Purification, crystallization and preliminary X‐ray investigation of aqualysin I, a heat‐stable serine protease

Aqualysin I, a thermostable protease found in the culture medium of Thermus aquaticus YT-1, has been purified to homogeneity using a combination of ion-exchange and affinity chromatography. It is a polypeptide with a molecular weight of 28 350 [Kwon, Terada, Matsuzawa & Ohta (1988). Eur. J. Biochem. 173, 491–497] and is most active at 343–353 K and pH about 10.0 [Matsuzawa, Tokugawa, Hamaoki, Mizoguchi, Taguchi, Terada, Kwon & Ohta (1988). Eur. J. Biochem. 171, 441–447]. Crystals of the enzyme are monoclinic, space group P21, with cell dimensions a = 40.80 (5), b = 64.39 (6), c = 45.51 (6) A and β = 109.1 (1)°. The asymmetric unit consists of a single molecule (Vm = 1.99 A3Da−1). The crystals are stable to X-radiation and scatter to at least 2.8 A resolution.