Some P450 sequences lack the invariant residues that are the signature of this gene family. P450 sequences lacking the conserved Cys codon providing the thiolate ligand to the heme have evolved several independent times in arthropods. A file of six types of sequences (Cys-less type or CLT1-6)is available here.

CLT1 - The first type evolved about 400 MYA and can be found as a full-length single sequence in 40 surveyed species of Polyneoptera and Condylognatha.FASTA type 1 - 40 sequences here

The figure below (Fig.13 from Dermauw et al., 2020) shows its distribution. It includes the CYP408 sequences described previously Sezutsu et al., 2013. This first type is from the CYP3 clan. It is most noticeable by the lack of recognizable Cys pocket motif with in some cases Ser at that position (see WebLogo) and is closely related to CYP360 sequences (consensus sequence below the WebLogo showing the difference in the P450 signature).

CLT2 - The second type evolved about 360 MYA in Heteroptera (“true bugs”), forming a monophyletic group of 16 sequences including CYP394A1 of Cimex lectularius, CYP394B1 of Rhodnius prolixus and CYP3221A1 of Murgantia histrionica belonging to the mito clan and closely related to CYP334 enzymes . These sequences lack the WxxxR motif but have ExxR and PERW motifs, yet lack a Cys pocket recognizable motif.FASTA type 2 - 16 sequences here

CLT3 - The third type is restricted to the aphid lineage (Aphidoidea - Adelgidae and Aphididae) and therefore evolved about 250MYA (Li et al., 2017). The aphid CYP6YC1 was initially called CYP6UN1 as a fragment in Aphis gossypii. While it has homologies with the CYP6 family, it is an odd member of the CYP3 clan outside the main (polyphyletic) CYP6 family. These sequences, found in single copies in 16 species are 30-40 codons shorter than most P450s. They have an N-terminal membrane-spanning region but miss parts of alpha helices B and C (no WxxxR motif), alpha F’ alpha G and H. They have ETLR in helix K, as well as the PxRF motif. The Cys pocket motif is truncated, and instead of the CYP2 clan typical PFxxGxRxCxG they have a P(H/Y)xxGxxSYIG motif with Tyr replacing Cys. This odd type of P450, while possibly tethered to the membrane, has a wide open active site with perhaps a large hydrophilic substrate. Mutants of CYP119 with Cys replaced by other amino acids can accomodate heme iron coordination (MacIntosh et al., 2015), and the aphid CYP6YC1 may therefore use either Tyr or the adjacent Ser (Coehlo et al., 2013) instead of the usual Cys as heme ligand. It is difficult to predict what type of reaction would be catalyzed by this P450.FASTA type 3 - 16 sequences here

CLT4 - the fourth type, derived from the CYP6 family, evolved about 220 MYA in a coleopteran clade that includes Tenebrionoidea, Cucujoidea and the “Phytophaga” (Chrysomeloidea and Curculionoidae). It includes CYP393A1 of Dendroctonus ponderosae and CYP348A1 of Tribolium castaneum. These short sequences have the ETLR and PDRF motifs and a conserved I helix lacking the usual Thr, and also lack the Cys in the heme binding pocket.FASTA type 4 - 15 sequences here

CLT5 - The fifth type is restricted to Orthoptera, more specifically to the family Acrididae, including Locusta migratoria CYP3136A1 (Wu et al.,2020) and TSA sequences from four other species . It is also derived from the CYP6 family and has a PFGxxxNDTG consensus with Asp replacing Cys.FASTA type 5 - 5 sequences here

CLT6 - The sixth type is found in Psocodea, and is related to the mito clan CYP334 enzymes, but appears to be different from the heteropteran type. This type has a conserved Cys, but displaced 6 codons 5’ from the consensus. The human body louse CYP3750A1 belongs to this type.FASTA type 6 - 5 sequences here

P450 sequences lacking the conserved Cys have also been noted in fungi (Chen et al.,2014).

The collembolan Sinella curviseta CYP3720A1 (CYP2 clan) lacks the conserved Cys and has a Ser instead. The other CYP3720 genes have the usual sequence with Cys. It also lacks the conserved WxxxR motif or I helix conserved Thr. This is fully supported by TSA data (GGYG01006102), and is not an error from incorrect intron junctions, yet no such gene was found in other Collembola (yet).

The CYP3651A1 gene (CYP3 clan) in the copepod ectoparasites Lepeophtheirus salmonis and L. nordmanni lacks a Cys motif.

The Mito clan CYP301 orthologs are conserved in Neoptera but they differ in their Cys pocket motif. Whereas CYP301B genes all have the usual PFxxGxxxCxG motif, all CYP301A genes have a PYxxGxxxCxG motif instead.

See also Dipteran CYP316A1 (CYP4 clan)

See also Lepidopteran CYP418A1 (Mito clan)

Horizontal gene transfer (HGT) is a rare event in arthropod P450s. The most conspicuous cases are found in Bradysia (sensu lato) species (Diptera, Sciaridae), Bradysia (Sciara) coprophila, Bradysia odoriphaga (cellarum) and Bradysia (Pseudolycoriella) hygida. A full account of HGT in the CYPome of Bradysia coprophila is provided in Feyereisen et al. (2023): Twenty-eight sequences, including one full length pseudogene, were found to be clear outliers as their best BlastP matches were not sequences from Diptera, or even from insects. These sequences belong to CYP clans 2, 3 and 4, and one sequence each to CYP19 and CYP53 clans.