The allosteric effector L-lactate induces a conformational change of 2x6-meric lobster hemocyanin in the oxy state as revealed by small angle x-ray scattering

Hemocyanins are multisubunit respiratory proteins found in many invertebrat es. They bind oxygen highly cooperatively. However, not much is known about the structural basis of this behavior. We studied the influence of the phy siological allosteric effector L-lactate on the oxygenated quaternary struc ture of the 2x6-meric hemocyanin from the lobster Homarus americanus employ ing small angle x-ray scattering (SAXS), The presence of 20 mM L-lactate re sulted in different scattering curves compared with those obtained in the a bsence of L-lactate. The distance distribution functions p(r) indicated a m ore compact molecule in presence of L-lactate, which is also reflected in a reduction of the radius of gyration by about 0.2 nm (3%), Thus, we show fo r the first time on a structural basis that a hemocyanin in the oxy state c an adopt two different conformations. This is as predicted from the analysi s of oxygen binding curves according to the "nesting" model. A comparison o f the distance distribution functions p(r) obtained from SAXS with those de duced hom electron microscopy revealed large differences. The distance betw een the two hexamers as deduced from electron microscopy has to be shortene d by up to 1.1 nm to agree well with the small angle x-ray curves.