Open tetrapyrroles

Schultz, L. W., Lie, L., Cegielski, M., and Hastings, J. W. (2005). Crystal structure of a pH-regulated luciferase catalyzing the bioluminescent oxidation of an open tetrapyrrole. Proc. Natl. Acad. Sci. USA 102 1378-1383. 

The first step in the two-step pathway, catalyzed by heme oxygenase (HO), converts heme to biliverdin, a linear (open) tetrapyrrole derivative (Fig. 22-25). The other products of the reaction are free Fe2+ and CO. The Fe2+ is quickly bound by ferritin. Carbon monoxide is a poison that binds to hemoglobin (see Box 5-1), and the production of CO by heme oxygenase ensures that, even in the absence of environmental exposure, about 1% of an individual s heme is complexed with CO. 

A third class of accessory pigment of more limited distribution are the open tetrapyrroles, sometimes called "plant bile pigments" because of their relationship to the pigments of animal bile (Fig. 24-24). 

Figure 23-23 Structures of the open tetrapyrroles of plants. See also Fig. 24-24. After Szalontai et al.271...

The open tetrapyrroles of algae and the chro-mophore of phytochrome (Chapter 23) are all derived from phycoerythrobilin, which is formed from biliverdin, as indicated in Fig. 24-24. The animal bile pigments have not been found in prokaryotes. How-... 

Figure 24-24 The degradation of heme and the formation of open tetrapyrrole pigments.

The complete structure of bilirubin was described as an open tetrapyrrole derivative of haem by H. Fischer in 1935. 

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