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Mechanism of the alpha-complementation reaction of E. coli beta-galactosidase deduced from fluorescence correlation spectroscopy measurements.

The kinetics of the alpha-complementation reaction of two protein fragments yielding active E. coli beta-galactosidase was measured using fluorescence correlation spectroscopy (FCS). The association reaction was extremely slow with an apparent association rate kapp of 207 M-1 s-1. This low association rate can be explained by a fast pre-equilibrium and slow subsequent steps involving at least two dimeric complexes. The subsequent formation of a tetrameric complex is probable and consistent with the experimental data. The complexes comprise two or four subunits, respectively, of the large fragment (EA)2 and in all cases only one small fragment, ED which has been labeled with Cy5. These kinetics have been compared to the association kinetics of ED to inactivated (EA)2. The kinetics were similar to the association with native (EA)2. The data support the observation that lyophilization of (EA)2 in a reducing environment which causes complete loss of enzymatic activity does not interfere with binding.