Calculating Km with substrate concentrations and Vmax

[mclee0033]

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I have a list of substrate concentrations and reaction velocities, including a substrate concentration where velocity doesn't increase anymore. So I have my Vmax.

This should be able to be done without a calculator as it's from an NBME. I know you can divide 1/substrate concentrations and make a Lineweaver-Burke plot, but there's no way to do that without a calculator and under time constraints. There's gotta be a better way! Right?

 

[tiedyeddog]

I have a list of substrate concentrations and reaction velocities, including a substrate concentration where velocity doesn't increase anymore. So I have my Vmax.

This should be able to be done without a calculator as it's from an NBME. I know you can divide 1/substrate concentrations and make a Lineweaver-Burke plot, but there's no way to do that without a calculator and under time constraints. There's gotta be a better way! Right?

Can you give the actual question? No, you shouldn't need to do complex equations... This can probably be solved via mental reasoning.

 

[mclee0033]

Ok:

Kid being evaluated for Lesch-Nyan syndrome. Use his HGPRT enzyme with saturating concentrations of phosphoribosyltransferase and varying amounts of guanine.

[Guanine] Velocity
0.06 0.26
0.2 0.60
.... ......
20 2.0
200 2.0

 

[mclee0033]

Answer choices are

< .06
.2 to .5
.5 to 1.0
1.0 to 2.0
8.0 to 20

 

[Wordead]

Step 1 never wants you to do super complicated stuff like this, man. Never.

Km = substrate at which Vmax is half. Divide vmax by 2, look for where Km is. That's it.

 

[tiedyeddog]

Ok:

Kid being evaluated for Lesch-Nyan syndrome. Use his HGPRT enzyme with saturating concentrations of phosphoribosyltransferase and varying amounts of guanine.

[Guanine] Velocity
0.06 0.26
0.2 0.60
.... ......
20 2.0
200 2.0

Answer choices are

< .06
.2 to .5
.5 to 1.0
1.0 to 2.0
8.0 to 20

Where did this question come from? An NBME, which one? I am not sure how to figure this out without using math more complicated than step 1 should take...

 

[DrDrummer]

It's from NBME 7. Vmax/2 = Km. You can see from the table (or draw a quick sketch) that the curve seems to level out around 2.0.

Therefore, half of Vmax is around 1.0. Look for the concentration that gives you that, and you have your Km.

Questions like that are why I wish I hadn't wasted my time with NBME 7.

 

[mclee0033]

Oh right of course! Thanks!

 

[sharklasers]

Step 1 never wants you to do super complicated stuff like this, man. Never.

Km = substrate at which Vmax is half. Divide vmax by 2, look for where Km is. That's it.

Yeah this is right.

In your question, what is the substrate concentration at velocity of 1?

 

[mclee0033]

It's from NBME 7. Vmax/2 = Km. You can see from the table (or draw a quick sketch) that the curve seems to level out around 2.0.

Therefore, half of Vmax is around 1.0. Look for the concentration that gives you that, and you have your Km.

Questions like that are why I wish I hadn't wasted my time with NBME 7.

Thanks!! And welll....then you prob won't wanna do NBME 15 either cause that's where I got it from haha.

 

[sharklasers]

You probably shouldn't be spoiling NBME questions like this. If you ahve a question about a particular NBME, post it in that topic. Or mention that it is from an NBME in the topic title...

 

[John Lannister]

.

 

[DrDrummer]

Thanks!! And welll....then you prob won't wanna do NBME 15 either cause that's where I got it from haha.

All the face-impacting-desk questions are blurring together in my brain. Two... days... left...

Honestly, I looked at that one for a long time and tried to remember what complicated math was involved, then remembered the picture explanation. I think that's the best way to remember it-- just draw a model graph and know what goes with what intersection/axis.