Location of the Heme in Horse Heart Ferricytochrome c by X-Ray Diffraction

Richard E. Dickerson,Mary L. Kopka,Joan Varnum,E. Margoliash,David Eisenberg,Jon Weinzierl

doi:10.1016/s0021-9258(18)99607-9

Richard E. Dickerson, Mary L. Kopka + Show 4 more

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https://doi.org/10.1016/s0021-9258(18)99607-9

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Abstract

Abstract A low resolution (4A) electron density map of horse heart cytochrome c crystals has been obtained by x-ray diffraction methods. The molecule is a prolate spheroid, approximately 25 x 25 x 37 A. It has a cleft or crevice along one side into which the heme is inserted, normal to the surface, with only one of the edges of the porphyrin ring exposed to solvent. The ligands occupying coordination positions 5 and 6 of the heme iron extend out from either side of the cleft. The thioether links binding the vinyl side chains of the heme to cysteinyl residues in positions 14 and 17 of the amino acid sequence are visible. One of the iron ligands can be identified from its shape and location as the imidazole side chain of the histidyl residue in position 18. The other is not an imidazole side chain and is probably located in the carboxyl-terminal half of the amino acid sequence. There is little or no α-helix present, the body of the molecule being an extended chain shell around a core of packed hydrophobic side chains.

Highlights

A considerable amount of knowledge is available with regard to the amino acid sequences, the activity, the physicochemical behavior, and the immunochemical properties of cytochromes c from numerous species
A tentative fitting of the amino acid sequence of horse heart cytochrome c [12] to the electron density contours, necessarily of doubtful validity at the present resolution, indicates that the sixth iron ligand most probably arises from the carboxyl-terminal half of the polypeptide chain and could readily accommodate the methionyl residue in position 80, as indicated by several lines of indirect evidence [15,16,17,18,19,20,21]
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Summary

Fmctionation procedure and specific

The values given in the table are averages activities of cyclase of five separate experiments. Of substrate converted per mg of proincreases greatly on treatment of the micalculated yields are based on the total. The latter reaction can be studied as an isolated process with the aid of 2,3-iminosqualene, a potent inhibitor of lanosterol formation from 2,3-oxidosqualene [5].

Location of the Heme in Horse Heart

Findings

Biosynthesis of Cardiolipin in Escherichia coli*