Recombinant human cathepsin X is a carboxymonopeptidase only: a comparison with cathepsins B and L

Abstract

The S-1 and S-2 subsite specificity of recombinant human cathepsins X was studied using fluorescence resonance energy transfer (FRET) peptides with the general sequences Abz-Phe-Xaa-Lys(Dnp)-OH and Abz-XaaArg-Lys(Dnp)-OH, respectively (Abz=ortho-aminobenzoic acid and Dnp=2,4-dinitrophenyl; Xaa=various amino acids). Cathepsin X cleaved all substrates exclusively as a carboxymonopeptidase and exhibited broad specificity. for comparison, these peptides were also assayed with cathepsins B and L. Cathepsin L hydrolyzed the majority of them with similar or higher catalytic efficiency than cathepsin X, acting as an endopeptidase mimicking a carboxymonopepticlase (pseudo-carboxymonopeptidase). in contrast, cathepsin B exhibited poor catalytic efficiency with these substrates, acting as a carboxydipeptidase or an endopeptidase. the S-1' subsite of cathepsin X was mapped with the peptide series AbzPhe-Arg-Xaa-OH and the enzyme preferentially hydrolyzed substrates with hydrophobic residues in the P-1' position.