Comments on Metazoan Cytochrome P450 Evolution
Feb. 19, 1999
The paper Metazoan Cytochrome P450 Evolution in Comparative Biochemistry and
Physiology 121C, 15-22 (1998) was written in June of 1997 and revised in Nov. 1997. It
did not appear in print until Nov. 1998. This long gestation time has caused some changes
to take place that affect some of the statements in this paper. I would like to point these out
1) The genome sequence of C. elegans, now essentially complete, contains 80 cytochrome
P450 genes, 10 more than identified in the paper. A complete version of Table 2 modified
from the paper is on this web site, as are the translated C. elegans P450 sequences.
80 C. elegans Cytochrome P450 sequences.
Please note that Osamu Gotoh has also translated these P450 sequences and finds that there
are some errors on the Genbank translations. I have found some of these, but not all of
them. These sequences will need to be revised over time.
2) The placement of nematodes on the tree of life has been revised. Nematodes are now
considered to be among the moulting animals Ecdysozoa. Evidence suggests that this
character evolved only once and forms a monophyletic group. (Aguinaldo AM, Turbeville
JM, Linford LS, Rivera MC, Garey JR, Raff RA, Lake JA, Nature 1997 May
29;387(6632):489-93 Evidence for a clade of nematodes, arthropods and other moulting
animals.) Therefore, the nematodes have been moved to cluster with the arthropods as
protostomes. They did not diverge before the protostome deuterostome split. This affects
several remarks that I made in the paper. On p. 16 I said "the divergence represented by
nematodes and mammals is more ancient than the divergence of insects and mammals. "
This is not true. Because of the revised location of nematodes on the tree of life, the
divergence between (nematodes, arthropods) and mammals should be the same, and it
should be the same as the divergence of protostomes and deuterostomes.
3) C. elegans does have one sequence that clusters on trees with mitochondrial P450s. It is
short and may be missing part of the I-helix. An extensive search for the missing I-helix
region in a genomic region between the N-terminal exons and the C-terminal exons did not
find the missing part. The sequence has been named CYP44 and the clone name is ZK177.
This sequence affects some statements about C. elegans mitochondrial P450s. The
statement on p. 19 "...that the mitochondrial and steroidogenic P450s arose after the split
between coelomates and other Bilateria represented by C. elegans. " is no longer true, since
C. elegans is now among the protostomes. The next statement "At least one mitochondrial
P450 arose before the split between protostomes and deutrostomes and this gave rise to the
mitochondrial clan. " is still true. CYP44 is this sequence in C. elegans. In Drosophila
CYP12 sequences are the only known mitochondrial P450 candidates, and in mollusks,
CYP10 is the only known sequence that clusters with the mitochondrial sequences of
mammals. The lack of steroidogenic P450s in C. elegans still supports the idea that this
ability arose in the deuterostome lineage, not in the protostome lineage. Ecdysteroid
synthesis in insects (i.e. ecdysone) must have evolved independently of the steroid
synthetic route in mammals, and will not use homologs of CYP11, 24, 27, 17, 19 and 21.
It is probable that ecdysone biosynthesis uses five different P450s. Microsomal P450s are
probable at the 7,8 desaturation step and the 25 hydroxylation. The localization of the
P450s involved in the 14 hydroxylation and the 6 oxidation to C=O is not clear, but they
are probably microsomal. C22 hydroxylation is accomplished by a mitochondrial P450.
The 2 position is hydroxylated in the mitochondria, but it does not seem to be hydroxylated
by a P450 enzyme. Conversion of ecdysone to 20 hydroxyecdysone may be carried out
by a mitochondrial P450, but this may not be true in every tissue. Conversion of 20
hydroxyecdysone to 20, 26 dihydroxyecdysone is apparently a microsomal P450 reaction.
Many of these P450s may be recruited from an existing P450 family, probably the 4 family
4) Mitochondrial P450s still seem to have arisen by a mistargeting event, but the placement
in time must now be before the protostome deuterostome split.
5) A new sequence from sponge (Porifera) has been released named CYP38, it is most like
CYP4 family members.
CYP38A1 Suberite domuncula (sponge) GenEMBL Y17816 (1789bp)
Two sea urchin (echinoderm) sequences CYP4C19 and CYP4C20 have been placed in the
CYP4 family. The CYP4 family is related to several bacterial eukaryotic-like P450s
CYP102 and CYP110. The earliest eukaryote ancestor probably had a CYP4 precursor
sequence. CYP51 is also seen in Mycobacterium tuberculosis and is required for
cholesterol biosynthesis, a eukaryotic feature. All eukaryotic P450s probably diverged
from either CYP4 or CYP51. It is not clear what the relationship is between these two