Fluorescence emission spectra for E92C and E121C EGK labeled with extrinsic fluorophores. Separate samples of each enzyme were labeled with dansyl as fluorescence donor, D, or fluorescein as fluorescence acceptor, A, as described in the text. The labeled proteins were mixed with equimolar concentrations of unlabeled protein or the other labeled protein to a total protein concentration of 1 µM (subunits). Emission spectra were recorded with an excitation wavelength of 336 nm. (A) E92C EGK. (B) E121C EGK. Legend: D, circles; A, open squares; D+A, filled squares; acceptor difference spectrum ((D+A) − D), X. (C) Donor difference spectra. Filled symbols show spectrum for D and open symbols show the donor difference spectrum ((D+A) − A). E92C, squares; E121C, circles.