Differential scanning calorimetry was used to measure changes in thermodynamic stability and aggregation for glycine 93 mutants of human copper, zinc-superoxide dismutase (SOD). Glycine 93 is a conserved residue at position i ؉ 3 of a tight turn and has been found to be a mutational hot spot in familial amyotrophic lateral sclerosis (fALS). The fALS-associated mutations, G93A, G93S, G93R, G93D, and G93V, were made in a pseudo wild-type background containing no free cysteines, which prevented the formation of aberrant disulfide bonds upon thermal unfolding, and enabled quantitative thermodynamic analysis of the effects of the mutations. Thermal unfolding was highly reversible for all the SODs in both the fully metallated (holo) and metal-free (apo) forms. The data for all the holo-SODs and for the apo-pseudo-wild-type SOD were well fit by a 2-state unfolding model for native dimer (N 2 ) to two unfolded monomers (2U), N 2 7 2U. The holo-and apo-forms of the mutants are significantly destabilized (by 1.5-3.5 kcal mol ؊1 monomer) relative to the corresponding forms of pseudo wild-type, with the relative stabilities being correlated with statistical preferences for amino acids in this structural context. Although van't Hoff (⌬H vH ) to calorimetric (⌬H cal ) enthalpy ratios are close to unity for all the holo-SODs and for apo-pseudo-wild-type, consistent with a 2-state transition, ⌬H vH is considerably larger than ⌬H cal for all the apo-mutants. This suggests that the mutations cause apo-SOD to have an increased propensity to misfold or aggregate, which may be linked to increased toxic mutant SOD aggregation in fALS. SOD2 is a highly stable homodimeric metalloenzyme (1-4). Each subunit forms a Greek key ␤-barrel structure that binds one zinc and one copper ion (5, 6); the zinc contributes to the structural stability of the active site, and the copper is involved in the redox dismutation of the toxic superoxide free radical (7). ALS is a common, rapidly progressive adult motor neuron disease for which there is very little in the way of treatment and no cure (8). Mutations in SOD were linked to fALS in 1993 (9); to date, over 110 SOD mutations have been associated with the disease. Although mutant SOD fALS represents only a few percent of all ALS cases, this is the major known cause of the disease (10, 11). Mutant SODs have a gain of toxic function, the molecular mechanism of which remains unresolved (8). Recent attention has focused on increased toxic misfolding of mutant SOD as a cause for fALS, analogous to other protein misfolding diseases (12-15). Increased misfolding has been correlated with decreased protein stability; hence, a better understanding of the thermodynamic effects of fALS mutations may lead to valuable insights into the disease process.Wild-type human SOD contains 4 cysteines per monomer; in the native protein two of these (Cys-57 and Cys-146) form a conserved intra-subunit disulfide bond, and the other two (Cys-6 and Cys-111) remain as free cysteines. Thermal unfolding of wild-type SOD is highly i...

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