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The role of tyrosine 67 in the cytochrome c heme crevice structure studied by semisynthesis

Tyr‐67 of mitochondrial cytochrome c is thought to be involved in important hydrogen bonding interactions in the hydrophobic heme pocket of the protein (Takano, T., Dickerson, R.E. (1981) J. Mol. Biol. 153:95–115). The role of this highly conserved residue in heme pocket stability was studied by comparing properties of semisynthetic (Phe‐67) and (p‐F‐Phe‐67) analogs with those of native cytochrome c and a “control” analog, (Hse‐65)cytochrome c. The (Phe‐67) and (p‐F‐Phe‐67) analogs have well‐developed 695‐nm visible absorption bands and are active in a cytochrome c oxidase assay. The reduction potentials of both analogs are lower than the native protein by approximately 50 mV. Although both analogs bind imidazole with higher affinity than the native protein, only the (p‐F‐Phe‐67) analog has a 3‐ to 5‐fold lower binding constant for cyanide. Only the (Phe‐67) analog was significantly more stable toward alkaline isomerization. These results are not consistent with stabilization of the native protein heme pocket via hydrogen bonding of Tyr‐67 to Met‐80. An alternative steric role for Tyr‐67 is proposed in which the residue controls the heme reduction potential by limiting the number of internal H2O molecules in the heme pocket. © 1992 Wiley‐Liss, Inc.