High concentrations of D-glyceraldehyde-3-phosphate dehydrogenase stabilize the enzyme against denaturation by low concentrations of GuHCl

It is known that denaturation of D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH, EC 1.2.1.12) in low concentrations of GuHCl, around 0.5 M, at 25 degrees C, leads first to a burst phase drop of activity, followed by slow unfolding with further loss of enzyme activity and aggregation. However, GAPDH at higher concentrations does not increase the aggregation in the slow phase as would be expected but decreases both the inactivation and aggregation of the enzyme instead. It seems that GAPDH at high concentrations protects the enzyme against GuHCl-denaturation. This protection is not a general effect of GuHCl binding by increased protein concentration but specific for GAPDH, as either bovine serum albumin or alpha-lactalbumin does not show any protection at similar concentrations. It is proposed that dissociation of tetrameric GAPDH into dimers in the early phase of denaturation in dilute GuHCl is reversible and further unfolding of the dimer to an aggregation prone species is irreversible and rate-limiting for the unfolding process. High concentrations of the enzyme shift the equilibrium towards the tetramer thus decrease the aggregation of GAPDH in dilute GuHCl.