Structure and activation pattern of pyruvate decarboxylase (PDC) from yeast was studied by synchrotron radiation X-ray solution scattering. The results give a direct proof that the reversible deactivation of PDC at pH 8.0 is accompanied by the dissociation of the tetrameric holoenzyme into dimeric halves. The kinetics of this process was followed. At pH 6.5 the dimeric halves reassociate to a tetramer even in the absence of cofactors. The changes of the scattering pattern upon binding of the substrate-like activator pyruvamide indicate that the structure expands in the course of the enzyme activation.
