STRUCTURE OF AN ENANTIOMERIC PROTEIN, D-MONELLIN AT 1.8 ANGSTROM RESOLUTION

The D-enantiomer of a potently sweet protein, monellin, has been cryst allized and analyzed by X-ray crystallography at 1.8 Angstrom resoluti on. Two crystal forms (I and II) appeared under crystallization condit ions similar, but not identical, to the crystallization conditions of natural L-monellin. There are four molecules per asymmetric unit in cr ystal form I and one in crystal form II. Crystal form I is not reprodu cible and is equivalent to that of monoclinic L-monellin. Intermonomer contacts in crystal form II are very different from those found in na tural 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-mo nellin are different from those of natural L-monellin. Most of these a pparent differences may be attributable to the crystal packing differe nces.