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Haem Accessibility in Monomeric Haemoglobins of Glycera dibranchiata and Petromyzon marinus, a Proton Magnetic Relaxation Study

The temperature dependence of the longitudinal magnetic relaxation rates of water protons in solutions of differently liganded monomeric haemoglobins from Petromyzon marinus (fraction V) and Glycera dibranchiata (fraction III) was measured. The results were compared with horse and bovine myoglobins and interpreted according to the model of chemical exchange of water molecules.This exchange takes place between a site within the haem-pocket (but non-identical to the sixth-ligand position) and the bulk of the solvent. Aquomethaemoglobin from Glycera dibranchiata only slightly enhances the realaxation rates of water protons between 0 °C and 40 °C and pH between 5.85 and 7.0. The finding is compatible with tight protein packing around the distal side of the haem. In the solutions of aquomethaemoglobin from Petromyzon marinus up to 30 °C, the solvent-proton relaxation rates are determined by the rate of chemical exchange of water molecules. At higher temperatures the fast exchange mechanism takes place, an effect not observed in solutions of horse and bovine myoglobins. The distance of cloasest approach of water protons to the ferric ion of Petromyzon marinus haemoglobin is at least 0.3 &Aring ; ; ; longer than in mamalian myoglobins. Binding of fluoride to the ferric haem-irons of all the haemoglobins examined thus far enhences the proton relaxation rates relative to their aquomet forms, while in their nitrosyl complexes slightly lower rates were measured. These data indicate the sensitivity of the protein structure to the nature of the sixth ligand. The accesibility of the unpaired electron(s) in all haemoglobins examined is in the order: NO- < aquomet < fluoromet-forms. From our previous and present data and from that found in literature, a scale of accessibilities of ferric haem-irons for the exchangable water molecules is compiled.

Glycera dibranchiata; Haemoglobin; Nitrosylmyoglobin; Nitrosylhaemoglobin; Petromyzon marinus; Proton magnetic relaxation

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