The Diversity And Biosynthesis Of Naturally Occurring 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 organ-

Heme, Chlorophyll, and Bilins: Methods and Protocols Edited by A.G. Smith and M. Witty ©2002 Humana Press, Totowa, NJ

isms (25,26). The structures of two of the most common of these bilins, phyco-cyanobilin (PCB) and phycoerythrobilin (PEB), are also shown in Figure 1. Other light-harvesting pigments include phycovi-olobilin, phycourobilin, 15,16-dihydro-biliverdin (DHBV), and mesobiliverdin (MBV) (25,26). In higher plants, the related bilin, phytochromobilin (POB) (Figure 1), serves as the chromophore of the phy-tochromes (17,58). This family of photore-ceptors is important throughout plant development and regulates such diverse processes as germination, growth, flowering, and the synthesis of the photosynthet-ic apparatus. Also, BV and BR may not just be waste products, since both have been demonstrated to have potent antioxidant activities (51), and BR has even been implicated in having a role in circadian regulation in humans (41). BV is used for pigmentation in reptiles (4), fishes (23), and insects, where the insecticyanin protein is bound to BV IX rather than the IXa isomer more commonly found in nature (27). BV is also found in the eggshells of a wide variety of birds (31). More remarkably still, the marine snail Aplysia californica uses

PEB, which it obtains from a diet of red seaweed, as a defensive ink pigment (43).

The biosynthetic relationship between the bilins discussed in this chapter is shown in Figure 2. The first committed step in the pathway for the synthesis of all bilins is the oxidation of protoheme to BV IXa by the enzyme heme oxygenase (5,34,42,53). Following this universal reaction, BV IXa can be reduced at one of three different positions in a somewhat phylogenetic-depen-dent manner. In animals, BV IXa is reduced at the C-10 position (see Figure 1) by the enzyme BV reductase to give BR IXa (32,50). In higher plants, the plastid-localized enzyme POB synthase, which is a

Figure 1. Chemical structures of the major bilins.

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