The structure of biliproteins is typically characterised by bilin chromophores arranged in linear tetrapyrrolic formation, and the bilins are covalently bound to apoproteins via thioether bonds.[2] Each type of biliprotein has a unique bilin that belongs to it (e.g. phycoerythrobilin is the chromophore of phycoerythrin and phycocyanobilin is the chromophore of phycocyanin). The bilin chromophores are formed by the oxidative cleavage of a haem ring and catalysed by haem oxygenases at one of four methine bridges, allowing four possible bilin isomers to occur. In all organisms known to have biliproteins, cleavage usually occurs at the α-bridge, generating biliverdin IXα.[2]