Proximity between fluorescent probes attached to four essential lysyl residues in phosphoenolpyruvate carboxylase

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Abstract
Phosphoenolpyruvate carboxylase, purified from maize leaves, is rapidly inactivated by the fluorescence probe dansyl chloride. The loss of activity can be ascribed to the covalent modification of an R-NH2 group, presumably the .epsilon.-NH2 group of lysine. Analysis of the data by the statistical method of Tsou [Sci. Sin. 11, 1535-1558 (1962)] provides clear evidence that at pH 8 eight R-NH2 groups can be modified in the tetrameric form of the enzyme, four of which are essential for catalytic activity. Essential groups are modified about five times more rapidly than the non-essential ones. The enzyme was completely protected against inactivation by Mg2+ plus phosphoenolpyruvate and consequently binding of the modifier to the essential groups is completely abolished. Hence the four essential groups seemed to be located, at or near the active site(s). One of the four essential groups was modified with dansyl chloride and the other three progressively with eosin isothiocyanate. In the doubly labeled protein non-radiative singlet-singlet energy transfer between dansyl chloride (donor) and eosin isothiocyanate (acceptor) was observed. The low variance (.+-. 5%) in the efficiency of energy transfer obtained at a particular acceptor stoichiometry (0.8-1.1, 1.9-2.1, 2.9-3.1) in triplicate samples provided confidence that the measured transfer efficiency may be interpreted as transfer between specific sites. The distances calculated from the efficiency of resonance energy transfer revealed two acceptor sites, equally separated, 4.8-5.1 nm from the donor site and the third site being 6.4 nm apart from the donor. Under conditions where the tetrameric enzyme dissociates into the monomers, no transfer of resonance energy between the protein-bound dansyl chloride and eosin isothiocyanate was observed. Most likely the four essential lysyl residues in the tetrameric enzyme are located in different subunits of the enzyme, hence each of the subunits would contain a substrate-binding site with one lysyl residue crucial for activity.

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