Suppression of Axial Methionine Fluxion in Hydrogenobacter thermophilus Gln64Asn Cytochrome c₅₅₂^†

Proteins in the cytochrome c (cyt c) family with His-Met heme axial ligation display diverse heme electronic structures as revealed by the NMR spectra of their oxidized (paramagnetic) forms. These variations in electronic structure are thought to result primarily from differences in heme axial Met orientation among cyt c species. The factors determining Met orientation in cyts c, however, remain poorly understood. An additional layer of complexity was revealed with the recent finding that the axial Met in Hydrogenobacter thermophilus cytochrome c₅₅₂ (Ht cyt c₅₅₂) is fluxional, sampling two conformations rapidly on the NMR time scale, resulting in an unusual compressed range of heme substituent hyperfine shifts [Zhong, L., Wen, X., Rabinowitz, T. M., Russell, B. S., Karan, E. F., and Bren, K. L. (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 8637−8642]. In this work, the ¹H NMR hyperfine shift pattern of Ht cyt c₅₅₂ is drastically altered by making the conservative heme pocket mutation Gln64Asn. The mutant (Ht Q64N) displays a pattern of heme hyperfine shifts with a remarkable resemblance to that of structurally homologous Pseudomonas aeruginosa cyt c₅₅₁, which has Asn at position 64 and a single heme axial Met conformation. NMR analysis reveals that Asn64 in Ht Q64N is positioned to interact with the axial Met61, whereas the Gln64 in wild-type Ht cyt c₅₅₂ is not. It also is found that the heme axial Met is not fluxional in Ht Q64N and has an orientation similar to that in P. aeruginosa cyt c₅₅₁. These results indicate that peripheral interactions with the axial Met play an important role in determining axial Met orientation and heme electronic structure in cyts c.