Vibrational circular dichroism measurements of ligand vibrations in haem and non-haem metalloenzymes

• Published in: Faraday discussions 1994, Vol.99 (99), p.327-39; discussion 383-400
• Main Authors: Bormett, R W, Smith, G D, Asher, S A, Barrick, D, Kurtz, Jr, D M
• Format: Article
• Language: English
• Subjects: Azides; Circular Dichroism; Cyanides; Heme - chemistry; Hemoglobins - chemistry; Ligands; Metalloproteins - chemistry; Myoglobin - chemistry; Vibration
• ISSN: 1359-6640; 1364-5498
• DOI: 10.1039/FD9949900327

We have measured the vibrational circular dichroism (VCD) spectra of the stretching vibrations of azide and cyanide ligated to the Fe3+ atoms of haemoglobin (Hb) and myoglobin (Mb). The antisymmetric azide-stretch of the low-spin haems have an anomalously large g-value of ca. -1 x 10(-3). In contrast, CN- has a g-value of ca. +2.4 x 10(-3). We also show, for the first time, that a significant VCD occurs for the azide ligand antisymmetric stretches of non-haem proteins; we measure a g-value of ca. -1 x 10(-4) for azide bound to haemerythrin. We have examined the mechanism of the VCD phenomenon by: (1) reconstituting Mb with haems substituted such that they insert differently in the haem pocket; (2) replacing the Fe3+ with Mn3+; (3) examining proteins where replacements occur for E-7 His and E-11 Val distal amino acids close to the haem and (4) examining an Mb mutant where the proximal F-8 His is replaced by Gly, and where an imidazole ligand inserts into the resulting crevice and binds to the haem in a way similar to that of the proximal histidine in the native protein. The VCD anisotropy appears insensitive to the haem substituent replacements used in this study. Exchange of the E-7 distal His or the E-11 Val has a dramatic effect on the g-value. Exchange of the F-8 proximal His reverses the sign of the g-value for the azide complex, but not for the cyanide complex. The work to date indicates that VCD has the potential to become a sensitive technique for examining the structure of metalloenzymes. Work is needed to determine the mechanism giving rise to the large g-values and to correlate the VCD spectrum with the metalloenzyme structure at the active site.