analyzed the charge on the RGD tripeptide as a function of pH, and found that th

Question

analyzed the charge on the RGD tripeptide as a function of pH, and found that the peptide was 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 contains R, G, and D amino acids, but not in equal numbers. The isoelectric point of the protein is measured 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

1. The tripeptide is R-G-D, which are arginine, glycine and aspartic acid. If 100 of these are in sequence, beginning with R at amino terminal and D at carboxyl terminal, the effect of ph will be such that there will be a shift to alkaline side. The increased Arginine is responsible for pI shift generally.

2. The pI as per question 1 is nearly around 7. 34, as calculated using a pI calculator. This can theoretically be explained on the same lines that additional L-Arginine or even lysine and histidine brings about a shift in the pKa and thus pI of the system/solution. So the exact number may not be available of the three amino acids in the sequence.

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