17 c histidine would be a good buffer at ph values

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Unformatted text preview: on the side ­ chain R ­group pI •  At pI (isoelectric pH), a molecule carries no net electric charge •  For amino acids without ionizable side chains, pK1 + pK 2 pI = 2 Amino acids with ionizable side chains Net charge •  His amino acid has 3 ionizable groups – 3 buffer regions. •  His side chain is protonated and charged +1 at lower pH •  pKR is near pH 7  ­ biological importance pI Amino acids with Ionizable side chains •  Each amino acid has different side chain pKa (pKR)  ­  many are not ionizable •  Glu side chain is protonated and charged 0 at low pH pI Q: Indicate whether the following statements concerning histidine are true or false. Refer to the following titration __ (a) At pH 1.82 there are equal amounts of form 1 and form 2 __ (b) The α-carboxyl group is half dissociated at pH 9.17 __ (c) Histidine would be a good buffer at pH values near 1.8 and 9.2 __ (d) Histidine has biological importance because the pKa of its side chain is close to physiological pH __ (e) Histidine’s pI is between pH 1.82 and 6.0 __ (f) Histidine’s pI is between pH 6.0 and 9.17 Amino Acid α ­COOH Aspartate Glutamate Data from Lehninger, 5th ed Table 3 ­1 — only approximate pKa is important; pKa varies inside protein CO O H pKa =1.8–2.4 CO O H CO O pKa = 3.65 CO O H pKa = 4.25 HN HisGdine CO O CO O CH C NH C H pH 1 2 3 4 HN pKa = 6.00 5 6 7 CH C N C H Amino Acid Data from Lehninger, 5th ed Table 3 ­1 — only approximate pKa is important; pKa varies inside prot...
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