Large intramolecular movement in human complement protein C3 induced by methylamine

The reaction of methylamine with complement protein C3, which involves cleavage of a labile thiol ester bond, yields a large intramolecular rearrangement. This is shown by small‐angle neutron and X‐ray scattering using a Fab antibody as a marker. For the C3(Fab) 1:1 complex, the methylamine reaction yields an increase in the radius of gyration, R , from 4.6 nm to 6.0 nm. In the absence of Fab the corresponding R values increase from 4.4 nm to 5.1 nm. It is estimated that the methylamine‐induced increase in R may correspond to a movement of the epitope to a position 5 nm away from the centre of gravity of the C3 molecule. In agreement with this finding, the maximum distance within the C3(Fab) complex increases from 16 nm to 22 nm as a result of the methylamine reaction. In order to explain this conformational change, it is tentatively suggested that the methylamine‐induced cleavage of the C3 thiol ester bond leads to a domain rotation within the C3 molecule. In agreement with this idea, the data is consistent with a model which enables a globular domain within the molecule to rotate without redistributing the molecular mass more than that corresponding to the radii of gyration observed.