QBook biochem query

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aru
Posts: 13
Joined: Thu Jul 05, 2018 12:41 pm

QBook biochem query

Post by aru » Sun Jan 13, 2019 8:34 pm

question 568, Nextstep MCAT Science Qbook, Biochemistry section, page 93
They have given a lineweaver Burk plot and asked to find the Km value. They haven't given the coordinate points for X-axis so I don't get it how did they got 3/2 or slope. Can you please explain me the math here please and how id they got the Km value of 1/2?
Kind Regards,
Arushana Ali.
NS_Tutor_Will
Posts: 192
Joined: Fri May 25, 2018 9:15 am

Re: QBook biochem query

Post by NS_Tutor_Will » Mon Jan 14, 2019 11:28 am

Thanks for your question!

On a Lineweaver-Burk plot, we can always find Km or Vmax from the slope, provided we have one of them. Remember, the reciprocal of the y-intercept will give us Vmax and the negative reciprocal of the x-intercept will give us Km. The equation we can use is:

slope = Km / Vmax

In this case, we can solve for slope using the points and coordinates we were given. I learned slope as rise over run... in this case we have a rise (y) of 3 and a run (x) of 2. So, our slope is 3/2 or 1.5. We also know our Vmax, since it is the reciprocal of the y-intercept: 1/3.

So:

slope = Km / Vmax

(3/2) = Km / (1/3)

(3/2)(1/3) = Km = 1/2

I hope this helps!
aru
Posts: 13
Joined: Thu Jul 05, 2018 12:41 pm

Re: QBook biochem query

Post by aru » Tue Jan 15, 2019 2:32 am

Thank you that makes sense. In the book the picture there was no labelling on X-coordinates given so the coordinates for X are 2,0 right? I have another question this is from Biochem page 115, question 700. What does steric probability means? It says cells can utilize cell uses enzymes to increase steric probability to drive nonspontaneous reaction what does it mean?
Kind Regards,
Arushana Ali.
NS_Tutor_Will
Posts: 192
Joined: Fri May 25, 2018 9:15 am

Re: QBook biochem query

Post by NS_Tutor_Will » Tue Jan 15, 2019 8:36 am

Steric probability is essentially referring to an enzyme's ability to orient molecules correctly so that a specific, catalyzed reaction can occur. Enzymes can contort, or bend, or orient substrates so that the activation energy is lower. I'm sure you're familiar with the concept, it's just a new vocab term to refer to it!
aru
Posts: 13
Joined: Thu Jul 05, 2018 12:41 pm

Re: QBook biochem query

Post by aru » Tue Jan 15, 2019 2:44 pm

Question 843, page 138, biohem question. I am not sure if I understood the question. Can you explain me how to approach this? I particularly did not understand the shifts of loop in response to depolarization. Your explanation will make much more sense.
Additionally, question 775, page 125 it says three phases of pentose phosphate pathway? Is it referring to the oxidative and non oxidative phase? If so what is the third phase then?
Also page 113, question 686 biochem the answer is Pka2 and explanation says side chain carboxylic acid is represented by Pka2. I thought Pka3 is the side chain Pka. Can you explain me this?
Kind Regards,
Arushana Ali.
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