What is the isoelectric point of the following tetrapetide that contains the amino acids represented by...

What is the isoelectric point of the following tetrapetide that contains the amino acids represented by the single letters “DREN”, such that D is at the amino-terminal end of the peptide and N is at the carboy-terminal end.

A. 3.86
B. 4.06
C. 6.53
D. 8.8
E. 12.48

Expert Solution

The polypeptide DREN can be translated to Asp-Arg-Glu-Asn.

Where:

Asp = Aspartic acid

Arg = Arginine

Glu = Glutamic acid

Asn = Asparagine

The first thing we should do when we face a problem like this is to look up for all the pka values for each aminoacid (including the one for the side chain in case the aminoacid has one). See the detailed procedure below for a table that includes these pka values for the above mentioned aminoacids.

Now that we have all those values, we should determine which of the aminoacid is located at the amino end (NH₃) and which one is located at the carboxyl end (COO). In this problem we are already given the information that Aspartic acid is at the amino end and that Asparagine is at the carboxyl end. This means that the other two aminoacids (Arginine and Glutamic acid) will only contribute with their side chains because their amino and carboxyl groups are present in the peptide bond.

So this means that Asparagine contributes with the pka of its carboxyl group and Aspartic acid contributes with the pka of its amino group. Arginine and Glutamic acid contribute with the pka of their side chains and let's not forget that aspartic acid has a side chain as well so we must consider it.

Now we have to list the values of the 5 pka mentioned above and we just simply locate the two closest ones and average them. See the detailed procedure below for more information.

So the correct answer, using significant figures, is B) 4.06

Detailed procedure:

queen_honey_blossom answered 7 months ago

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