Ascorbate peroxidase neofunctionalization at the origin of APx-R and APx-L: evidences from basal Archaeplastida
Ascorbate peroxidases (APx) are class I members of the non-animal peroxidases superfamily, a large group of evolutionarily related enzymes. Through mining in public databases, our group has previously identified two unusual subsets of APx homologs, disclosing the existence of two uncharacterized families of class I peroxidases, which were named ascorbate peroxidase-related (APx-R) and ascorbate peroxidase-like (APx-L). As APx, APx-R proteins possess all catalytic residues required for peroxidase activity. Nevertheless, these proteins do not contain residues known to be critical for ascorbate binding, implying that members of this family must use other substrates while reducing hydrogen peroxide. On the other hand, APx-L proteins not only lack ascorbate-binding residues, as do not contain any residue known to be essential for peroxidase activity, in contrast with every other member of the non-animal peroxidase superfamily, which is composed by over 10,000 proteins distributed among bacteria, archaea, fungi, algae, and plants. Through a molecular phylogenetic analysis performed with sequences derived from basal Archaeplastida, we now show the existence of hybrid proteins, which combine features of these three families. Analysis performed on public databases show that the prevalence of these proteins varies among distinct groups of organisms, accounting for up to 33% of total APx homologs in species of green algae. The analysis of this heterogeneous group of proteins sheds light on the origin of APx-R and APx-L, through a process characterized by the progressive deterioration of ascorbate-binding sites and catalytic sites towards neofunctionalization.