Exosite determinants of serpin specificity

Abstract

Serpins form an enormous superfamily of 40-60-kDa proteins found in almost all types of organisms, including humans. Most are one-use suicide substrate serine and cysteine proteinase inhibitors that have evolved to finely regulate complex proteolytic pathways, such as blood coagulation, fibrinolysis, and inflammation. Despite distinct functions for each serpin, there is much redundancy in the primary specificity-determining residues. However, many serpins exploit additional exosites to generate the exquisite specificity that makes a given serpin effective only when certain other criteria, such as the presence of specific cofactors, are met. With a focus on human serpins, this minireview examines use of exosites by nine serpins in the initial complex-forming phase to modulate primary specificity in either binary serpin-proteinase complexes or ternary complexes that additionally employ a protein or other cofactor. A frequent theme is down-regulation of inhibitory activity unless the exosite(s) are engaged. In addition, the use of exosites by maspin and plasminogen activator inhibitor-1 to indirectly affect proteolytic processes is considered.

FIGURE 1.

Examples of cofactor involvement. A, antithrombin (AT) inhibition of IIa resulting from bridging heparin as part of a heparan sulfate proteoglycan (HSPG). B, HCII inhibition of IIa at the membrane surface mediated by bridging dermatan sulfate (DS) and the N-terminal tail of HCII, which is displaced by dermatan sulfate binding. ExoI and ExoII, exosites I and II, respectively. C, specific protein Z-bound ZPI inhibition of membrane-associated fXa. Whereas the final complex between ZPI and fXa dissociates from protein Z (PZ), evidence is lacking on whether it also dissociates from the membrane surface (as shown).