Hydrogen exchange demonstrates three domains in aldolase unfold sequentially

Rabbit muscle aldolase is a homotetramer in which the subunits have a classical alpha/beta-barrel structure and Mr 39,212 Da. We have previously reported that aldolase incubated in 3 M urea has three unfolding domains distinguished by their different unfolding rates. The unfolding rates of these domains were determined from isotope patterns in the mass spectra of peptic fragments derived from aldolase incubated in 3 M urea and pulse labeled in (2)H2O. The present study extends this investigation to more thoroughly characterize the structures of these unfolding intermediates. Mass spectra of intact monomers had four envelopes of isotope peaks corresponding to four structural forms of aldolase. Analysis of the present results suggests that these structural forms consist of native aldolase and three forms that have one to three domains unfolded. The molecular masses of these four structural forms indicate that there are 107 residues in each of the three unfolding domains of aldolase. Present results also show that aldolase remains a tetramer in 4 M urea, even though hydrogen exchange and circular dichroism indicate that it has lost most of its secondary and tertiary structure. The abundances of unfolded domains, which were determined from mass spectra of either intact aldolase or its peptic fragments, were used to determine the abundances of specific, partially unfolded forms of aldolase. Kinetic modeling of the abundances of these structures suggests that these structures are formed sequentially as aldolase unfolds in urea.