1) What is the overall charge state for the tetrapeptide DREAM at physiological

Question

                    1) What is the overall charge state for the tetrapeptide DREAM at physiological pH. The pK values for N-terminal, C-terminal, side chain of asp, arg, glu are                     9.0, 3.5, 3.9, 12.5, and 4.1, respectively.
                    Note: DREAM is the sequence of Asp-Arg-Glu-Ala-Met                 

                    2) Calculate fractional saturation of Mb (K = 2.8) at pO2 = 5, 10, 15, 20.                 

                                     Full answers will receive full credit.

Explanation / Answer

At pH<pKa, we have protonation of the acid/base. When pH>pKa, we have deprotonation. When a base is protonated, it has a +1 charge. No charge when deprotonated. When an acid is deprotonated, it has -1 charge. No charge when protonated.

At pH1:
Amino group is protonated (NH3+, giving a +1 charge), carboxy group is protonated (No charge), both arginine side chain is protonated (2+ charge since it is basic, just like amino group), lysine is protonated (+1 charge), aspartic acid is protonated (no charge). Other amino acids are not acidic/basic so no charge.
Hence net charge:1+2+1=+4
At pH7:
Amino group is protonated (NH3+, giving a +1 charge), carboxy group is deprotonated (-1), both arginine side chain is protonated (2+ charge since it is basic, just like amino group), lysine is protonated (+1 charge), aspartic acid is deprotonated (-1 charge). Other amino acids are not acidic/basic so no charge.
Hence net charge: 1-1+3-1= +2
At pH14:
Amino group is deprotonated (NH2, no charge),carboxy group is deprotonated (-1), both arginine side chain are deprotonated (no charge), lysine is deprotonated (no charge), aspartic acid is deprotonated (-1 charge). Other amino acids are not acidic

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