The interhexameric contacts in the four-hexameric hemocyanin from the tarantula Eurypelma californicum. A tentative mechanism for cooperative behavior.

Arthropod hemocyanins (Hcs) are regular assemblies of 1, 2, 4, 6 or 8 hexameric protein molecules. They transport oxygen and can bind it in a cooperative manner. The hexameric X-ray structures of Panulirus interruptus (spiny lobster) and of Limulus polyphemus (horseshoe crab) subunit II Hc were solved recently by the groups of Hol (Groningen, The Netherlands) and Magnus (Cleveland, U.S.A.). They related cooperativity to a rotational movement of a domain within a subunit and of the two trimers mutually inside the hexamer. In our study a model was derived for the structure and related to the function of the four-hexameric Hc from the tarantula Eurypelma californicum by combining data from electron microscopy and image processing, from the X-ray diffraction studies mentioned earlier and from amino acid sequence studies. Interhexameric contacts were determined at the level of secondary structure elements and in some cases of single amino acids. Loops, undefined in the X-ray structures of the hexamers, were often involved in these contacts. In one case the contact was formed between four parallel alpha-helices, two from each hexamer. Based on these findings a mechanism is proposed for the transmission of cooperativity between the hexamers, in which the concept of "helical friction" plays a key role.