THE AVERAGE ATOMIC VOLUME AND DENSITY OF PROTEINS

The density of proteins is an important quantity. For ab initio phasin g in X-ray crystallography, such as low resolution envelope techniques [1, 2], an accurate protein density is essential to get a correct pre diction of the protein volume from the amino acid sequence. An accurat e value will also facilitate the determination of the number of protei n subunits in the unit cell. There has long been a general consensus t hat the mean density of a protein is 1.35 g/cm(3) [3], but this value should be revised. The present study, based on X-ray crystallographic coordinates of 28 different proteins, shows that the mean density of p roteins is 1.22 +/- 0.02 g/cm(3). The protein density was calculated u sing the Voronoi construction with over 80% of the total number of ato ms in the protein molecules taken into account.