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

AbstractBilins 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 ConceptsBilins 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.