Question

1. How do the titration curves differ between nonionizable and ionizable R groups? Why

2.Is the pKa of a Histidine side chain always the same as that of the free amino acid? Why or Why not?3. What is eliminated when two amino acids are combined to form a dipeptide?

Biochemistry

Answer 1

1) --COOH is a weak acid(proton donor and --NH₂ is a weak base (proton acceptor) ,they undergo dissociation and ionization at neutral pH. If pH = 7  the amino group of an amino acid will be protonated and the carboxylic acid group will be deprotonated. This is called the zwitterion form.In strongly acidic solutions the carboxylic acid group will also be protonated, while in strongly basic solutions both the carboxylic acid group and the amino group will both be unprotonated.

All amino acids with a single α-amino group, a single α-carboxyl group, and an R group that does not ionize have simple titration curves with 2pKa values.. Amino acids with an ionizable R group have more complex titration curves with three stages corresponding to the three possible ionization steps; thus they have three pK_a values. The third stage for the titration of the ionizable R group merges to some extent with the others

i) amino acids with non-ionizable R group have 2 pKa ii) amino acids with ionizable R group have 3 pKa values values because it has 2 ionization steps because it has 3 ionization steps

If the R-group contains an acidic or basic(ionizable) group, the titration curve will have 3 equivalence points and 3 pKa's.

2) Histidine has three acidic groups of pKa's 1.82 (carboxylic acid) 6.04 (pyrrole NH) and 9.17 (amino NH). Histidine can exist in the four forms depending on the solution pH.

The imidazole ring in the side chain of the free amino acid i.e histidine loses its proton at about pKa 6. when Histidine is incorporated into proteins, the pKa is raised to 7, because the histidine side chain can exchange protons near physiological pH.

pKa value of imidazole side chain in histidine ranges from 6.5-7.4 in proteins while pKa of imidazole ring is 6 in free amino acid(histidine)

The pKa of a Histidine side chain is not always the same as that iof the free amino acid. It changes when Histidine is incorporated into proteins.

3) Since amino acids have both acid(carboxlic group) and basic(amine) groups, two molecules can therefore react with each other to form a molecule containing a peptide linkage

This reaction is called a condensation reaction because two amino acid molecules join together and one molecule of water is eliminated.The resulting molecule is called a dipeptide.

Water molecule is eliminated when two amino acids combine to form a dipeptide.