Question

proteses into pyepries ond the peride Src is an important signaling protein that is regulated by phosphorylation. A purified solution of Src is digested with proteases into peptides, and the peptide mixture is then incubated with extracts from lymphoma cells known to have tyrosine kinase activity. The peptides are analyzed by mass spec to identify any Src peptides that are phosphorylated. Src peptide fragment Asn-Gln-Tyr-Thr is identified a) Draw basic stick figures showing the ionization states of peptide Asn-Gln-Tyr- Thr as the pH is raised from acidic to basic. Indicate the pKa values of the different ionizable groups on your figure.

Answer 1

The given peptide consists of two basic amide side chains which possess quite a high pKa due to the inertness of amide groups arising from their tautomerism while Tyr contains a phenolic side-chain and Thr a hydroxyl side-chain, of which ionize only the phenolic chain gets deprotonated and that too only in very strongly basic conditions. Other ionizable groups are the amine group of aspargine and acid group of threonine which possess consideraly low pKa values. Thus, the structure of the given tetrapeptide in (strongly) acidic and basic conditions are as shown below.

Acidic OH NH OH 8.72 H3lN HO H2N Basic o 10.46 NH2 O 2.09 H2N eo H2N

Another possible ionization under highly basic conditions, above pH 14 will be that of the hydroxyl group of threonine which has a pKa of 13 which is comparable to that of tyrosine's phenolic residue whose pKa is 10.46. However, the ionization of threonine's hydroxyl group seldom occurs but since the pH is not specified here it is taken to be ionizable under the "basic" conditions mentioned.