Transpeptidation?

[nejikon]

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Can anyone explain why is this so? I thought transpeptidation was about forming peptide bonds???

 

[yeasureyoubetcha]

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Can anyone explain why is this so? I thought transpeptidation was about forming peptide bonds???

Looking at the choices Val is small nonpolar, Asp is - chg carboxy group, Phe is large aromatic, Arg carries a + charge. Peptidyl transferase is involved in forming peptide bonds within the ribosome. Transpeptidation as explained is a different process and knowing what it is is not necessary to answer the question. The free amino group of the side chain is participating in the cross linking. They're basically asking what other + chg AA could also participate

 

[NextStepTutor_2]

View attachment 201762
Can anyone explain why is this so? I thought transpeptidation was about forming peptide bonds???

Hi @nejikon Transpeptidation is a name for a reaction involving the transfer of one or more amino acids from one peptide chain to another, or of a peptide chain itself, like in bacterial cell wall synthesis. That being said, its not really needed for the question and serves to confuse the explanation.

We have a +-charged AA residue (Lys) taking part in the bridges and the question asks for another potential AA that could participate in these linkages. The more similar to Lys the AA is, the more likely it is to serve a similar function in the cell-wall process described in the question stem.

A: Valine, hydrophobic, unlikely since it is unlike Lys and unlikely to bond with Lys to form these links.
B: Asp, - charged which makes it unlike Lys, but could be part of the attachment site opposite the + residues.
C: Phe, same as Val so its out for the same reasons. The AAMC often does this, put 2 similar answers. They cannot both be correct, so they must both be wrong.

We are left with D, an AA with identical chemical properties to Lys.

 

[aldol16]

Can anyone explain why is this so? I thought transpeptidation was about forming peptide bonds???

Transpeptidation is exactly what it sounds like. You take an amino acid in a peptide bond with partner A and transfer it to partner B. Remember that amino acids form peptide bonds via their backbone atoms. Side chains don't matter. What you're thinking of is the amine on Lys attacking the carboxylic acid side chain of Asp. That's not a peptide bond. It's an amide bond, but not a peptide bond in the strictest definition of the term. So the question here is asking basically which amino acids have similar properties.