Question

Cholecystokinin (CCK) was the first hormone that was demonstrated to affect appetite. It is released from the duodenum and jejunum upon consumption of food, and it acts to create a feeling of "fullness". After its discovery, it was hoped that CCK could be used somehow as an anti-obesity drug. However, it appears that decreases in meal size caused by CCK are offset by increases in meal frequency The active hormones are derived by cleavage of the cholecystokinin precursor protein into smaller peptides. In humans, CCK8 is an octapeptide with the sequence DYMGWMDF-NH2. Notice that the peptide is modified: the last residue does not have a free carboxyl group. Instead, it has been amidated (see figure below) Calculate the pl of immature CCK8 Calculate the pl of mature CCK8.

Answer 1

Immature CCK8: Predicted isoelectric point with free carboxylic group: 3.17

Mature CCK8: Predicted isolelectric point with free amidate group:3.93 (as calculated from online calculator at http://isoelectric.ovh.org/calculate.php)

Amino acid	Pk1	Pk2	side chain pk	pI
Asp	1.88	9.6	3.65	2.98
Tyr	2.20	9.11	10.07	5.63
Met	2.28	9.21		5.74
Gly	2.34	9.60		6.06
Trp	2.38	9.39		5.88
Met	2.28	9.21		5.74
asp	1.88	9.6	3.65	2.98
Phe	2.58	9.24		5.91