OVERALL STRUCTURE OF PNP is highlighted in this computer-generated image colored by atom type. The enzyme consists of three identical lobes, or subunits, arrayed around a central axis. The subunits are distinguished by distinctly colored ribbons representing the protein backbone of the subunits. The three active sites are shown filled by a purine nucleoside and a phosphate ion (structures depicted y large spheres), the substances normally acted on by PNP.