it found?
tells how the concentration of a single reactant affects the rate of the reaction; is the exponent on a specific reactant in the rate law
what is the equation to find the units of k, the rate constant?
k units = M^1-n x t^-1
how is rate related to activation energy?
there is an inverse relationship between rate (v) and activation energy (delta G double dagger), so the lower the activation energy, the faster the rate of the reaction and vice versa
what do enzyme kinetics depend on? what is the issue with this?
[S], substrate concentration is difficult to measure
what do we measure instead of [S] for enzyme kinetics?
[P], concentration of products
how can rate also be determined? (a formula)
dP/dT
how to we determine initial rate? (Vo)
Vo = dP/dT
what does initial rate (Vo) increase with?
increasing initial [S]
what does each axis represent in the michaelis-menten plot?
X axis is inital [S]
Y axis is initial velocity [Vo]
what is the slope of each line on the michaelis menten plot?
Vo
what is Vmax? where is it found on a michaelis menten plot?
the max velocity of a reaction, the top of the curve on a Michaelis-menten plot
why does Vmax exist for enzymatic reactions?
because you only have so many enzymes, so once they are all bound to subrates, you can’t make more product even if you have more substrates
what does michaelis menten kinetics describe
how enzymes function
what are michaelis menten kinetics only valid for? (3)
- steady state conditions
- single enzyme with single active site
- single substrate
what do michaelis menten kinetic relate?
relate inital velocity of rxn to max velocity
what is the michaelis constant? (Km)
Km = [S] at 1/2 Vmax
what is Km NOT equal to?
Km is NOT EQUAL TO 1/2Vmax, is the concentration of the substrate AT 1/2 Vmax
how does Km relate to binding affinity?
inversely so
lower Km means higher binding affinity and
higher Km means lower binding affinity
how do you find the lineweaver burk equation?
take the reciprocal of the michaelis menten equation and rearrange some
what does the lineweaver burk equation allow?
allows the michaelis menten equation to be plotted linearly
describe the x and y axes, x and y intercepts, and slope on the plot of the lineweaver burk equation
x axis: 1/[S]
y axis: 1/Vo
x intercept: -1/Km (negative because on a -x value)
y intercept: 1/Vmax
slope is Km/Vmax
describe the slope of a lineweaver burk plot with a high [Et]
lower slope
describe the slope of a lineweaver burk plot with a low [Et]
higher slope
what does a lower [E] do the Vmax?
decreass Vmax
what are the 5 assumptions of the michaelis menten model?
- [S]»_space;> [E]
- [Et] (or [E] + [ES]) is constant
- steady state is reached quickly ([ES] is constant)
- starts with no products
- product release is fast
what does the michaelis menten assumption that the reaction starts with no products allow (2)?
- no backwards of EP to ES
- can remove k-2
what does the michaelis menten assumption that product release is fast allow (2)?
- that interconversion of EP to E + P is negligible
- can remove EP, k3, and k-3 from equation
with all 5 michaelis menten model assumptions in place, what does the enzymatic reaction equation look like?
just go write it this one is hard
E + S (double arrow with k1 on top and k-1 on bottom) ES (single arrow with k2 on top) E + P
describe steady state conditions (5)
- rate of formation of [ES] = rate of breakdown of [ES]
- [ES] is constant
- [E] is constant
- [S] is decreasing
- [P] is increasing
what is [S]?
substrate
what is [P]?
product
what is [ES]?
enzyme substrate complex
what is [E]?
free enzyme/apoenzyme
what is [Et]?
total enzyme, [E] + [ES]
describe [Et] in michaelis menten kinetics
constant
since the only way product can be formed under michaelis menten kinetics is through [ES], what is the inital velocity of these reactions? how does this change Vmax knowing that the reaction is limited by the amount of enzyme and that Vmax occurs when all enzyme is in the form [ES]? (so [Et] = [ES])?
Vo = k2x[ES] where k2 is the rate constant for ES –> E + P; but given the assumptions in the second part, Vmax = k2[Et]
wha is kcat? what does it tell?
the catalytic constant; tells how fast an enzyme works once [ES] has been formed
what does kcat not tell/account for? (2)
- doesn’t fully account for enzyme efficiency
- doesn’t tell binding efficiency (bc [ES] must already be formed for kcat to be determined)
what are the units of kcat?
1/sec or per second
what is the specificity constant?
kcat/Km, a true measure of catalytic efficiency (in units of 1/Ms)
what is used to describe enzyme activity and why? (2)
- kcat measures turnover rate
- Km describes [ES] formation
when comparing enzymes, how do you tell which enzyme has the highest binding affinity?
look for the lowest Km
when comparing enzymes, how do you tell which makes product the fastest?
look for the highest kcat
when comparing enzymes, how do you tell which is most efficient?
look for the highest Kcat/Km
describe k-1 and k2 when product formation is super slow
k-1»_space; k2; rate of product formation is so slow it doesn’t even happen because [ES] dissociates before [P] even made
what is Kd? (3)
- the dissociation constant (units M)
- how readily the substrate dissociates from the ES complex
- inversely related to binding affinity
relate Km and Kd when rate of product formation is super slow
Km = Kd
relate k2 and k-1 when rate of product formation is super fast and also relate Km and Kd
k2»_space; k-1 and
Km DOES NOT EQUAL Kd
if [S] «_space;Km, relate [Et] and [E]
if [S]»_space; Km, the [Et] = [E], or all enzymes are free/apo
if [S]»_space; Km, what governs enzyme efficiency and why?
k1, which describe the ensyme substrate complex formation (the rate-limiting step in this case) because the enzyme cannot work faster than the rate of diffusion
how can temperature and pH effect enzyme function?
- can either denature and cause loss of function or
- activate enzyme if only works in certain conditions
