Three-Dimensional Solution Structure of Oryzacystatin-I, a Cysteine Proteinase Inhibitor of the Rice, Oryza sativa L. japonica,

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Abstract
The three-dimensional structure of oryzacystatin-I, a cysteine proteinase inhibitor of the rice, Oryza sativa L. japonica, has been determined in solution at pH 6.8 and 25 °C by 1H and 15N NMR spectroscopy. The main body (Glu13−Asp97) of oryzacystatin-I is well-defined and consists of an α-helix and a five-stranded antiparallel β-sheet, while the N- and C-terminal regions (Ser2−Val12 and Ala98−Ala102) are less defined. The helix-sheet architechture of oryzacystatin-I is stabilized by a hydrophobic cluster formed between the α-helix and the β-sheet and is considerably similar to that of monellin, a sweet-tasting protein from an African berry, as well as those of the animal cystatins studied, e.g., chicken egg white cystatin and human stefins A and B (also referred to as human cystatins A and B). Detailed structural comparison indicates that oryzacystatin-I is more similar to chicken cystatin, which belongs to the type-2 animal cystatins, than to human stefins A and B, which belong to the type-1 animal cystatins, despite different loop length.