Answer to Question #48298 in Biochemistry for ivan

Question

20 Lysine 15 Arginine and three histidine how much should be added asp and glu that pI is 9.93 , ie we have a zwitter ion.

Answer 1

Assuming the following pKa for the aminoacids:
Arg (10.5), Lys (12.9), His (6.1), Asp (3.9), Glu (4.3).
At the pI the net charge of the molecule is zero.
At 9.93 Lys (pKa=12.9) and Arg (pKa=10.5) are charged positively, His has zero charge (pKa=6.1). Thus, overall positive charge is 20+15=35. Thus, ideally we need 35 Asp/Glu to compensate this positive charge, and the protein will have pI of 9.93.
Comment: real pI of this protein would be less, so in fact we need less Asp/Glu to compensate the positive charge. pI also depends on pKa values used, and folding of the protein also affects its pI.