MMP3 - Structure

Structure

Most members of the MMP family are organized into three basic, distinctive, and well-conserved domains based on structural considerations: an amino-terminal propeptide; a catalytic domain; and a hemopexin-like domain at the carboxy-terminal. The propeptide consists of approximately 80–90 amino acids containing a cysteine residue, which interacts with the catalytic zinc atom via its side chain thiol group. A highly conserved sequence (. . .PRCGXPD. . .) is present in the propeptide. Removal of the propeptide by proteolysis results in zymogen activation, as all members of the MMP family are produced in a latent form. The catalytic domain contains two zinc ions and at least one calcium ion coordinated to various residues. One of the two zinc ions is present in the active site and is involved in the catalytic processes of the MMPs. The second zinc ion (also known as structural zinc) and the calcium ion are present in the catalytic domain approximately 12 Å away from the catalytic zinc. The catalytic zinc ion is essential for the proteolytic activity of MMPs; the three histidine residues that coordinate with the catalytic zinc are conserved among all the MMPs. Little is known about the roles of the second zinc ion and the calcium ion within the catalytic domain, but the MMPs are shown to possess high affinities for structural zinc and calcium ions. The hemopexin-like domain of MMPs is highly conserved and shows sequence similarity to the plasma protein, hemopexin. The hemopexin-like domain has been shown to play a functional role in substrate binding and/or in interactions with the tissue inhibitors of metalloproteinases (TIMPs), a family of specific MMP protein inhibitors. In addition to these basic domains, the family of MMPs evolved into different subgroups by incorporating and/or deleting structural and functional domains. For example, MMP-2 and MMP-9, also known as gelatinases, incorporated the three repeats homologous to the type-II module of fibronectin into the catalytic domain that has been shown to be involved in binding to denatured collagen or gelatin. This domain, known as the gelatin binding domain or fibronectin type-II-like domain, is unique to the gelatinases, and so these enzymes are regarded as a separate subgroup among members of the MMP family. Incorporation of a hydrophobic stretch of approximately 25 amino acids, representing a putative transmembrane domain at the carboxy terminus and recognition motif (RXKR) for furin-like convertases at the end of the propeptide domain, is a characteristic of the membrane-type MMPs (MT-MMPs) except MT4-MMP (vide infra). MMP-11 also contains this furin recognition motif and, similar to the MT-MMPs, it is processed into the active form intracellularly. Additional insertion to the three basic MMP domains also includes a proline-rich 54 amino acid insertion in MMP-9 with sequence similarity to the α2 chain of collagen V. Finally, MMP-7 lacks the hemopexin-like domain and represents the smallest member of the MMP family.