Question

You then perform a titration of the myoglobin solution to find a pKa value for its heme group. While the details aren’t important for this question, you are able to measure the absorbance of the protein change as a function of pH. You obtain the following data points:

Based on this data, what is the pKa? (Hint – I only want one value – you choose the most accurate way to calculate this based on your experience)

pH	Abs
2	0.200
2.5	0.218
3	0.271
3.5	0.398
4	0.609
4.5	0.811
5	0.922

What is the pKa, which type of line/curve fit did you choose, and why is it the most accurate? Should the pI and the pKa be the same value? Yes or No, and any reason why this is true?

Answer 1

SOLUTION:

If we plot Absorbance vs pH, the point of inflection is pI. Based on the above data, the pI of myoglobin is 4.0. Since, pKa and pI are connected by the formula: pI = 1/2 (pKa1 + pKa2). If pKa1 = pKa2, then pI = 1/2 * (2pKa). Thus, pI = pKa.

pI and pK values can not be the same as for a molecules like amino acids there should be two or more pK values ( for every ionizable group present in amino acid) and the pI value is the average of the pK values.