Structure of a novel farnesylated bilin from an insect – formation by α‐cleavage of heme A of mitochondrial cytochrome c oxidases?

Biliproteins are present in almost all forms of life, and many of them play vital roles in photobiology. The bilin ligand of a recently characterized 500‐kDa biliprotein from an insect has been isolated and its structure elucidated with chemical and spectroscopic techniques ( UV –visible, IR , MS , NMR , and CD ). This blue pigment, named CV ‐bilin, represents a unique high molecular mass derivative of biliverdin  IX α, with an unusual 10 E ‐configuration and a molecular mass of 852 Da, corresponding to C 48 H 60 N 4 O 10 . The high mass of this open‐chain tetrapyrrole results from the presence of an epoxi‐dihydroxyethylfarnesyl substituent at C ‐18 and a hydroxymethyl substituent at C ‐13. This substitution pattern exactly reflects that of heme  A of mitochondrial cytochrome  c oxidases with a hydroxyethylfarnesyl chain and a formyl group at corresponding positions of the cyclic tetrapyrrole. As no other natural product is known to show these structural features (heme  O , the precursor of heme  A , has a methyl group at C ‐13), this bilin is presumed to be derived from heme  A by cleavage of the α‐methine bridge and oxidative modifications at C ‐13 and the hydroxyethylfarnesyl chain. Possibly, a bilin structurally related to this insect bilin is also produced in other organisms as a result of mitochondrial turnover or degradation. As CV ‐bilin in complex with a specific protein is accumulated at the end of larval life, stored in the pupa, and finally transferred to the oocytes, a possible role of the free or protein‐bound pigment in egg or embryonic development is discussed.