Fluorescence spectroscopic analysis of ligand binding to kringle 1 + 2 + 3 and kringle 1 fragments from human plasminogen

The ligand binding of kringle 1 + 2 + 3 and kringle 1 from human plasminogen has been investigated by fluorescence spectroscopy. Analysis of fluorescence titration of kringle 1 + 2 + 3 with 6-aminohexanoic acid shows that this fragment, besides the high-affinity lysine-binding site with Kd = 2.9 microM, contains two additional lysine-binding sites which differ in binding strength (Kd = 28 microM and Kd = 220 microM). This strongly suggests the existence of a lysine-binding site in each of the first three kringles. 6-Aminohexanoic acid, pentylamine, pentanoic acid and arginine were used for investigation of the ligand specificity of isolated kringle 1 prepared by pepsin hydrolysis of kringle 1 + 2 + 3. It has been established that kringle 1 has high affinity to 6-aminohexanoicacid, pentylamine and arginine (Kd values are 3.2 microM, 4.8 microM and 4.3 microM, respectively). At the same time pentanoic acid did not bind with kringle 1. These facts indicate, firstly, a broad ligand specificity of kringle 1 and, secondly, the paramount importance of the positively charged group of the ligand for its interaction with lysine-binding site of this kringle.