Kinetics (4) Flashcards
Semester 1 year 1
How do you calculate the rate from the Michaelis Menten equation when the substrate concentration is much greater than Km?
-denominator of [S] + Km ≈ [S]
-rate = (Vmax x [S]) / [S]
-rate = Vmax
How do you calculate the rate from the Michaelis Menten equation when the substrate concentration is much smaller than Km?
-denominator of [S] + Km ≈ Km
-rate = (Vmax x [S]) / Km
-rate = (Kcat / Km) x [E]total x [S]
-final equation from subbing in Vmax equation
What does the line look like on a graph when substrate concentration is much smaller than Km?
-linear
-the equation is the same as equation for straight line
What is Kcat / Km?
-second order rate constant describing reaction of E + S to give enzyme + product (includes both steps)
-it’s a specificity constant
What is the importance of Kcat / Km?
Only valid way of comparing enzymes with different substrates
How do you calculate the rate from the Michaelis Menten equation when the substrate concentration is equal to Km?
-denominator of [S] + Km = 2[S]
-rate = (Vmax x [S]) / 2[S]
-rate = 1/2 Vmax
What is Km?
The concentration of substrate that produces half the maximal rate
Why is a graph of rate against substrate concentration not normally used and what are the alternative representations?
-difficult to see what’s going on at low [S]
-use either log [S] or a double reciprocal plot
Describe a plot of rate against log [S]
-data points at low [S] more spread out
-harder to visualise Kcat / Km
Describe a double reciprocal plot
-plot 1/v against 1/[S]
-the line is straight
-gradient = Km / Vmax
-y intercept = 1/Vmax
What are competitive inhibitors?
-molecules that bind to the enzyme and prevent substrate binding
-often bind in active site
How do you overcome a competitive inhibitor and how does this effect Km and Kcat?
-need a higher [S] to compete against inhibitor so Km increases
-once substrate is in active site, it reacts as normal, so Kcat doesn’t change
On a double reciprocal plot, how does a line with a competitive inhibitor differ to the line without?
It has a shallower gradient
What are allosteric inhibitors?
-molecules that bind to the enzyme but not in the active site
-generally change the shape of active site
What do allosteric inhibitors affect?
-can affect binding + chemistry - both Km or Kcat can change
-can also activate, so change can be in either direction
