WILD-TYPE ALPHA(1)-ANTITRYPSIN IS IN THE CANONICAL INHIBITORY CONFORMATION

alpha(1)-Antitrypsin is the archetypal member of the serine proteinase inhibitor or serpin superfamily. Members of the family show structura l homology based on a dominant A beta-sheet and a mobile reactive cent re loop. Our recent crystal structure of alpha(1)-antitrypsin stabiliz ed with a point mutation showed the loop to be in a canonical inhibito ry conformation in the absence of significant insertion into the A bet a-sheet. It could be argued that the stabilizing mutation may Induce t he reactive centre loop to adopt an artificial, and unrepresentative, conformation and the finding seems to be at variance with studies asse ssing rates of peptide insertion into the A beta-sheet and limited pro teolysis of the reactive loop. Here we present a 2.9 Angstrom structur e of recombinant wild-type alpha(1)-antitrypsin with no stabilizing mu tations. Again, the reactive loop is in a canonical conformation in th e absence of significant insertion into the A beta-sheet. A stabilizin g salt bridge between P-5 glutamate and arginine residues 196, 223 and 281, already identified in the mutant, provides strong evidence that this conformation is not an artefact of crystallization but represents the conformation of the circulating inhibitor in vivo. Comparison wit h the structure of alpha(1)-antitrypsin stabilized with the Phe51Leu m utation indicates that the increased thermal stability of the mutant r esults from enhanced packing of aromatic residues in the hydrophobic c ore of the molecule. The structure of wild-type alpha(1)-antitrypsin r eveals a hydrophobic pocket between s2A and helices D and E that is fi lled on reactive loop insertion and the formation of biologically rele vant loop-sheet polymers. This pocket may provide a target for rationa l drug design to prevent the formation of polymers and the associated plasma deficiency, liver cirrhosis and emphysema. (C) 1998 Academic Pr ess Limited.