Unusual proximal heme pocket geometry in the deoxygenated Thermobifida fusca: A combined spectroscopic investigation

The spectroscopic properties of the deoxygenated truncated hemoglobin from the actinobacterium Thermobifida fusca have been investigated by means of extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and near infrared spectroscopies both at room and cryogenic temperatures. At room temperature the near infrared charge transfer band III occurs at 772 nm, a value that is unusually high for a canonical deoxygenated hemoglobin species, and can only be found as a transient species after photolysis in vertebrate hemoglobins and myoglobins or under strongly dehydrating conditions. EXAFS and XANES quantitative analyses, carried out in parallel with deoxygenated horse myoglobin, revealed an unusually short iron-histidine distance 1.90 ± 0.03 Å, significantly shorter than the deoxygenated horse myoglobin distance of 2.11 ± 0.02 Å. These findings provide novel structural basis for discussing the fine structural geometry of the proximal site, and eventually mapping the coordinates of the metal with respect to the pyrrole nitrogens and the proximal histidine nitrogen.