Our previous study showed that the solubility and crystal growth rate of the protein subtilisin changed with the substitution of small numbers of surface amino acid residues. Structural and energetic comparisons of crystal structures of two subtilisin mutants were conducted to explore the reason for changes in the growth rate of subtilisin crystals. Unique lattice contact patches were determined for the two mutants. The loss of solvent accessible surface area (ASA), the average hydrophobicity and the number of hydrogen bonds and salt bridges were calculated to quantify surface properties of the contact patches. The structural comparison showed that the three amino acid mutations (Purafecr(R) --> Properase(R)) are all in contact patches and provide extra atomic contacts. For Properase(R) subtilisin, the number of contacting residues and the loss of ASA increased. Binding energetic calculations, based on the detailed protein structures, were performed to determine non-electrostatic interaction contributions for the required crystallographic orientation and the number of energetically favored, false-binding orientations. The agreement and disparity between molecular structure and macroscopic crystallization behavior are discussed. (C) 2003 Elsevier Science B.V. All rights reserved.