In structural biology, a beta-propeller is a type of all-ß protein architecture characterized by 4 to 8 blade-shaped beta sheets arranged toroidally around a central axis. Each sheet typically has four antiparallel ß-strands twisted so that the first and fourth strands are almost perpendicular to each other. The enzyme's active site is often found in the cleft formed in the center of the propeller by loops connecting the successive four-sheet motifs. Murzin proposed a geometric model to describe the structural principles of the beta propeller. According to this model the seven bladed propeller was the most favoured arrangement in geometric terms.