Question

A researcher isolates a new protein and finds that it contains 0.695% tyrosine by weight upon amino acid analysis. The molecular mass of tyrosine is 181 gmol"!. What is the minimum molecular mass for the protein assuming only one Tyr residue per protein molecule? mmin = g.mol-1 The researcher estimates the molecular mass of the protein using gel electrophoresis. From this estimated molecular mass, the researcher concludes the protein contains 3 Tyr residues. What is the estimated molecular mass of the protein as shown by gel electrophoresis? mest = g-mol ?

Answer 1

According to the question, the protein contains 0.695% of tyrosine by weight and it is given that the molecular weight of tyrosine is 181 g mol^-1.

First part: Minimum molecular mass of protein assuming there is only one tyrosine residue in protein:

Let the weight of protein is X gram per mole. Assuming there is only one tyrosine per protein we can say that 181 gram of tyrosine is present per X gram of protein.

Alternatively we can say that 1 mol of tryrosine (as mol. wt of tyrosine is 181 g per mol) is present per mol of protein.

Since tyrosine is 0.695 % of X ( Weight of protein in gram per mol)

So
(X x 0.695) = 100 = 181gmol-1
or
X = 26043.16gmol-1

This is weight of protein in gram per mole assuming that there is only one mol of tyrosine residue per mole of protein.

Second part: Let the molecular weight of protein estimated on gel electrophoresis is Y gram per mol.

Since the researcher estimated that it has three residues in one protein or we can say that there are 3 mol of tyrosine present per mol of protein.

So we can write that (3 x 181 g) of tyrosine is present in one mol of protein or Y gram of protein.

that is

(Y X 0.695) = 100 = 3 x 1819

or

Y = 78129.49gmol-1