Original scientific paper

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

B. Benko ; Macromolecular Biophysics Laboratory, Institute of Immunology, Rockefellerova 10, 41000 Zagreb, Croatia, Yugoslavia
S. Maričić ; Macromolecular Biophysics Laboratory, Institute of Immunology, Rockefellerova 10, 41000 Zagreb, Croatia, Yugoslavia

Abstract

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 relaxation rates of water protons between O 0c and
40 °c and pH between 5.85 and 7.0. This 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 closest approach of water protons to the ferric ion
of Petromyzon haemoglobin is at least 0.3 A longer than in mammalian
myoglobins.
Binding of fluoride to the ferric haem-irons of all the haemoglobins
examined thus far enhances 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 accessibility of the unpaired electron(s) in all the 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 the ferric haem-irons for the
exchangeable water molecules is compiled.

Keywords

Hrčak ID:

195943

URI

https://hrcak.srce.hr/195943

Publication date:

8.2.1979.

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