Biophysical techniques developed during the last three decades have provided an increasingly detailed description of the internal processes associated with ligand capture and release by haem proteins. Myoglobin has long been the paradigm for these studies. More recently, cytochrome P450cam (the camphor-metabolizing cytochrome P450 from Pseudomonas putida) has also received considerable interest. In spite of sharing the same prosthetic group, the Fe(II)-haem, these proteins are structurally unrelated and they perform different functions. Recent works show that both proteins exhibit a common feature: a series of permanent or fluctuating, mostly hydrophobic, cavities of the protein matrix are providing transient docking sites as well as migration, escape and possibly entry pathways for the ligand. Remarkably, these systems of cavities connect the distal and the proximal regions of the haem, a disposition that may contribute to ligand capture enhancement.

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