AAMC QBank Chem 37

[jconnell82760]

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Ignoring stereochemistry how many different tripeptides can be produced by amino acids A, B, and C below.
Answer options:
1, 3, 6, 9


Correct answer: 6
Okay maybe I'm completely overthinking this, but when I wrote them out, I wrote out 6, but then noticed that some are the same. ex: ABC/CBA and BAC/CAB, so I only counted those as 1 per pair and put 3 as the correct answer. Why would CBA not be the same tripeptide as ABC? Also, how should I check myself on these to make sure I'm not overthinking it?

 

[BTC0821]

Ignoring stereochemistry how many different tripeptides can be produced by amino acids A, B, and C below.
Answer options:
1, 3, 6, 9


Correct answer: 6
Okay maybe I'm completely overthinking this, but when I wrote them out, I wrote out 6, but then noticed that some are the same. ex: ABC/CBA and BAC/CAB, so I only counted those as 1 per pair and put 3 as the correct answer. Why would CBA not be the same tripeptide as ABC? Also, how should I check myself on these to make sure I'm not overthinking it?

Consider how amino acids are made by underlying nucleotide base pairs. Now if one were to reverse the order of these base pairs the whole way through, the products would be totally off. For example you have two tripeptides with base pairs

AUG|GGC|CGA
AGC|CGG|GUA

Notice how they're in the same order but different direction. One of these has a start codon (AUG) at the beginning, and the other one doesn't have a start codon at all. This is why they're different.

 

[JustinM88]

is this the kind of math were we just do 3 factorial?

 

[Verity]

The N- and C-termini are your answer 🙂

These are not the same:
N-ABC-C
N-CBA-C

If the termini/linkages were the same, you'd be correct in thinking ABC and CBA were the same molecule, just rotated in space. BUT, because there are different termini on a peptide, the molecules are different.

 

[aldol16]

ABC implies that you're linking up the N-terminus of A with the C-terminus of B and similar with B and C. CBA implies that the N-terminus of B is being linked with the C-terminus of A. Opposite terminals.

 

[NextStepTutor_1]

Hi @jconnell82760 -

The Wikipedia page on tripeptides has a nice color-coded image that should help you visualize why ABC is not the same as CBA. As other posters have pointed out, this is because we write amino acid sequences moving from the N-terminus on the left to the C-terminus on the right, which also corresponds to the directionality of translation.

Hope this helps clarify things & best of luck!