Crystallization and preliminary X-ray diffraction analysis of hemextin A: a unique anticoagulant protein from Hemachatus haemachatus venom.

Hemextin A was isolated and purified from African Ringhals cobra (Hemachatus haemachatus). It is a three-finger toxin that specifically inhibits blood coagulation factor VIIa and clot formation and that also interacts with hemextin B to form a unique anticoagulant complex. Hemextin A was crystallized by the hanging-drop vapour-diffusion method by equilibration against 0.2 M ammonium acetate, 0.1 M sodium acetate trihydrate pH 4.6 and 30% PEG 4000 as the precipitating agent. The crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 49.27, b = 49.51, c = 57.87 A and two molecules in the asymmetric unit. They diffracted to 1.5 A resolution at beamline X25 at BNL.