Expression, purification and characterization of the recombinant kringle 2 and kringle 3 domains of human plasminogen and analysis of their binding affinity for ω‐aminocarboxylic acids

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Abstract
The kringle 2 (E161T/C162S/EEE[K2HPg/C169S]TT) and the kringle 3 (TYQ[K3HPg]DS) domains of human plasminogen (HPg) were expressed in Escherichia coli in an expression vector with the phage T5 promotor/operator element N250PSN250P29 and the cDNA sequence for a hexahistidine tail to facilitate the isolation of the recombinant protein. A coagulation factor Xa (FXa)‐sensitive cleavage site was introduced to remove the N‐terminal histidine tag. In r‐K2, mutations E161T and C162S were introduced to enhance the FXa cleavage yield and C169S to replace the cysteine residue, participating in the inter‐kringle disulfide bridge between kringles 2 and 3. Recombinant proteins were isolated by affinity chromatography on Ni2+‐nitrilotriacetic acid/agarose and refolded under denaturing and reducing conditions followed by a non‐denaturing and oxidising environment. The free thiol group in position 297 in r‐K3 was selectively alkylated with iodoacetamide. The hexahistidine tail was successfully removed with FXa. The N‐terminal sequence, the amino acid composition and the molecular mass analyses are in agreement with the expected data. The correct arrangement of the disulfide bonds was verified by sequence analysis of the corresponding thermolytic and subtilisin fragments. r‐K2 exhibits weak binding to lysine‐Bio‐Gel. The weak binding affinity of r‐K2 for ω‐aminocarboxylic acids is confirmed by intrinsic fluorescence titration with 6‐aminohexanoic acid (NH2C5COOH) indicating a Kd of approximately 401 μM. In contrast, r‐K3 seems to be devoid of a binding affinity for ω‐aminocarboxylic acids. Considering earlier determined Kd values of kringle 1, kringle 4 and kringle 5, the binding affinity of HPg kringle domains for NH2C5COOH is proposed to decrease in the following order, kringle 1 > kringle 4 > kringle 5 > kringle 2 > kringle 3.