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
H. Hartmann et al., 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, J BIOL CHEM, 276(23), 2001, pp. 19954-19958
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.