Extracellular endonuclease of Serratia marcescens. 1. Three-dimensional structure of crystalline protein at 1.7 angstrom resolution

A refined crystal structure of Serratia marcescens nuclease is reported to an R factor of 17.3% and R-free factor of 22.2% at a 1.7 Angstrom resolutio n. The structure includes 3678 nonhydrogen atoms of the enzyme and 443 boun d water molecules. The average deviations of bond lengths and valent angles from their standard values are 0.011 Angstrom and 1.8 degrees, and the max imal deviations are 0.07 Angstrom and 11 degrees, respectively. A new topol ogical model of the enzyme molecule is proposed, indicating high symmetry o f the monomer in its N- and C-terminal regions.