Molecular Structure of the Arthropod Hemocyanins

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Abstract

Hemocyanin is an extracellular, blue protein that occurs in high concentrations in the blood of many arthropods, including spiders, scorpions, horseshoe crabs, crustaceans, and at least two centipedes. Serving as an ### oxygen carrier, it is functionally equivalent to hemoglobin, but performs reversible oxygen binding between two copper ions. Hemocyanin is composed of a number of subunits that assemble in an extremely large macro-molecular entity. These particles, which are similar in size to viruses or ribosomes, exhibit a complex allosteric behavior during oxygen binding. There is growing evidence that this functional plasticity has evolved upon, and answers to, ecophysiological constraints. Arthropod hemocyanins are cubical molecules which, in the electron microscope, differ largely from the cylindrical particles found in mollusks (Fig. 1). Molluscan hemocyanins are decamers, didecamers, or multidecamers of polypeptides of Mr up to 450000 that carry up to eight binuclear copper sites. In contrast, arthropod hemocyanins are hexamers (1 × 6) or oligohexamers (n × 6) of polypeptides of Mr 75000, each containing only a single such copper site (Fig. 2). For earlier reviews, see e.g. Kobert (1903), Quagliariello (1923), Redfield (1934), Ghiretti (1962), Van Holde and Van Bruggen (1971), Bonaventura et al. (1977), Wood (1980), Van Holde and Miller (1982), Ellerton et al. (1983), Brunori et al. (1985), and Markl (1986).