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#1
01-05-2014
 USMLE Forums Scout Steps History: 1+CK+CS+3 Posts: 17 Threads: 9 Thanked 12 Times in 4 Posts Reputation: 22
enzyme kinetics..

consider a reaction that can be catalyzed by one of two enzymes, A and B, with the following kinetics,

Km (M) Vmax (mmmol/min)
A. 5 x 10-6 20
B. 5 x 10-4 30

6. At concentration of 5x10-4 M substrate, the velocity of the reaction catalyzed by enzyme A will be

A. 10
B. 15
C. 20
D. 25
E. 30

kindly answer with explanation because i'm just stuck with this....your response will be appreciated.. thankuu
 The above post was thanked by: fizA (05-21-2014)

#2
01-06-2014
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 138 Threads: 14 Thanked 86 Times in 45 Posts Reputation: 96

first of all rule out the last two options because the enzyme cant work beyond it Vmax which is 20.
then rule out the first option bcoz thats the velocity at Km i.e 5 x10-6 .
u r left with B and C now the value 5 x 10-4 is far above the Km so i m `guessing` the enzyme is working at its Vmax. they dont expect you to solve the complicated calculations on exam but if you want to make sure then solve the problem with lineweaver-burk equation and u will get the answer C. 20
 The above post was thanked by: excellence (01-09-2014), fizA (05-21-2014), shine4ever (01-06-2014)
#3
01-07-2014
 USMLE Forums Addict Steps History: 1 + CK Posts: 143 Threads: 12 Thanked 59 Times in 38 Posts Reputation: 69

Quote:
 Originally Posted by slacker.mle first of all rule out the last two options because the enzyme cant work beyond it Vmax which is 20. then rule out the first option bcoz thats the velocity at Km i.e 5 x10-6 . u r left with B and C now the value 5 x 10-4 is far above the Km so i m `guessing` the enzyme is working at its Vmax. they dont expect you to solve the complicated calculations on exam but if you want to make sure then solve the problem with lineweaver-burk equation and u will get the answer C. 20
i understood that Vmax of A =20 so at Km its 20/2= 10,
but i didnt get what is this value: 5 x10-6?? can you please clear it out?
thank you

#4
01-09-2014
 USMLE Forums Addict Steps History: 1 + CK Posts: 143 Threads: 12 Thanked 59 Times in 38 Posts Reputation: 69

Quote:
 Originally Posted by excellence i understood that Vmax of A =20 so at Km its 20/2= 10, but i didnt get what is this value: 5 x10-6?? can you please clear it out? thank you
i found the q on kaplan LN, its clear now, i just got confused due to the no. typing in the post

5x 10-6 represent the conc of the S= Km for enz A, so at 5x10-4 definitely the enz will work at its vmax which is 20 so C is the correct answer.
 The above post was thanked by: CallingDrAsh (01-09-2014)

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