Alpha₁-antitrypsin is a member of a family of protease inhibitors known as the serpins. Mutations in these molecules can lead to disease, not only because the biologic activity of the protease in tissue is increased, but also because the mutations result in misfolded (i.e., conformationally abnormal) protease molecules that accumulate in tissue. This review article summarizes the action of these protease inhibitors and how mutations lead to their accumulation in particular neurodegenerative disorders such as prion encephalopathies and Alzheimer's disease.

A 20-year effort has succeeded in identifying the initiating step in the molecular interactions that release the vasopressor peptide angiotensin, and hence control blood pressure. Angiotensins are generated by cleavage of a larger protein, angiotensinogen, by the enzyme renin. Crystal structures of angiotensinogen alone and bound to renin now show that a large conformational change is required to expose the renin cleavage site. This transition is regulated by oxidation, and women with pre-eclampsia — which is associated with high blood pressure — have higher levels of the more active oxidized form. Angiotensins have a crucial role in blood pressure regulation and are generated by cleavage of a larger protein, angiotensinogen, by the enzyme renin. Structures of angiotensinogen alone and in complex with renin show that a large conformational change is required to expose the renin-cleavage site. The authors also show that this transition is regulated by oxidation and that women with pre-eclampsia have higher levels of the more active, oxidized, form. Blood pressure is critically controlled by angiotensins1, which are vasopressor peptides specifically released by the enzyme renin from the tail of angiotensinogen—a non-inhibitory member of the serpin family of protease inhibitors2,3. Although angiotensinogen has long been regarded as a passive substrate, the crystal structures solved here to 2.1 Å resolution show that the angiotensin cleavage site is inaccessibly buried in its amino-terminal tail. The conformational rearrangement that makes this site accessible for proteolysis is revealed in our 4.4 Å structure of the complex of human angiotensinogen with renin. The co-ordinated changes involved are seen to be critically linked by a conserved but labile disulphide bridge. Here we show that the reduced unbridged form of angiotensinogen is present in the circulation in a near 40:60 ratio with the oxidized sulphydryl-bridged form, which preferentially interacts with receptor-bound renin. We propose that this redox-responsive transition of angiotensinogen to a form that will more effectively release angiotensin at a cellular level contributes to the modulation of blood pressure. Specifically, we demonstrate the oxidative switch of angiotensinogen to its more active sulphydryl-bridged form in the maternal circulation in pre-eclampsia—the hypertensive crisis of pregnancy that threatens the health and survival of both mother and child.

The hormones that most directly control tissue activities in health and disease are delivered by two noninhibitory members of the serpin family of protease inhibitors, thyroxine-binding globulin (TBG) and corticosteroid-binding globulin. The structure of TBG bound to tetra-iodo thyroxine, solved here at 2.8 Å, shows how the thyroxine is carried in a surface pocket on the molecule. This unexpected binding site is confirmed by mutations associated with a loss of hormone binding in both TBG and also homologously in corticosteroid-binding globulin. TBG strikingly differs from other serpins in having the upper half of its main β-sheet fully opened, so its reactive center peptide loop can readily move in and out of the sheet to give an equilibrated binding and release of thyroxine. The entry of the loop triggers a conformational change, with a linked contraction of the binding pocket and release of the bound thyroxine. The ready reversibility of this change is due to the unique presence in the reactive loop of TBG of a proline that impedes the full and irreversible entry of the loop that occurs in other serpins. Thus, TBG has adapted the serpin inhibitory mechanism to give a reversible flip-flop transition, from a high-affinity to a low-affinity form. The complexity and ready triggering of this conformational mechanism strongly indicates that TBG has evolved to allow a modulated and targeted delivery of thyroxine to the tissues.

Cortisol is transported in the blood by corticosteroid-binding globulin (CBG), a non-inhibitory member of the serpin family of serine protease inhibitors. Recent structural advances reveal how CBG acts as a releasing-agent as well as a carrier of cortisol. Taken together, the structures of the various forms of CBG and of the closely related thyroxine binding-globulin, show how the inherent conformational mechanism of the serpins has been adapted to modulate hormone release to the tissues by changes in binding affinities. A deduction from this, of the temperature dependence of hormone binding, is remarkably borne out with CBG, with a doubling in plasma free cortisol as the body temperature rises to 39°C. Another insight, against a dogma in the corticosteroid field, is that the proteolytic cleavage of CBG in inflammation results in a partial and not a complete loss of cortisol binding. This becomes of medical importance in conjunction with recent evidence of a pool of the circulating cleaved-form of CBG. It is now evident that tissue levels of free cortisol are buffered by two responsive plasma pools, intact CBG with a high binding-affinity and, particularly in inflammation and sepsis, a further pool of cleaved-CBG with a ten-fold lower affinity. The new molecular understandings, as well as providing insights into the differential release of circulating hormones, also open prospects for therapeutic interventions and draw attention to the potential of CBG and TBG as vehicles for the targeted delivery of drugs.