© 2006 The Japanese Biochemical Society.
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The Hemocyanin of the Shamefaced Crab Calappa granulata: Structural-Functional Characterization
1 Dipartimento di Scienze Applicate ai Biosistemi, Università di Cagliari, Cittadella Universitaria, I-09042 Monserrato (CA), Italy; 2 Istituto di Biochimica e Biochimica Clinica, Facoltà di Medicina, Università Cattolica, Largo F. Vito 1, I-00168 Roma, Italy; 3 Istituto per la Chimica del Riconoscimento Molecolare, CNR, Largo F. Vito 1, I-00168 Roma, Italy; and 4 Dipartimento di Biologia Animale ed Ecologia, viale Poetto 1, I-09126, Cagliari, Italy
* To whom correspondence should be address. Tel: +39 070 6754523, Fax: +39 070 6754523; E-mail: pelleg{at}unica.it
Arthropod hemocyanins (Hcs) transport and store oxygen and are composed of six subunits, or multiples thereof depending on the species. Calappa granulata Hc is found as a mixture of dodecamers (95%) and hexamers (5%). Removal of calcium ions and alkaline pH induce an incomplete partially reversible dissociation of dodecameric Hc. Two-dimensional electrophoretic pattern of dissociated Hc indicated a large heterogeneity in Hc subunit: most differences are likely to be explained by post-translational modifications. Dodecameric Hc showed a large Bohr effect (
log P50/pH = –0.95) and a normal cooperativity (h50 values = 2.7 ± 0.2) in the presence of 10 mM CaCl2. The hexameric molecule displayed lower Bohr effect and cooperativity than the dodecamer. Lactate effect on the oxygen affinity (log P50 = 0.55) and the increase of lactate concentrations in animals kept in emersion were related to the increased oxygen requirements that occur during hypoxia in vivo. Calcium affects oxygen affinity only at high concentrations: this Hc appeared to lack the calcium high-affinity binding sites found in other species. The effect of temperature on both oxygen affinity and cooperativity was measured in the absence and presence of 10 mM lactate, allowing calculation of the exothermic contribution of lactate binding (H = –25 kJ mol–1).
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