X-Ray Crystallographic Studies of Pepsin

Pepsin was the first object used to obtain x-ray photographs of protein crystals (1) and the second protein to be crystallized (2). J. Northrop developed the method of porcine pepsin crystallization in this well known investigation (2). Two of the crystal forms of pepsin that have been described, the first hexagonal form has been studied by J. Bernal and D. Crowfoot (1), and by M. Perutz (3). The unit cell of hexagonal pepsin crystals was too large, however, for studies of it to continue at the initial stage of the development of protein crystallography. The second form of the crystal was more suitable for x-ray crystallographic studies.