Construction, expression, and purification of recombinant kringle 1 of human plasminogen and analysis of its interaction with omega-amino acids

Abstract

An Escherichia coli expression vector, containing the alkaline phosphatase promoter and the stII heat-stable enterotoxin signal sequence, along with the cDNA of the kringle 1 (K1) region of human plasminogen (HPg), has been employed to express into the periplasmic space amino acid residues 82-163 (E163----D) of HPg. This region of the molecule contains the entire K1 domain (residues C84-C162) of HPg, as well as two non-kringle amino-terminal amino acids (S82-E83) that are present in their normal locations in HPg and a carboxyl-terminal amino acid, D163, that results from mutation of the E163, normally present at this location in the HPg amino acid sequence. After purification of r-K1 by chromatographic techniques, we have investigated its omega-amino acid binding properties by titration calorimetry, intrinsic fluorescence, and differential scanning microcalorimetry (DSC). The antifibrinolytic agent, epsilon-aminocaproic acid (EACA), possesses a single binding site for r-K1. The thermodynamic properties of this interaction, studied by calorimetric titrations of the heats of binding with this ligand, reveal a Kd of 12 +/- 2 microM at 25 degrees C and pH 7.4, a corresponding delta G of -6.7 +/- 0.1 kcal/mol, a delta H of -3.6 +/- 0.1 kcal/mol, and a delta S of 10.5 +/- 0.8 eu. The intrinsic fluorescence of r-K1 decreases by approximately 44% when its binding site is saturated with EACA, and titrations of this perturbation with EACA lead to calculation of a Kd of approximately 13 microM, a value in good agreement with that obtained from titration calorimetric analysis. EACA represents the strongest binding ligand of a variety of simple aliphatic omega-amino acids examined. A cyclic analogue of EACA, trans-4-(aminomethyl)cyclohexanecarboxylic acid, interacts with r-K1 with an approximate 12-fold tighter Kd (1.0 +/- 0.2 microM). Investigations by DSC, at pH 7.4, demonstrate that a significant stabilization of the r-K1 structure occurs when EACA binds to this domain. The temperature of maximum heat capacity change (Tm) in the thermal denaturation of r-K1 increases from approximately 340.8 to 359.1 K as a consequence of EACA binding. These studies demonstrate that a fully functional EACA-binding kringle from HPg can be expressed and secreted in E. coli, purified by techniques that do not require refolding, and investigated as an independent structural unit.