Substrate specificity determinants of human macrophage elastase (MMP-12) based on the 1.1 angstrom crystal structure

The macrophage elastase enzyme (MMP-12) expressed mainly in alveolar macrop hages has been identified in the mouse lung as the main destructive agent a ssociated with cigarette smoking, which gives rise to emphysema, both direc tly via elastin degradation and indirectly by disturbing the proteinase/ant iproteinase balance via inactivation of the alpha1-proteinase inhibitor (al pha1-PI), the antagonist of the leukocyte elastase. The catalytic domain of human recombinant MMP-12 has been crystallized in complex with the broad-s pecificity inhibitor batimastat (BB-94). The crystal structure analysis of this complex, determined using X-ray data to 1.1 Angstrom and refined to an R-value of 0.165, reveals an overall fold similar to that of other MMPs. H owever, the S-shaped double loop connecting strands III and IV is fixed clo ser to the beta -sheet and projects its His172 side-chain further into the rather hydrophobic active-site cleft, defining the S3 and the S1-pockets an d separating them from each other to a larger extent than is observed in ot her MMPs. The S2-site is planar, while the characteristic S1'-subsite is a continuous tube rather than a pocket, in which the MMP-12-specific Thr215 r eplaces a Val residue otherwise highly conserved in almost all other MMPs. This alteration might allow MMP-12 to accept P1' Arg residues, making it un ique among MMPs. The active-site cleft of MMP-12 is well equipped to bind a nd efficiently cleave the AlaMetPhe-LeuGluAla sequence in the reactive-site loop of al-PI, as occurs experimentally. Similarities in contouring and pa rticularly a common surface hydrophobicity both inside and distant from the active-site cleft explain why MMP-12 shares many substrates with matrilysi n (MMP-7). The MMP-12 structure is an excellent template for the structure- based design of specific inhibitors for emphysema therapy and for the const ruction of mutants to clarify the role of this MMP. (C) 2001 Academic Press .