Biosynthesis and Analysis of Bilins

The term bilin is a collective one to describe a broad group of open chain tetrapyrroles and derives from the name “bile pigments” as the first of these compounds to be characterized were isolated from animal bile. These bilins, biliverdin (BV) and bilirubin (BR), are the sequential products of heme degradation (their green and yellow pigmentation can be detected during the discoloration of a bruise), with BR being conjugated to glucuronic acid to expedite excretion. The structures of BV and BR are shown in Figure 1 , and their biochemistry is still the best understood of the bilins today. However, we now know that there is a great diversity of naturally occurring bilins that have a wide range of different functions. In cyanobacteria and two groups of algae, the rhodophytes (red algae) and the cryptomonads, a tremendous variety of bilins are utilized for light harvesting through covalent attachment to the phycobiliproteins, which comprise the photosynthetic apparatus of these organisms ( 25 , 26 ).