Minireview: Recent progress in hemocyanin research

Abstract

This review summarizes recent highlights of our joint work on the structure, evolution, and function of a family of highly complex proteins, the hemocyanins. They are blue-pigmented oxygen carriers, occurring freely dissolved in the hemolymph of many arthropods and molluscs. They are copper type-3 proteins and bind one dioxygen molecule between two copper atoms in a side-on coordination. They possess between 6 and 160 oxygen-binding sites, and some of them display the highest molecular cooperativity observed in nature. The functional properties of hemocyanins can be convincingly described by either the Monod-Wyman-Changeux (MWC) model or its hierarchical extension, the Nested MWC model; the latter takes into account the structural hierarchies in the oligomeric architecture. Recently, we applied these models to interpret the influence of allosteric effectors in detailed terms. Effectors shift the allosteric equilibria but have no influence on the oxygen affinities characterizing the various conformational states. We have shown that hemocyanins from species living at different environmental temperatures have a cooperativity optimum at the typical temperature of their natural habitat. Besides being oxygen carriers, some hemocyanins function as a phenoloxidase (tyrosinase/catecholoxidase) which, however, requires activation. Chelicerates such as spiders and scorpions lack a specific phenoloxidase, and in these animals activated hemocyanin might catalyse melanin synthesis in vivo. We propose a similar activation mechanism for arthropod hemocyanins, molluscan hemocyanins and tyrosinases: amino acid(s) that sterically block the access of phenolic compounds to the active site have to be removed. The catalysis mechanism itself can now be explained on the basis of the recently published crystal structure of a tyrosinase. In a series of recent publications, we presented the complete gene and primary structure of various hemocyanins from different molluscan classes. From these data, we deduced that the molluscan hemocyanin molecule evolved ca. 740 million years ago, prior to the separation of the extant molluscan classes. Our recent advances in the 3D cryo-electron microscopy of hemocyanins also allow considerable insight into the oligomeric architecture of these proteins of high molecular mass. In the case of molluscan hemocyanin, the structure of the wall and collar of the basic decamers is now rapidly becoming known in greater detail. In the case of arthropod hemocyanin, a 10-Å structure and molecular model of the Limulus 8 × 6mer shows the amino acids at the various interfaces between the eight hexamers, and reveals histidine-rich residue clusters that might be involved in transferring the conformational signals establishing cooperative oxygen binding.

Interactive effects of metal pollution and temperature on metabolism in aquatic ectotherms: implications of global climate change

“…In arcid bivalves Scapharca inaequivalvis, blood oxygen transport mediated by hemoglobin-containing erythrocytes is also impaired by Cd exposure, although in this case the mechanism involves a decrease in hematocrit and hemocyte count, and not in oxygen affinity (Weber et al 1990). Given that the oxygen affinity of respiratory pigments, including hemoglobins and hemocyanins, tends to decrease with increasing temperatures (Burnett 1992, Weber & Vinogradov 2001, Decker et al 2007), the combined action of trace metals, high temperature and low oxygen solubility in the water may strongly limit oxygen uptake and delivery in the respiratory organs of aquatic ectotherms.…”

Molecular insight into lignocellulose digestion by a marine isopod in the absence of gut microbes

“…These prophenoloxidases are copper-containing enzymes that are closely related to the oxygen transporter hemocyanin. Interestingly, it has been demonstrated that the hemocyanins can be readily converted into phenoloxidases by conformational changes induced by limited proteolysis or chemical treatment (45). In vivo activation of both insect and crustacean hemocyanins has also been reported as an antimicrobial defense response (46,47).…”

Enhanced structural stability of Concholepas hemocyanin increases its immunogenicity and maintains its non‐specific immunostimulatory effects

“…Their basic structure is composed of ten subunits that are self-assembled into a hollow cylinder known as a decamer. In gastropods, decamers can self-associate face-to-face to form stable dimers or didecamers [1,2]. Inoculating mammals with hemocyanins induces a strong immune response that is characterized by a Th1 cytokine profile.…”