The optical spectrum of heme a is red-shifted in aa sub(3)-type cytochrome c oxidases compared to isolated low-spin heme A model compounds. Early spectroscopic studies indicated that this may be due ...to hydrogen-bonding of the formyl group of heme a to an amino acid in the close vicinity. Here we show that most of the optical spectral shift of native heme a is due to a hydrogen-bonding interaction between the formyl group and arginine-54 in subunit I of cytochrome aa sub(3) from Paracoccus denitrificans, and that a smaller part is due to an electrostatic interaction between the D ring propionate of heme a and arginine-474.
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