Gold Standard CBT 10 Bio question *Spoiler Alert*

[tcon91]

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Hey Everyone, I got this question wrong on GS-10 and I think it's ridiculous. Just wanted to see if Gold Standard's answer truly is right. Here it is....

22) If individuals with PKU disease lack the protein phenylalanine hydroxylase, what would best explain their being able to metabolize small amounts of phenylalanine?

1. Non-specific enzymes cleave phenylalanine.
2. Phenylalanine catalyzes reactions in the liver.
3. Phenylalanine is broken down mechanically in the formation of chyme in the stomach.
4. Phenylalanine is defecated from the body.

My answer was D (#4) however this is incorrect according to GS. The correct answer is A.

Here is their explanation...

Proteins can undergo cleavage by non-specific enzymes. The cleaved products are then used as metabolites by the body.

I thought all enzymes ARE specific hence the "lock and key" theory.


Can anyone help me out? Who's right here?

 

[justadream]

@tcon91

Not all enzymes are so specific that they can only react with 1 exact type of molecule. For example, if you alter one small part of the substrate, the original enzyme may still be able to act on the substrate.

As for your answer, how does defacating phenylalanine show that you can metabolize it? If swallow a piece of bubble gum, I'm pretty sure you will defacate that. However, that doesn't mean you can metabolize it.

 

[tcon91]

@tcon91

Not all enzymes are so specific that they can only react with 1 exact type of molecule. For example, if you alter one small part of the substrate, the original enzyme may still be able to act on the substrate.

As for your answer, how does defacating phenylalanine show that you can metabolize it? If swallow a piece of bubble gum, I'm pretty sure you will defacate that. However, that doesn't mean you can metabolize it.

Good explanation. I see where I went wrong. Thank you so much!

 

[kraskadva]

Hey Everyone, I got this question wrong on GS-10 and I think it's ridiculous. Just wanted to see if Gold Standard's answer truly is right. Here it is....

22) If individuals with PKU disease lack the protein phenylalanine hydroxylase, what would best explain their being able to metabolize small amounts of phenylalanine?

1. Non-specific enzymes cleave phenylalanine.
2. Phenylalanine catalyzes reactions in the liver.
3. Phenylalanine is broken down mechanically in the formation of chyme in the stomach.
4. Phenylalanine is defecated from the body.

My answer was D (#4) however this is incorrect according to GS. The correct answer is A.

Here is their explanation...

Proteins can undergo cleavage by non-specific enzymes. The cleaved products are then used as metabolites by the body.

I thought all enzymes ARE specific hence the "lock and key" theory.


Can anyone help me out? Who's right here?

Good point by @justadream

Also, lock and key is a mostly defunct theory now. Induced fit is in.
Enzymes are specific, but some are more specific than others. There's a continuum where some only look 2-3 atoms away from the bond they are affecting (so "non-specific" in terms of the macromolecules they work on), and at the other end some look at very large sections of the molecule.

So #4 has no relevance to the answer (as there's no metabolism happening), same for #2.
#3 is completely untrue if you know about what happens in the stomach (tissues broken down to proteins, nucleic acids and fats, but not beyond that)
And #1 is the only thing that involves metabolism of phenylalanine, per se.