The crystal structure of ribonuclease. I

Abstract

A detailed investigation of the monoclinic form of crystalline ribonuclease has been undertaken. The unit cell dimensions of the crystals in the wet and air-dried states are as follows: a b c β a sin β (A) (A) (A) (°) (A) wet crystal 30*90 38-80 54-06 106-0 29-30 dry crystal 29*10 30-08 51-03 114-0 26-4 The densities of the wet and air-dried crystals were found to be 1.220 and 1.269 respectively, and the air-dried crystals contained 13.6 % by weight of water. From the latter observations and the dry-cell dimensions, the molecular weight of ribonuclease, assuming two molecules in the unit cell, was calculated to be 13400. This figure is in reasonable agreement with the determinations of molecular weight made by osmotic pressure and sedimentation-diffusion methods. The Patterson projection maps along the three axes, a, band c, show vectors of 9.5 and 4.5 to 5.0 A, which are characteristic of protein crystals. In the case of the c-axis projection map it has been possible to identify peaks separated by 9.5 A lying on an hexagonal net. Perpendicular to this plane there is a vector repeat of 4.9 A lying parallel to the c axis of the unit cell. These vector distances are similar to those obtained from measurements on other proteins, where 9*5 A is thought to be the distance between polypeptide chains lying parallel to one another in an hexagonal array and 4.5 to 5 A are repeat units along the chains. The attempt has been made to interpret the Patterson vector projection maps in term s of sets of points chosen from the vector diagrams in such a manner that there is agreement between calculated and observed structure factors. Phase angles were thus obtained, and these were used with observed | F | values to calculate electron-density maps on the c- and a-axis projections of the unit cell. In all this work the minimum of hypothesis about protein structure has been employed. The selection of points made it possible to construct a vector model of ribonuclease, which suggests that it contains five polypeptide chains, each with a minimum length of about 46 A, lying parallel to one another on an hexagonal network and separated from each other by distances of about 9.5 A. These preliminary investigations are insufficient to establish any connexions between the ends of the polypeptide chains, or of the nature of the intramolecular fold of the chains themselves. The dimensions of the molecule are approximately 18 x 30 x 48 A, and the evidence favours the existence of two amino-acids per 4.9 A repeat unit along the axes of the polypeptide chains, assuming these chains to contain the a-type fold. The molecule contains something between 90 and 110 amino-acid residues. Independent confirmation of the direction of the polypeptide chains in the crystal has been provided from polarized infra-red absorption studies of the —N—H — combination bands of the internal hydrogen bonds of the polypeptide chains.