Abstract
Hemocyanins are giant oxygen transport proteins found in the hemolymph of several invertebrate phyla. They constitute giant multimeric molecules whose size range up to that of cell organelles such as ribosomes or even small viruses. Oxygen is reversibly bound by hemocyanins at binuclear copper centers. Subunit interactions within the multisubunit hemocyanin complex lead to diverse allosteric effects such as the highest cooperativity for oxygen binding found in nature. Crystal structures of a native hemocyanin oligomer larger than a hexameric substructure have not been published until now. We report for the first time growth and preliminary analysis of crystals of the 24-meric hemocyanin (MW = 1.8 MDa) of emperor scorpion (Pandinus imperator), which diffract to a resolution of 6.5 Å. The crystals are monoclinc with space group C 1 2 1 and cell dimensions a = 311.61 Å, b = 246.58 Å and c = 251.10 Å (α = 90.00°, β = 90.02°, γ = 90.00°). The asymmetric unit contains one molecule of the 24-meric hemocyanin and the solvent content of the crystals is 56%. A preliminary analysis of the hemocyanin structure reveals that emperor scorpion hemocyanin crystallizes in the same oxygenated conformation, which is also present in solution as previously shown by cryo-EM reconstruction and small angle x-ray scattering experiments.
Highlights
Hemocyanins are giant oxygen transport protein complexes in the hemolymph of invertebrates with molecular masses up to 8 MDa [1]
The molluscan hemocyanin subunit consists of a concatenation of seven to eight paralogous functional units (FU) of approx. 50 kDa each on a single polypeptide chain
Arthropod hemocyanins occur as hexamers (MW
Summary
Hemocyanins are giant oxygen transport protein complexes in the hemolymph of invertebrates with molecular masses up to 8 MDa [1]. Among hemocyanins the four-hexamer hemocyanin of the emperor scorpion (Pandinus imperator) is known for its exceptionally high cooperativity with Hill coefficients (nH) commonly ranging between 6–7 [24] Owing to these properties hemocyanins are used as model proteins to better understand their underlying mechanisms both on a functional and structural level. In recent years hemocyanins have gained interest due to the fact that they may serve as ecdysone carriers or act as phenoloxidases after specific activation [27,28]. With the development of cryo-EM reconstruction several low and medium resolution structures of arthropod hemocyanin multimers have been solved in recent years. Despite all the advances made with cryo-EM a hemocyanin multimer structure with atomic resolution most probably will only be solved by crystallographic analysis. The molecular mass in the asymmetrical unit of emperor scorpion hemocyanin crystals is one of the largest recorded in the PDB databank comparable to ribosomes, human DNA-dependent protein kinase or erythrocruorin [39,40,41]
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