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by a bilin 2,3-reductase to 3(Z)-PEB (9). This later enzyme is presumably related to POB synthase, as both enzymes accomplish the same reaction. 3(Z)-PEB then undergoes a series of isomerizations to produce the 3 (E)-isomers of both PEB and PCB (5,9).

The complexity of bilin synthesis is fur ther increased by recent evidence that the reduction of BV IXa is not simply related to broad phylogenetic group. The green alga Mesotaenium caldariorum is able to synthesize 3(Z)-PCB directly from 3(Z)-POB (67). Such a pathway suggests a reasonable biosynthetic alternative to the red algal pathway for PCB synthesis, and

Figure 2. The biosynthetic relationship of the bilins discussed in the text. The full pathway shown has been constructed from biosynthetic pathways identified in a number of different organisms with no single organism containing all pathways. Heme oxygenase is present in animals, plants, algae, and cyanobacteria. Of the four reductases, BV reductase has been detected in animals and cyanobacteria, POB synthase (bilin 2,3-reductase) in plants and green algae, bilin 15,16-reductase in red algae and possibly cyanobacteria, and bilin 18!,182-reductase in green algae alone. There are two isomerase activities. 3(Z),3(E) cis-trans-isomerase (31,32-isomerase) appears to be present in plants and algae, while (15,16),(181,182)-isomerase activity has only been detected in red algae. Broken arrows indicate possible biosynthetic routes that have yet to be shown experimentally. Abbreviations: BR, biliru-bin; BV, biliverdin; DHBV, dihydrobiliverdin; PCB, phycocyanobilin; PEB, phycoerythrobilin; POB, phytochromobilin.

Figure 2. The biosynthetic relationship of the bilins discussed in the text. The full pathway shown has been constructed from biosynthetic pathways identified in a number of different organisms with no single organism containing all pathways. Heme oxygenase is present in animals, plants, algae, and cyanobacteria. Of the four reductases, BV reductase has been detected in animals and cyanobacteria, POB synthase (bilin 2,3-reductase) in plants and green algae, bilin 15,16-reductase in red algae and possibly cyanobacteria, and bilin 18!,182-reductase in green algae alone. There are two isomerase activities. 3(Z),3(E) cis-trans-isomerase (31,32-isomerase) appears to be present in plants and algae, while (15,16),(181,182)-isomerase activity has only been detected in red algae. Broken arrows indicate possible biosynthetic routes that have yet to be shown experimentally. Abbreviations: BR, biliru-bin; BV, biliverdin; DHBV, dihydrobiliverdin; PCB, phycocyanobilin; PEB, phycoerythrobilin; POB, phytochromobilin.

indeed the authors report the unpublished observation that Cyanidium caldarium has POB synthase activity. Since the yeast Pichia pastoris also has this activity (66), bilin 2,3-reductases may be considerably more widespread than first thought. To complicate matters further, it now appears that cyanobacteria also possess a BV (bilin 10,11-)reductase (49), while two more novel enzymes (a bilin 4,5-reductase and a bilin 121,122-dehydrogenase) have been proposed to account for the synthesis of the five additional phycobilins identified to date (5).

While the diversity of bilins and their biosynthesis is a fascinating topic, this chapter is primarily concerned with the methods required to study bilin biosynthesis in general. In particular, I will focus on a few specific steps in bilin synthesis in photosynthetic organisms, only addressing the biochemistry of these pathways in animals for comparative purposes. It is intended that the methods described here could be applied equally well to the biosynthetic steps that have yet to be characterized.

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