Bilin Metabolism in Plants: Structure, Function and Haem Oxygenase Regulation of Bilin Biosynthesis


Bilins are open‐chain tetrapyrroles with a wide range of visible and nearly visible‐light absorption and emission properties. The linear tetrapyrrole molecules function as chromophores of the light‐harvesting phycobiliproteins and phytochrome‐mediated light sensing in photosynthetic organisms. They are derived from the cyclic precursor haem. The initial step in bilin biosynthesis is the conversion of haem into biliverdin (BV IX α) catalysed by haem oxygenase, which is subsequently reduced to specific bilins by ferredoxin‐dependent bilin reductases (FDBRs). Bilins usually bound to apoproteins via single or double covalent bonds to form a macromolecular complex phycobilisomes. The attachment of apoproteins to bilin is an autocatalytic process, but bilin lyases are required for the specific attachment of bilin chromophores to phycobiliprotein apoproteins. Besides the biosynthesis, structure and functions of bilins, this article also aims to recapitulate and discuss the current progress in the field of bilins and to emphasise the emerging areas.

Key Concepts

  • Bilins are open‐chain tetrapyrrole non‐metallic colour compounds formed as a metabolic product of protoporphyrin IX.
  • Biliverdin IX α is the common precursor of all naturally occurring bilins.
  • Haem oxygenase (HO) and ferredoxin‐dependent bilin reductases (FDBRs) are the two key enzymes involved in the biosynthesis of bilins.
  • Phycobiliproteins assemble with bilins to form phycobilisomes, which help in light harvesting and energy transfer.
  • Bilin plays a significant role in various physiological processes, namely, photosynthesis, respiration, light perception, signalling, cell defence against oxidative stress, nitrate and sulfate assimilation and programmed cell death.

Keywords: bilins; haem oxygenase; ferredoxin dependent bilin reductases; phycobiliproteins; Biliverdin IXα

Figure 1. Biosynthesis of several types of bilins in different photosynthetic organisms from the cyclic precursor haem by two key enzymes haem oxygenases (HOs) and ferredoxin‐dependent bilin reductases (FDBRs). Biliverdin IXα is the common precursor of all bilin pigments formed from haem catalysed by an enzyme haem oxygenase, which is further reduced to phytobilins by different ferredoxin‐dependent bilin reductases (FDBRs).
Figure 2. Overview of the reaction catalysed by ferredoxin dependent bilin reductases and representation of the attachment of bilins to apoprotein phycobiliproteins through one or two thioether bonds.
Figure 3. Phytochrome signal transmission in prokaryotic (Cph) and eukaryotic phytochromes. Bilins and light exhibit reverse effects on the sensor domain, leading to variations in the activity of the regulatory domain. Abbreviations: D, aspartate; H, histidine; P, phosphate group. Reproduced with permission from Montgomery . © Elsevier.
Figure 4. Schematic representation of cytoprotective role of bilins in plant tolerance to various environmental stress stimuli.
Figure 5. Representation of the signalling mechanism of bilins and their intermediates in plants. Two lineages that is, haem and Mg proto IX, share the common precursor protoporphyrin IX located in plastids. Protoporphyrin IX plays an important role in the retrograde signalling from plastids to nucleus by the regulation of photosynthesis‐associated nuclear genes (PhANG) expression (Barajas‐Lopez de et al., ). Zhang et al. and Larkin . Reproduced with permission of Springer Nature.


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Shekhawat, Gyan Singh, Parihar, Suman, Mahawar, Lovely, Khator, Khushboo, and Bulchandani, Neha(Apr 2019) Bilin Metabolism in Plants: Structure, Function and Haem Oxygenase Regulation of Bilin Biosynthesis. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0028352]