CRYSTAL-STRUCTURES OF ACUTOLYSIN-A, A 3-DISULFIDE HEMORRHAGIC ZINC METALLOPROTEINASE FROM THE SNAKE-VENOM OF AGKISTRODON-ACUTUS

Acutolysin A alias AaHI, a 22 kDa hemorrhagic toxin isolated from the snake venom of Agkistrodon acutus, is a member of the adamalysin subfa mily of the metzincin family and is a snake venom zinc metalloproteina se possessing only one catalytic domain. Acutolysin A was found to hav e a high-activity and a low-activity under weakly alkaline and acidic conditions, respectively. With the adamalysin II structure as the init ial trial-and-error model, the crystal structures were solved to the f inal crystallographic X-factors of 0.168 and 0.171, against the diffra ction data of crystals grown under pH 5.0 and pH 7.5 conditions to 1.9 Angstrom and 1.95 Angstrom resolution, respectively. One zinc ion, bi nding in the active-site, one structural calcium ion and some water mo lecules were localized in both of the structures. The catalytic zinc i on is coordinated in a tetrahedral manner with one catalytic water mol ecule anchoring to an intermediate glutamic acid residue (Glu143) and three imidazole N-epsilon 2 atoms of His142, His146 and His152 in the highly conserved sequence H(142)E(143)XXH(146)XXGXXH(152). There are t wo new disulfide bridges (Cys157-Cys181 and Cys159-Cys164) in acutolys in A in addition to the highly conserved disulfide bridge Cys117-Cys19 7. The calcium ion occurs on the molecular surface. The superposition showed that there was no significant conformational changes between th e two structures except for a few slight changes of some flexible resi due side-chains on the molecular surface, terminal residues and the ac tive-site cleft. The average contact distance between the catalytic wa ter molecule and oxygen atoms of the Glu143 carboxylate group in the w eakly alkaline structure was also found to be closer than that in the weakly acidic structure. By comparing the available structural informa tion of the members of the adamalysin subfamily, it seems that, when l owering the pH value, the polarization capability of the Glu143 carbox ylate group to the catalytic water molecule become weaker, which might be the structural reason why the snake venom metalloproteinases are i nactive or have a low activity under acidic conditions. (C) 1998 Acade mic Press.