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#1
08-23-2014
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Question in biochemistry

Consider a reaction that can be catalyzed by one of two enzymes. A and B, with the following kinetics.
A. km= 5 x (10)-6 , Vmax = 20
B. km= 5 x (10)-4 , Vmax = 30
At a concentration of 5 x (10)-4 substrate, the velocity of the reaction catalyzed by enzyme A will be:
a) 10
b) 15
c) 20
d) 25
e) 30

The answer is C because at the concentration of 5 x (10)-4 M, enzyme A is working at its Vmax, which is 20 mmol/min.

I read the explanation but it still didn't make sense to me! What I know that according to what I studied and to what is given in this question that at 5 x (10)-4, enzyme A will be having concentration > Km so that its velocity will be definitely more than its half Vmax which is 10 so I was searching for a value more than 10! So why not the other choices?!
 The above post was thanked by: israaoday (08-25-2014), olayiwola-aims (08-23-2014)

#2
08-23-2014
 USMLE Forums Scout Steps History: Not yet Posts: 15 Threads: 3 Thanked 7 Times in 4 Posts Reputation: 17

Hey i will try to explain to you.
EnzymeA's km is 5×10-6 i e at this substrate concentrarion enzymeA converts substrate at a rate of 1/2 maxvelocity(its maxvelocity being 20)i e 1/2×20.
So at a conc of 5×10-6. Velocity=10
For a conc of 5×10-4. Velocity should be 1000,but maxvelocity of reaction is 20 itself.so velocity at this increased concentration doesnt reach 1000 but remains at 20 itself(Vmax)
It is because,though substrate conentratn has increased enormously enzymeA conc remained same whos max capacity to convert substrate is at a rate of 20mmol/min.
I hope you have understood.

Last edited by sai steps; 08-23-2014 at 08:20 AM.
 The above post was thanked by: Albatoul Khaled (08-23-2014)
#3
08-23-2014
 USMLE Forums Scout Steps History: Not yet Posts: 33 Threads: 20 Thanked 3 Times in 2 Posts Reputation: 13

Thanks but one thing isn't it the relation between Km and Vmax linear relation?!

Last edited by Albatoul Khaled; 08-23-2014 at 01:23 PM. Reason: need more explanation
#4
08-23-2014
 USMLE Forums Scout Steps History: Not yet Posts: 33 Threads: 20 Thanked 3 Times in 2 Posts Reputation: 13

Sorry spelling mistake. I meant is the relation between Km and Vmax linear relation?! What I know it is not!
Anyhow thanks. I appreciate trying to help!
#5
08-23-2014
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hello khaled

As the concentration of substrate increases, the enzyme becomes saturated with substrate. As soon as the catalytic site is empty, more substrate is available to bind and undergo reaction. The rate of formation of product now depends on the activity of the enzyme itself, and adding more substrate will not affect the rate of the reaction to any significant effect.
The rate of reaction when the enzyme is saturated with substrate is the maximum rate of reaction, Vmax.
The relationship between rate of reaction and concentration of substrate depends on the affinity of the enzyme for its substrate. This is usually expressed as the Km (Michaelis constant) of the enzyme, an inverse measure of affinity.

so i think vmax remains the same even at increase in substrate concentration.!
i.e;20
 The above post was thanked by: DrNewB (08-24-2014), israaoday (08-25-2014)
#6
08-24-2014
 USMLE Forums Scout Steps History: Not yet Posts: 15 Threads: 3 Thanked 7 Times in 4 Posts Reputation: 17

Quote:
 Originally Posted by Albatoul Khaled Sorry spelling mistake. I meant is the relation between Km and Vmax linear relation?! What I know it is not! Anyhow thanks. I appreciate trying to help!
Yes Albatoul you are right! The relation is not linear but a curve.thats what i have concluded indirectly.
If it is a linear relation the velocity of reaction might have reached 1000,which is not happening because velocity also depends on enzyme concentration.but the enzyme concentration is constant here and hence it becoms saturated.
So enzymeA is limiting the velocity to 20 which is its maximum capacity to convert the substrate,though the substrate conc has been increased enormously.
Thanku
 The above post was thanked by: DrNewB (08-24-2014), MDS91 (09-24-2014)

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