Carbohydrate gluing is a strategy for supramolecular clamping of submultiples in annelid extracellular multi-subunit...

A key to understanding the mysterious hierarchic organization of annelid multi-subunit extracellular hemoglobin (giant Hb, composed of 144 globin chains and about 36 nonheme chains called linkers) is knowing the role of linkers in holding together the entire two-tiered hexagonal form. Here, the effects of added monosaccharides on the dissociation of giant Hb from the marineworm Perinereis aibuhitensis were monitored using dynamic light scattering (DLS), transmission electron microscopy (TEM), and circular dichroism (CD) measurements. Changes in Stokes radius and more clearly the distribution analysis of the Hb based on the DLS measurements showed that Hb preferentially dissociates into hexagonal units (called submultiples), which was consistent with the results of TEM and CD measurements. The results thus show that linkers specifically "clamp" submultiples together to organize the two-tiered form through carbohydrate gluing. Thus, a submultiple behaves like an ordinary protein, whereas the intact Hb behaves like a miniature supramolecular system. This clamp model is plausible because it inherently involves catastrophe of the molecular stoichiometry at the two-tiered hexagonal formation level because carbohydrates are under posttranslational regulation and therefore contain structural ambiguity.