Chapter 5 - Regulation of Proteases by Protein Inhibitors of the Serpin Superfamily

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The serpins comprise an ancient superfamily of proteins, found abundantly in eukaryotes and even in some bacteria and archea, that have evolved to regulate proteases of both serine and cysteine mechanistic classes. Unlike the thermodynamically determined lock-and-key type inhibitors, such as those of the Kunitz and Kazal families, serpins use conformational change and consequent kinetic trapping of an enzyme intermediate to effect inhibition. By combining interactions of both an exposed reactive center loop and exosites outside this loop with the active site and complementary exosites on the target protease, serpins can achieve remarkable specificity. Together with the frequent use of regulatory cofactors, this permits a sophisticated time- and location-dependent mode of protease regulation. An understanding of the structure and function of serpins has suggested that they may provide novel scaffolds for engineering protease inhibitors of desired specificity for therapeutic use.

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Overview of the Serpin Superfamily

The serpin superfamily of protein protease inhibitors is an ancient family of proteins that are widely distributed in all the three kingdoms of life, the archea, the bacteria, and the eukaryotes, as well as in some viruses.1, 2 They represent a particularly notable advance in the evolution of proteins designed to control the activity of the proteolytic enzymes that regulate a multitude of biologic processes. Indeed, the serpins differ fundamentally from other families of protein protease

Mechanism of Action

Unlike most other protein protease inhibitors, serpins are capable of inhibiting proteases belonging to different mechanistic classes, as well as to different clades within a given class.1 While the most numerously documented examples are of serpins inhibiting serine proteases of the chymotrypsin family, there are examples of serpins inhibiting furin-like serine proteases44 and of serpins inhibiting cysteine proteases of both the cathepsin45 and caspase families.46 Furthermore, some individual

Rates of Reaction, Specificity, and Regulation

Canonical inhibitors form reversible noncovalent complexes with target proteases. The extent of inhibition is determined by the affinity of the complex and the concentration of each species. In contrast, the Michaelis complex between a serpin and protease, once formed, usually leads irreversibly to the kinetically trapped intermediate. The effectiveness of inhibition by a serpin is thus determined by the rates at which the Michaelis complex is formed and undergoes acylation, modified by the SI

Recombinant Serpins for Replacement Therapy

Normal and variant serpins have been engineered as recombinant proteins for the purposes of treating diseases whose pathogenesis is associated with dysfunctional protease regulation. As discussed earlier, serpin deficiencies can result in tissue damage and disease due to an overactivity of the proteases they regulate. One treatment for such diseases is replacement of the deficient serpin, usually with a recombinant protein to restore the natural protease/inhibitor balance.

Lung diseases that

Concluding Remarks

The serpin superfamily of protein protease inhibitors has evolved a unique conformational trapping mechanism of protease inhibition that is fundamentally different from that of the canonical lock-and-key inhibitors and that allows for novel modes of protease regulation. A common theme is the use of both RCL and exosite determinants on the serpin or on associated cofactors to lure a protease target to form an initial Michaelis encounter complex at an appropriate time and place. For serpins such

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