A phylogenetic study of cytochrome b561 proteins

As an antioxidant and cofactor to numerous metabolic enzymes, ascorbate has an essential role in plants and animals. Cytochromes b561 constitute a class of intrinsic membrane proteins involved in ascorbate regeneration. Despite their importance in ascorbate metabolism, no evolutionary analysis has been presented so far on this newly described protein family.

Cytochromes b561 have been identified in a large number of phylogenetically distant species, but are absent in fungi and prokaryotes. Most species contain three or four cytochrome b561 paralogous proteins, and the encoding genes usually have four or five exons. At the protein level, sequence similarities are rather low between cytochromes b561 within a single species (34-45% identity), and among phylogenetically distant species (around 30% identity). However, particular structural features characterizing this protein family are well conserved in members from all species investigated. These features comprise six transmembrane helices, four strictly conserved histidine residues, probably coordinating the two heme molecules, and putative ascorbate and monodehydro-ascorbate (MDHA) substrate-binding sites. Analysis of plant cytochromes b561 shows a separation between those from monocotyledonous and dicotyledonous species in a phylogenetic tree.

All cytochromes b561 have probably evolved from a common ancestral protein before the separation of plants and animals. Their phyletic distribution mirrors the use of ascorbate as primary antioxidant, indicating their role in ascorbate homeostasis and antioxidative defense. In plants, the differentiation into four cytochrome b561 isoforms probably occurred before the separation between monocots and dicots.