Abstract
<title>Abstract</title> <p>The vast diversity of P450 superfamily proteins across species from different kingdoms significantly complicates the understanding of P450 phylogeny. To minimize the risk of phylogenetic inaccuracies, this study was focused exclusively on proteobacterial proteins. The FASTA sequences dataset was compiled to encompass diverse types of P450 enzymes from various phyla within Proteobacteria. In the resulting unrooted maximum likelihood phylogenetic tree, the main subtrees were arranged in the following ascending order: CYP74 clan (9), CYP5164 clan (7), CYP152 family (4), a group of unclassified P450s (3), CYP7 clan (0), CYP4 clan (0), and CYP51 clan (0). The numbers in parentheses indicate the number of amino acids in the cysteine pocket insertions of the corresponding P450s. This ascending order of subtrees corresponds to a decrease in the size of cysteine pocket insertions among the P450s in these subtrees. Thus, the data inspire two conclusions. First, the CYP74 proteins are the ancestors of other proteobacterial P450s. Second, the nine-amino-acid “insertion” in CYP74 proteins is not an insertion per se but a rudimentary structural feature distinguishing ancestral proteins from their descendants. During the evolutionary transition to aerobic metabolism, this “insertion” was gradually lost. This deletion was necessary to enable P450s to interact with redox partners. As previously hypothesized, P450s first emerged on early Earth under an anoxic atmosphere. The independence of CYP74s from oxygen and redox partners further supports their ancestral role within the P450 superfamily.</p>