Crystal structure of a functional unit from Octopus hemocyanin

Area of Science:

• Biochemistry
• Structural Biology
• Evolutionary Biology

Background:

• Hemocyanins are large oxygen-transport proteins in arthropods and molluscs.
• They are multisubunit proteins with copper-containing active sites for oxygen binding.
• Octopus hemocyanin has ten subunits, each with seven oxygen-binding functional units.

Purpose of the Study:

• To determine the X-ray structure of the octopus hemocyanin carboxyl-terminal functional unit (Odg).
• To analyze the structural features and active site of Odg.
• To compare mollusc hemocyanin structure with arthropod hemocyanin.

Main Methods:

• X-ray structure determination of Odg at 2.3 A resolution.
• Analysis of Odg's tertiary and quaternary structures.
• Comparison of Octopus hemocyanin structure with horseshoe crab hemocyanin.

Main Results:

• Odg possesses two structural domains: a copper-binding domain and a beta-sandwich domain.
• Six histidine residues ligate the copper atoms in the active site.
• Octopus hemocyanin exhibits distinct tertiary folds compared to arthropod hemocyanin, despite a similar active site.

Conclusions:

• The tertiary folds of mollusc and arthropod hemocyanins are distinct.
• The active site structure is conserved, suggesting convergent evolution.
• Structural comparison reveals insights into hemocyanin evolution.