KINETIC AND SPECTROSCOPIC EVIDENCE OF CATION-INDUCED CONFORMATION CHANGES IN YEAST K+-ACTIVATED ALDEHYDE DEHYDROGENASE

The activity, stability and spectroscopic properties of yeast K+-activated aldehyde dehydrogenase were measured at various times after removal from, and after returning to, a solution containing K+. Enzyme activity is rapidly lost on removal of most of the K+ and rapidly regained if K+ is replaced immediately. These activity changes are slower than likely rates of K+ dissociation and association. These rapid changes in concentration result in altered enzyme stability with enzyme in K+ more stable. UV difference spectra are produced whenever enzyme in an activating environment (K+ or Tl+) is compared with enzyme in a non-activating environment (Tris+ or Li+). These spectral changes occur within 10s. The saturation characteristics with K+ are hyperbolic for all 3 phenomena of activation, stabilization and spectral change, with estimated apparent dissociation constants (Ks) for K+ or 7.5 mM, 5.5 mM and 6 mM, respectively. Continued incubation of enzyme in the absence of K+ results in the accumulation of an enzyme form that reactivates only slowly on replacing K+. Stability characteristics in various concentrations of K+ over equivalent time scales are consistent with the existence of additional conformations. Spectroscopic evidence also indicates such additional slow conformation changes. Results were interpreted in terms of 2 separate conformation transitions induced or stabilized by K+.