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AAMC Section Bank CP
Nicole_5521
#1 Posted : Saturday, July 31, 2021 3:37:09 PM
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Hi,

For Q48, could you explain why there needs to be a saturated substrate concentration in order to measure kcat?

For Q51, I'm confused why peptide C has a charge of -1? Doesn't it have a charge of 0 because H (+1), E (-1), E (-1), K (+1), R (+1), D (-1)?

For Q57, how come the volume changes are getting multiplied to the concentration? I'm confused because is concentration=moles/volume, so if the volume increases doesn't that make the concentration smaller? But, we are multiplying the concentration to make it larger here?

Thanks!
INSTR_Molly_129
#2 Posted : Saturday, July 31, 2021 9:38:56 PM
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Hi Nicole,

I'm unfortunately unable to see some of the questions that you are referring to, but I will answer 57 which matches what you are describing here. Hopefully another moderator could assist.

Q57: For the experiment, the commercial concentration was diluted from 0.1 mL to 25 mL, so the concentration had decreased by a factor of 250x. Then in the assay, 1 mL of the enzyme and 1 mL of the substrate was mixed, so concentration was further decreased by a factor of 2x. Overall, the concentration was decreased 250 x 2, which is 500.

Hope that helps!

Molly
Nicole_5521
#3 Posted : Saturday, July 31, 2021 10:02:19 PM
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Hi Molly,

If the concentration decreases, how come you would multiply the concentration by 500 in the answer choice? Wouldn't you have to divide to show it decreasing?

Thanks!
INSTR_Katrina_128
#4 Posted : Tuesday, August 03, 2021 5:12:30 PM
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Q48: This is asking about Michaelis-Menten kinetics.

There are two sections of the Michaelis-Menten graph: when the enzyme is unsaturated and when the enzyme is saturated.

https://lh3.googleuserco...C_8dmTGCAPjrzITeYrT44mFI

It is MUCH easier to measure the rate constant of the catalyst (kcat) when [S] is saturated, because you're in that flat region, where kcat stops changing.

When [S] is too low, then you need to find the slope of the line, and relate it to kcat/KM, meaning you also need to know KM. Not fun. So, dump a ton of substrate in until the rate of product formation doesn't change, and you know you're in the right spot!

Q51: Be careful! At pH = 7, histidine is UNCHARGED (neutral).

Here are the total charged species:

Two glutamic acids and one aspartic acid (-3).

One lysine and one arginine (+2).

Overall: -1.

Q57: I think you're thinking about it backwards. Here, they're asking you to start with the diluted concentration, and then figure out the more concentrated stuff. That's why we're multiplying!
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