Structure of an enantiomeric protein, D-monellin at 1.8 A resolution.

The D-enantiomer of a potently sweet protein, monellin, has been crystallized and analyzed by X-ray crystallography at 1.8 A resolut ion. Two crystal forms (I and II) appeared under crystallization conditions similar, but not identical, to the crystallization conditions of natural L-monellin. There are four molecules per asymmetric unit in crystal form I and one in crystal form II. Crystal form I is not reproducible and is equivalent to that of monoclinic L-monellin. Intermonomer contacts in crystal form II are very different from those found in natural L-monellin crystals. The backbone trace of D-monellin resembles very closely the mirror image of that of L-monellin, but the N- and C-terminus backbones as well as several side-chain conformations of D-monellin are different from those of natural L-monellin. Most of these apparent differences may be attributable to the crystal packing differences.