Metal ion substitution in the catalytic site greatly affects the binding of sulfhydryl-containing compounds to leucyl aminopeptidase

Bovine lens leucyl aminopeptidase (bILAP), a homohexameric metallopeptidase preferring bulky and hydrophobic amino acids at the N-terminus of (di)peptides, contains two Zn2+ ions per subunit that are essential for catalytic activity. They may be replaced by other divalent cations with different exchange kinetics. The protein readily exchangeable site (site 1) can be occupied by Zn2+, Mn2+, Mg2+, or Co2+, while the tight binding site (site 2) can be occupied by Zn2+ or Co2+. We recently reported that introduction of Mn2+ into site 1 generates a novel activity of bILAP toward CysGly [Cappiello, M., et al. (2004) Biochem. J. 378, 35-44], which in contrast is not hydrolyzed by the (Zn/Zn) enzyme. This finding, while disclosing a potential specific role for blLAP in glutathione metabolism, raised a question about the features required for molecules to be a substrate for the enzyme. To clarify the interaction of the enzyme with sulfhydryl-containing derivatives, (Zn/Zn)- and (Mn/Zn)bILAP forms were prepared and functional-structural studies were undertaken. Thus, a kinetic analysis of various compounds with both enzyme forms was performed; the crystal structure of (Zn/Zn)bILAP in complex with the peptidomimetic derivative Zofenoprilat was determined, and a modeling study on the CysGly-(Zn/Zn)bILAP complex was carried out. This combined approach provided insight into the interaction of bILAP with sulfhydryl-containing derivatives, showing that the metal exchange in site I modulates binding to these molecules that may result in enzyme substrates or inhibitors, depending on the nature of the metal.