Control of the stability of Hydrogenobacter thermophilus cytochrome C(552) through alteration of the basicity of the N-terminal amino group of the polypeptide chain.

In the denatured state of Hydrogenobacter thermophilus cytochrome c(552) (HT), the N-terminal amino group of the polypeptide chain is coordinated to the heme Fe in place of the axial Met, the His-N(term) form being formed [Tai, H., Munegumi, T., Yamamoto, Y. Inorg. Chem. 2009, 48, 331-338]. Since the His-N(term) form can be considered as an ordered residual structure in the denatured protein, its stability significantly influences the energy of the denatured state. In this study, the His-N(term) forms of the wild-type HT and its mutants possessing a series of amino acid residues at the N-terminal, such as N1D, N1E, and N1G, have been characterized to elucidate the physicochemical properties of the N-terminal residue responsible for the control of the thermodynamic stability of the His-N(term) form. The study revealed that the thermodynamic stability of the His-N(term) form depends highly on the basicity of the N-terminal amino group of the polypeptide chain in such a manner that an increase in the pK(a) value of the N-terminal amino group by 1 unit results in stabilization of the bond between heme Fe and the N-terminal amino group (Fe-N(term) bond) in the His-N(term) form by ∼4 kJ mol(-1). The empirical hard and soft acid and base principle could account for the observed relationship between the pK(a) value of the N-terminal amino group and the stability of the Fe-N(term) bond in the His-N(term) form. In addition, the study demonstrated that the overall stability of the protein can be manipulated through the energy of the denatured protein by changing the thermodynamic stability of the His-N(term) form. Consequently, the overall stability of the protein has been shown to be controlled through alteration of the basicity of the N-terminal amino group of the polypeptide chain. These findings provide new insights into the stabilizing interactions in the denatured protein, which are relevant as to characterization of the protein stability and folding.