Dimerization occurs during the reversible acid inactivation of 2-oxo-4-hydroxyglutarate aldolase from Escherichia coli

Abstract

Exposure of Escherichia coli 2-oxo-4-hydroxyglutarate aldolase (4-hydroxy-2-oxoglutarate glyoxylate-lyase, EC 4.1.3.16) (molecular weight = 63000) to phosphoric acid at pH 1.6 for 10 min at 4δC causes 95% or greater inactivation. No significant effect on the rate or extent of inactivation is caused by varied aldolase concentrations or the presence of exogenous proteins. Chloride ion (50–100 mM) or 10 mM 2-oxo-4-hydroxyglutarate markedly decreases both the rate and extent of inactivation; good protection is also afforded by 10 mM pyruvate, glyoxylate, glyoxal, 2-oxoglutarate or 2-oxobutyrate. Whereas native aldolase has two free and three buried sulfhydryl groups, all five are exposed in the acid-inactivated enzyme and the molecular weight of this species at pH 1.6 is 126000. Ultraviolet absorbance difference spectra, circular dichroism spectra and ultracentrifugation studies establish that the inactivation process is characterized by an alteration of secondary and tertiary structure as well as an aggregation to a dimer of the native molecule. Reactivation of enzyme activity to 60–80% of the original level is seen within 20 min at pH 6 to 8; examination of inactivation/reactivation as a function of pH indicates that these two processes occur via kinetically distinct pathways. Native and reactivated enzymes are identical in molecular weight, sulfhydryl titer, K m and α-helix content.