HIUase (5-hydroxyisourate hydrolase) catalyzes the second step in a three-step ureide pathway in which 5-hydroxyisourate (HIU), a product of the uricase (urate oxidase) reaction, is hydrolyzed to 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU). HIUase has high sequence similarity with transthyretins and is a member of the transthyretin-like protein (TLP) family. HIUase is distinguished from transthyretins by a conserved signature motif at its C-terminus that forms part of the active site. In HIUase, this motif is YRGS, while transthyretins have a conserved TAVV sequence in the same location. Most HIUases are cytosolic but in plants and slime molds, they are peroxisomal based on the presence of N-terminal periplasmic localization sequences. HIUase forms a homotetramer with each subunit consisting of eight beta-strands arranged in two sheets and a short alpha-helix. The central channel of the tetramer contains two independent binding sites, each located between a pair of subunits.