Small-angle x-ray-scattering studies on the substructure of Helix pomatia hemocyanin

Abstract

Hemocyanin of the snail helix pomatia was studied by small-angle X-ray scattering in 0.15 M phosphate buffer pH 6. The outer portion of the scattering curve showed minima, which indicated that the hemocyanin molecule is composed of a large number of identical (or very similar) nearly spherical subunits. From the position of the minima the radius of gyration Ru of the subunit was calculated to be 1.55 nm; this corresponds a diameter of 4 nm assuming a spherical shape. The number of the subunits is discussed, too; a number of 330 can be given as lower limit, a number of 528 as upper limit (most propable value 350 — 400). Various models were put up by arranging the subunits (diameter 4 nm) in different ways to hollow cylinders. The theoretical scattering curves of these models were compared with the experimental one and a model equivalent in scattering is suggested for the hemocyanin Helix pomatia. In this model, which fits very well to the experimental curve, 360 subunits form a hollow cylinder. The model shows also the essential features found by electron microscopy by VAN BRUGGEN. The essential features are an arrangement of the subunits in 6 layers vertical to the cylinder axis and a 5-fold screw axis parallel to it.