Val-Ala-Ala-His-Glu-X-Gly-His-X-X-Gly-X-X-His
The sequence Val-Ala-Ala-His-Glu-X-Gly-His-X-X-Gly-X-X-His contains two amino acids with pKa values of approximately 7.0. Glutamic Acid and Histidine are these two amino acids. Histidine’s pKa is greater than Glutamic Acid’s so Histidine would be the one that is deprotonated in solutions with pH 7.0 and lower. Calculated by comparing the pKa values for Glutamic Acid, Histidine and Histidine, the pI of this sequence can be found at 6.52. Specifically, Glutamic Acid has a pKa value of 4.25 while Histidine has a pKa value of 6.0 (Graf and Navarro, 2016, Navarro et.al., 2021). Therefore, the pI of the Val-Ala-Ala-His-Glu-X-Gly-His-X-X-Gly-X-X-His sequence is 6.52 (4.25 + 6.0)/2. The pH of any solution above 6.52 will cause Histidine to deprotonate, and the pH of any solution below 6.52 will cause Glutamic acid deprotonation. Cont…