J. Biol. Chem., Vol. 258, Issue 7, 4195-4201, Apr, 1983
The complete amino acid sequence of rabbit phenobarbital-induced liver microsomal cytochrome P-450
FS Heinemann and J Ozols
The complete amino acid sequence of the major phenobarbital-induced
cytochrome P-450 (P-450LM2) from rabbit liver microsomes has been
determined. The protein contains 489 amino acid residues in a single
polypeptide chain and has Mr = 55,464. The sequence was compared with the
amino acid sequence of P-450CAM and the nucleotide sequence of cDNA
obtained from phenobarbital-induced rat liver cytochrome P-450 mRNA. These
comparisons suggest that, despite functional similarities, the structural
homology between microbial and microsomal cytochromes P-450 is limited to a
single 8-residue region, and, in contrast, the structure of inducible
microsomal cytochrome P-450 isozymes is highly conserved among mammalian
species. Furthermore, we propose that the thiolate heme ligand of
cytochrome P-450 is contributed by a cysteinyl residue near the COOH
terminus, position 434 in the rabbit P-450LM2 sequence, based on the
homology in this region with P-450CAM. The NH2 terminus of the protein from
residues 1-310 is characterized by 8 hydrophobic segments 18-23 residues
long, each of which is terminated by a cluster of charged amino acid
residues. Residues 320-443 comprise a hydrophilic region which contains the
putative heme binding cysteinyl residue as well as segments of homology
with a constitutive rabbit cytochrome P-450 isozyme. The sequence data
suggest that cytochrome P- 450LM2 contains multiple transmembranous
segments as well as a hydrophilic cytoplasmic domain. The hydrophilic
domain contains regions of homology with several other cytochromes P-450,
and thus appears to have an essential role in the biological function of
the protein.