Relative orientation of close-packed beta-pleated sheets in proteins.

When beta-pleated sheets pack face to face in proteins, the angle between the strand directions of the two beta-sheets is observed to be near -30 degrees . We propose a simple model for beta-sheet-to-beta-sheet packing in concanavalin A, plastocyanin, gamma-crystallin, superoxide dismutase, prealbumin, and the immunoglobin fragment V(REI). This model shows how the observed relative orientation of two packed beta-sheets is a consequence of (i) the rows of side chains at the interface being approximately aligned and (ii) the beta-sheet having a right-handed twist. The special amino acid composition of residues at the beta-sheet-to-beta-sheet interfaces makes the contact surfaces essentially smooth and hydrophobic.