Molecular models of the uricase tetramer and of the PEGylated uricase pegloticase containing strands of 10 kDa polyethylene glycol (PEG) linked to each uricase tetramer. (a) Schematic model of the uricase tetramer, based on the crystal structure of Aspergillus flavus uricase. Each subunit is shown in a different color (red, blue, green, or yellow). (b) Space-filling model of the A. flavus uricase tetramer, showing the characteristic tunnel (or barrel) structure of the native enzyme tetramer. (c) Space-filling model of A. flavus uricase tetramer, rotated around the vertical axis so that the tunnel is not visible. (d) Space-filling model of the uricase tetramer in the same orientation as in (b) but to which nine strands of 10 kDa PEG per uricase subunit are attached. The structures of the PEG strands (shown in various shades of gray) were generated as described in . The scale of (d) is about half that of (a-c). Figure 5 and the legend are reprinted with permission from .