Matrix metalloproteinases

Synthesis and transcriptional regulation

Matrix metalloproteinases belong to the class of inducible enzymes. This means that their normal tissue concentration is maintained at a very low level, and large-scale synthesis is triggered only in response to specific stimuli.

The formation of MMPs is tightly controlled at the transcriptional stage. The main regulatory molecules capable of initiating or modulating the transcription of metalloproteinase genes include:

• Cytokines (e.g., interleukin-1, IL-1);
• Sex hormones (estrogen, progesterone);
• Growth factors (epidermal growth factor EGF, platelet-derived growth factor PDGF);
• Prostaglandins.

In addition to chemical signals, phagocytosis and mechanical stress on cells can also act as stimulators of MMP gene expression.

Activation and inhibition mechanisms

A small reserve of metalloproteinases is always present in the extracellular matrix, but they exist in an inactive form (as zymogens).

Enzyme activation occurs post-translationally via partial proteolysis. For the enzyme to become active, an inhibitory fragment must be cleaved off. This process is catalyzed by activators such as plasmin, urokinase, and other proteases. Additionally, the MMP family is characterized by autoactivation, a situation where already active metalloproteinase molecules cleave and activate their inactive counterparts.

Inhibition of activity is carried out by small specific proteins known as tissue inhibitors of matrix metalloproteinases (TIMPs).

• They exhibit high specificity specifically for MMPs.
• They form stable complexes with most members of this enzyme family.
• They are capable of binding to both active and inactive forms of metalloproteinases, reliably blocking their function.

In turn, TIMP inhibitors themselves can be degraded during proteolysis. Cleavage of the inhibitors is carried out by enzymes such as trypsin, chymotrypsin, stromelysin-3, and neutrophil elastase, leading to the release and a sharp increase in MMP activity at the lesion site.

Example of MMP function: Collagenase

A typical representative of the matrix metalloproteinase family is collagenase.

Reaction mechanism: Collagenase precisely cleaves peptide bonds right within the rigid helical region of the collagen molecule. The cleavage typically occurs between amino acids glycine and leucine, or glycine and isoleucine.

Result of action: The long collagen molecule breaks down into two fragments corresponding to approximately 3/4 and 1/4 of the original length. Following this cleavage, these fragments irreversibly lose their tight helical structure. The unfolded protein chain becomes vulnerable and accessible to hydrolysis by other, less specific proteases—gelatinases and various peptidases.

In healthy tissue, collagenase exhibits virtually no activity for three reasons:

1. Synthesis as a zymogen: the enzyme is secreted by cells as an inactive precursor—procollagenase.
2. Presence of tissue inhibitors: TIMP proteins rapidly bind any accidentally activated molecule.
3. Compartmentalization: the enzyme is secreted not randomly, but strictly locally and only when there is a valid necessity (e.g., for tissue remodeling or at an inflammation site).

Physiological and pathological role

The balance between MMP synthesis, their activators, and inhibitors is critically important for the body.

Normally, metalloproteinases ensure:

• Processes of embryonic development (requiring continuous tissue remodeling);
• Formation of an adequate immune response.

In pathology, when excessive, uncontrolled MMP activity is observed, they act as destructive factors:

• They provoke disruption of permeability and destruction of the blood-brain barrier;
• They cause severe destruction of articular cartilage, which is a key link in the pathogenesis of rheumatoid arthritis and osteoarthritis;
• They participate in the development of various cardiovascular diseases by remodeling the vascular wall and myocardial tissue.

Class and exam questions

What else to study on this topic

• Mechanism of procollagenase activation via partial proteolysis
• The role of cytokines (IL-1) and growth factors (EGF, PDGF) in the regulation of MMP genes
• Interaction of TIMPs with active and inactive forms of metalloproteinases
• Pathogenesis of blood-brain barrier destruction by MMPs
• Involvement of MMPs in the development of rheumatoid arthritis and osteoarthritis

Other topics in the section "Biochemistry of the Extracellular Matrix"

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