Stability of quaternary structure and mode of dissociation of fructose diphosphate aldolase isoenzymes

Abstract

A highly sensitive subunit exchange assay was used to study the relative strengths of interactions between different subunit types (A [rabbit chicken muscle] and C [chicken brain]) of fructosediphosphate aldolase and to determine the mode of dissociation of aldolase tetramers in vitro. Interactions between C subunits within C4 tetramers were considerably more resistant to disruption than were interactions between A subunits in A4 tetramers with regard to increasing concentrations of H+, OH- or urea. Slight dissociation of A4 was also observed in 1.2 M magnesium chloride. The quaternary structure of aldolase C4 may be inherently more stable than that of aldolase A4. Also, the symmetrical heterotetramer A2C2 was more resistant to urea-mediated dissociation than was the aldolase A4 homotetramer; this suggests that, even when in heteromeric combination, C subunits have a stabilizing influence on the quaternary structure of aldolase tetramers. In no case was evidence found for a stable dimeric intermediate in the dissociation of aldolase tetramers to monomers. These observations are considered in terms of the tetrahedral arrangement of subunits in the aldolase tetramer. The general applicability of the subunit exchange assay described here for studying the subunit structure and mode of dissociation of oligomeric enzymes is discussed.