A Possible Role for the Covalent Heme−Protein Linkage in Cytochrome c Revealed via Comparison of N-Acetylmicroperoxidase-8 and a Synthetic, Monohistidine-Coordinated Heme Peptide

N-Acetylmicroperoxidase-8 (1) contains heme and residues 14-21 of horse mitochondrial cytochrome c (cyt c). The two thioether bonds linking protein to heme in cyt c are present in 1, and the native axial ligand His-18 remains coordinated to iron. As an approach to probing structural or functional roles played by the double covalent heme-protein linkage in cyt c, we have initiated a study in which the properties of 1 are compared with those of a synthetic mono-His coordinated heme peptide containing a single covalent linkage (2). One consequence of the greater conformational restriction imposed on peptide conformation in 1 is that His-Fe(III) coordination is approximately 1.4 kcal/mol more favorable in 1 than in 2. This highlights a clear advantage conferred to cyt c by having two covalent heme-protein linkages rather than one: greater thermodynamic stability of the protein fold. EPR (11 K) and resonance Raman (298 K) studies reveal that 1 and 2 exhibit a thermal high-spin/low-spin ferric equilibrium but that low-spin character is considerably more pronounced in 1. In addition, the thioether 2-(methylthio)ethanol (MTE) coordinates 0.5 kcal/mol more strongly to 1 than to 2 in 60:40 H(2)O/CH(3)OH and only triggers the expected conversion of iron to the low-spin state characteristic of ferric cyt c in the case of 1. This demonstrates that the axial ligand field provided by an imidazole and a thioether is too weak to induce a high-spin to low-spin conversion in a ferric porphyrin. Our results suggest that a conformationally constrained double covalent heme-protein linkage, as exists in 1 and its parent protein cyt c, is an effective solution that nature has evolved to circumvent this limitation. We propose that the stronger His-Fe(III) coordination enabled by such a linkage serves to markedly enhance the effective ligand field strength of His-18. Our studies with 1 and 2 suggest that a double covalent linkage in cyt c may also enable energetically more favorable trans ligation of Met-80 than would be possible if only a single linkage were present. This would serve to further increase the stability of the protein fold and perhaps to increase the effective ligand field strength of Met-80 as well.