Beta-Galactosidase Alpha Complementation: Properties of the Complemented Enzyme and Mechanism of the Complementation Reaction

Intracistronic alpha complementation involving Escherichia coli beta-galactosidase occurs between the cyanogen bromide peptide CB2, derived from residues 3-92 of beta-galactosidase (Langley, K.E., Fowler, A.V., and Zabin, I. (1975), J. Biol. Chem. 250, 2587), and the defective beta-galactosidase from the Z-deletion mutant strain M15. The M15 protein, a dimer, lacks residues 11-41 of beta-galactosidase (Langley, K.E., Villarejo, M.R., Fowler, A.V., Zamenhof, P.J., and Zabin, I. (1975), Proc. Natl. Acad. Sci. U.S.A. 72, 1254). The complemented enzyme formed from purified components has a molecular weight of 533 000+/-25 000, is therefore tetrameric, and has a probable stoichiometry of 1 CB2:1 M15 monomer. The complemented enzyme has the same Km for substrate as wild type enzyme, but is less stable to heat or urea treatment. The overall equilibrium constant for the complementation reaction is approximately 1-2 X 10(9) M-1. Initial velocity studies indicate saturation kinetics when either component is fixed and limiting, with an apparent Kd of about 10(-6) M. A first-order rate constant of 0.05-0.1 min-1 was estimated. The kinetics favor a model of rapid complex formation, followed by slow conformational change, as the mechanism of activation. Ultraviolet difference spectroscopy indicated an increased absorbance in the 290-300 nm region as a result of the complementation reaction. The kinetics of the increase suggest that two processes, one rapid and the other slower, could be responsible. The temperature dependence of complementation (Ea approximately 24 000 cal) is also consistent with the rate-determining step being a conformational change.