10-A cryoEM structure and molecular model of the Myriapod (Scutigera) 6x6mer hemocyanin: understanding a giant oxygen transport protein.

Oxygen transport in Myriapoda is maintained by a unique 6x6mer hemocyanin, that is, 36 subunits arranged as six hexamers (1x6mers). In the sluggish diplopod Spirostreptus, the 1x6mers seem to operate as almost or fully independent allosteric units (h approximately 1.3; P(50) approximately 5 torr), whereas in the swift centipede Scutigera, they intensively cooperate allosterically (h approximately 10; P(50) approximately 50 torr). Here, we show the chemomechanical basis of this differential behavior as deduced from hybrid 6x6mer structures, obtained by single-particle cryo-electron microscopy of the Scutigera 6x6mer (10.0 A resolution according to the 0.5 criterion) and docking of homology-modeled subunits from Scutigera and two diplopods, Spirostreptus and Polydesmus. The Scutigera 6x6mer hemocyanin is a trigonal antiprism assembled from six smaller trigonal antiprisms (1x6mers), thereby exhibiting D3 point group symmetry. It can be described as two staggered 3x6mers or three oblique 2x6mers. Topologically, the 6x6mer is subdivided into six subunit zones, thereby exhibiting a mantle (24 subunits) and a core (12 subunits). The six hexamers are linked by 21 bridges, subdivided into five types: two within each 3x6mer and three between both 3x6mers. The molecular models of the 6x6mer reveal intriguing amino acid appositions at these inter-hexamer interfaces. Besides opportunities for salt bridges, we found pairs of carboxylate residues for possible bridging via a Ca(2+) or Mg(2+) ion. Moreover, we detected histidine clusters, notably in Scutigera, allowing us to advance hypotheses as to how the hexamers are allosterically coupled in centipede hemocyanin and why they act more independently in diplopod hemocyanin.