Haem conformation of amphibian nytrosylhaemoglobins detected by XANES spectroscopy.

We investigated for the first time the haem stereochemistry in the nitrosylated derivative of two amphibian haemoglobins, Xenopus laevis and Ambystoma mexicanum, by means of X-ray absorption spectroscopy technique with the aim to explain the relationships between the active site structure and physiological function of these proteins, compared to that from humans. Our results show that while the Fe site local structure of human HbNO is modulated by an allosteric effector such as IHP shifting the T-R equilibrium towards the T-state, the Fe site local structure of amphibians HbNO is stabilized in a particularly tensed T-state also without IHP.