Biochemical and Biological Attributes of Matrix Metalloproteinases

Abstract

Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases that are involved in the degradation of various proteins in the extracellular matrix (ECM). Typically, MMPs have a propeptide sequence, a catalytic metalloproteinase domain with catalytic zinc, a hinge region or linker peptide, and a hemopexin domain. MMPs are commonly classified on the basis of their substrates and the organization of their structural domains into collagenases, gelatinases, stromelysins, matrilysins, membrane-type (MT)-MMPs, and other MMPs. MMPs are secreted by many cells including fibroblasts, vascular smooth muscle (VSM), and leukocytes. MMPs are regulated at the level of mRNA expression and by activation of their latent zymogen form. MMPs are often secreted as inactive pro-MMP form which is cleaved to the active form by various proteinases including other MMPs. MMPs cause degradation of ECM proteins such as collagen and elastin, but could influence endothelial cell function as well as VSM cell migration, proliferation, Ca²⁺ signaling, and contraction. MMPs play a role in tissue remodeling during various physiological processes such as angiogenesis, embryogenesis, morphogenesis, and wound repair, as well as in pathological conditions such as myocardial infarction, fibrotic disorders, osteoarthritis, and cancer. Increases in specific MMPs could play a role in arterial remodeling, aneurysm formation, venous dilation, and lower extremity venous disorders. MMPs also play a major role in leukocyte infiltration and tissue inflammation. MMPs have been detected in cancer, and elevated MMP levels have been associated with tumor progression and invasiveness. MMPs can be regulated by endogenous tissue inhibitors of metalloproteinases (TIMPs), and the MMP/TIMP ratio often determines the extent of ECM protein degradation and tissue remodeling. MMPs have been proposed as biomarkers for numerous pathological conditions and are being examined as potential therapeutic targets in various cardiovascular and musculoskeletal disorders as well as cancer.

Keywords: Cell signaling; Collagen; Extracellular matrix; Protein degradation; Proteinases; Remodeling.

Fig. 1

Major MMPs subtypes and their structure. A typical MMP consists of a propeptide, a catalytic metalloproteinase domain, a linker peptide (hinge region), and a hemopexin domain. The propeptide has a cysteine switch PRCGXPD whose cysteine sulfhydryl (–SH) group chelates the active site Zn²⁺, keeping the MMP in the latent proMMP zymogen form. The catalytic domain contains the Zn²⁺ binding motif HEXXHXXGXXH, two Zn²⁺ ions (one catalytic and one structural), specific S1, S2,…Sn and S1′, S2′,…Sn′ pockets, which confer specificity, and two or three Ca²⁺ ions for stabilization. Some MMPs show exceptions in their structures. Gelatinases have 3 type-II fibronectin repeats in the catalytic domain. Matrilysins have neither a hinge region nor a hemopexin domain. Furin-containing MMPs such as MMP-11, 21 and 28 have a furin-like pro-protein convertase recognition sequence in the propeptide C-terminus. MMP-28 has a slightly different cysteine switch motif PRCGVTD. Membrane-type MMPs (MT-MMPs) typically have a transmembrane domain and a cytosolic domain. MMP-17 and -25 have a glycosylphosphatidylinositol (GPI) anchor. MMP-23 lacks the consensus PRCGXPD motif, has a cysteine residue located in a different sequence ALCLLPA, may remain in the latent inactive proform through its type-II signal anchor, and has a cysteine-rich region and an immunoglobulin-like proline-rich region.

Fig. 2

MMP-substrate interaction. MMP-3 is used as an example, and slight variations in the MMP-substrate-interaction and the positions of the conserved His and Glu may occur with other MMPs. Only the MMP catalytic domain is illustrated, and the remaining part of the MMP molecule is truncated by squiggles. A) In the quiescent MMP molecule, the catalytic Zn²⁺ is supported in the HEXXHXXGXXH-motif by binding to the imidazole rings of the 3 histidines His201, 205, 211. Additionally, the methionine-219 (Met219) in the conserved XBMX Met-turn acts as a hydrophobic base to further support the structure surrounding the catalytic Zn²⁺. In preparation of MMP for substrate binding, an incoming H₂O molecule is polarized between the MMP acidic Zn²⁺ and basic glutamate-202 (Glu202). B) Using H⁺ from free H₂O, the substrate carbonyl group binds to Zn²⁺, forming a Michaelis complex. This allows the MMP S1, S2, S3, …Sn pockets on the right side of Zn²⁺ and the primed S1′, S2′, S3′, …Sn′ pockets on the left side of Zn²⁺ to confer specific binding to the substrate P1, P2, P3, … Pn and the primed P1′, P2′, P3′, … Pn′ substituents, respectively. The MMP pockets are organized such that the S1 and S3 pockets are located away from the catalytic Zn²⁺, while the S2 pocket is closer to Zn²⁺. C) The substrate-bound H₂O is freed, the Zn²⁺-bound O from the Glu-bound H₂O executes a nucleophilic attack on the substrate carbon, and the Glu202 extracts a proton from the Glu-bound H₂O to form an N-H bond with the substrate N, resulting in a tetrahedral intermediate. D) Freed H₂O is taken up again, and the second proton from Glu-bound H₂O is transferred to the substrate, forming an additional N-H bond. As a result, the substrate scissile C-N bond breaks, thus releasing the N portion of the substrate while the carboxylate portion of the substrate remains in an MMP-carboxylate complex. Another H₂O is taken up, thus releasing the remaining carboxylate portion of the substrate, and the MMP is prepared to attack another substrate (A).

Fig. 3

Representative roles of MMPs in physiological processes.

Fig. 4

Representative roles of MMPs in pathological conditions. COPD, chronic obstructive pulmonary disease; MI, myocardial infarction