abstract: by oligonucleotide-directed mutageneses, 13 substitutions of amino acids at the carboxy-terminal region of rat liver cytochrome p-450d were done as follows: (a) phe-449→tyr; (b) gly-450→ser; (c) leu-451→ser; (d) gly-452→glu; (e) lys-453→glu; (f) arg-454→leu; (g) arg-455→gly; (h) cys-456→tyr; (i) cys-456→his; (j) ile-457→ser; (k) gly-458→glu; (l) glu-459→ala; (m) ile-460→ser. the co-bound reduced forms of the wild type and mutants b—g. j, l, and m gave soret peaks at 448 nm. the co complex of mutant a gave a soret peak at 445 nm. the intensities of the co-bound forms of mutants a, c, d, and j were very small compared with that of the wild-type complex. the co-reduced forms of mutants h, i, and k did not give a soret peak around 450 nm at all. the 448-nm peak of mutant f was unstable and quickly disappeared with the concomitant appearance of a peak at 420 nm. these findings, together with data in the literature, indicate that (1) invariant cys-456 at the carboxy-terminal region of p-450d is an axial ligand of the heme iron of the eukaryotic p-450’s; (2) hydrophobic amino acids such as phe-449, leu-451, gly-452, and other hydrophobic amino acids such as ile-457 and gly-458 next to the axial ligand are apparently very important for the apoprotein to hold and/or incorporate the heme plane at the active site of p-450; and (3) arg-454 interacts with the heme propionate as his-355 of p-450camdoes [poulos, t. l., finzel, b. c., gunsalus, i. c., wagner, g. c., & kraut, j. (1985) j. biol. chem. 260, 16122–16130].
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