CRYSTAL-STRUCTURE OF NARBONIN AT 1.8-ANGSTROM RESOLUTION

The three-dimensional structure of narbonin, a seed protein from Vicia narbonensis L, has been determined at 1.8 Angstrom resolution. Phase information was obtained by multiple isomorphous replacement and optim ized anomalous dispersion. The narbonin structure was initially traced with only 17% amino-acid sequence information and preliminarily refin ed to a crystallographic R-factor of 16.5%. It is now refined to 15.9% using full sequence information derived from cDNA and after the addit ion of more solvent molecules. The monomeric molecule of narbonin is a n eight-stranded parallel beta-barrel surrounded by alpha-helices in a beta/alpha-topology similar to that first observed in triose phosphat e isomerase. Differences exist in the N-terminal part of the polypepti de chain, where the first helix is replaced by a loop and the second b eta-strand is followed by an additional antiparallel alpha-sheet place d parallel on top of alpha-helices alpha 3 and alpha 4. Two short addi tional secondary structures are present. The first, an alpha-helix, is situated between the seventh beta-strand and the following helix, and the second, which is a 3(10) helix, between the eighth strand and the C-terminal helix. The most striking observation is the lack of a know n enzymatic function for narbonin, because all TIM-like structures kno wn so far are enzymes.