The charge on the RGD tripeptide is neutral at the isoelectric point of 6.3. A)

Question

The charge on the RGD tripeptide is neutral at the isoelectric point of 6.3.

A) What is the isoelectric point of an RGD protein that consists of 100 RGD sequences in a row?

B) Now consider a large protein that only cont ains R, G, and D amino acids, but not in equal numbers. The isoelectric point of the protein is me asured to be 7.4. Can you determine the relative numbers of the three different amino acids in the protein? If so, what are they? If not, what information about the amino acid content can you obtain from the isoelectric point?

Explanation / Answer

Ans. #A. The pI of 100 RGD sequence in a row = 6.3

Each tripeptide RGD unit has neutral charge at pH 6.3.

Adding two such units by peptide bond involves the –COOH terminal of first unit and the –NH2 group of the second unit. So, there is no new ionizable group created or destroyed during joining two or more units in a row.

Therefore, the pI of such units in a row remains the same as that of the single RGD unit.

#B. No, the relative numbers of R, G and D amino acids can’t be determined.

The charge on N-terminal, C-terminal and the side chains of R and D residues account for pI of the peptide. Change in the N-terminal residue, for example if G is replaced by R or D, would also lead to change in pI.

# The following information would be required-

I. The N-terminal residue

II. The C-terminal residue

III. Because of having a neutral side chain, the number of G residues does not affect the pI of the peptide.

IV. Only after I and II are known, the relative number of R and D can be determined.

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