The crystal structure of papain C: I. Two-dimensional fourier syntheses

Abstract

Papain was crystallized metal-free from water-methanol solution. Two monoclinic crystal forms, A and B, and two orthorhombic forms, C and D, were found. Form C belongs to the space group P 2 1 2 1 2 1 . It has four papain molecules per unit cell. Crystals of form C containing heavy atoms were obtained by soaking the metal-free crystals in solutions of p -chloromercuribenzoate, Na 2 PtCl 6 , K 2 HgI 4 and Na 3 IrCl 6 . The metal-free and metal-containing crystals were essentially isomorphous. X-ray reflections were recorded on film with precession cameras and copper Kα radiation. The heavy atoms were located by means of two-dimensional difference Patterson and Fourier syntheses. It appeared that the papain molecule has one site available for p -chloromercuribenzoate binding, while the inorganic heavy ions were attached to the molecule in two positions, which were approximately the same for all three ions. One of the latter positions is close to the p -chloromercuribenzoate binding site. Site occupancy factors were rather low. The X-ray intensities were put on an absolute scale by comparison with anthracene and with basic beryllium acetate crystals. The scale derived from anthracene was 1·22 times the scale determined with basic beryllium acetate. Fourier syntheses with 5 A resolution are presented for the [100] and [001] projections of the papain C crystal structure. The positions of the molecules in the unit cell could be inferred, assuming their shape to be spherical.