Crystal structure of narbonin at 1.8 Å resolution

The three-dimensional structure of narbonin, a seed protein from Vicia narbonensis L, has been determined at 1.8 Å resolution. Phase information was obtained by multiple isomorphous replacement and optimized anomalous dispersion. The narbonin structure was initially traced with only 17% amino-acid sequence information and preliminarily refined 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 addition of more solvent molecules. The monomeric molecule of narbonin is an eight-stranded parallel β-barrel surrounded by α-helices in a β/α-topology similar to that first observed in triose phosphate isomerase. Differences exist in the N-terminal part of the polypeptide chain, where the first helix is replaced by a loop and the second β-strand is followed by an additional antiparallel α-sheet placed parallel on top of α-helices α3 and α4. Two short additional secondary structures are present. The first, an α-helix, is situated between the seventh β-strand and the following helix, and the second, which is a 3₁₀ helix, between the eighth strand and the C-terminal helix. The most striking observation is the lack of a known enzymatic function for narbonin, because all TIM-like structures known so far are enzymes.