Retarded protein folding of deficient human α1‐antitrypsin D256V and L41P variants

α1‐Antitrypsin is the most abundant protease inhibitor in plasma and is the archetype of the serine protease inhibitor superfamily. Genetic variants of human α1‐antitrypsin are associated with early‐onset emphysema and liver cirrhosis. However, the detailed molecular mechanism for the pathogenicity of most variant α1‐antitrypsin molecules is not known. Here we examined the structural basis of a dozen deficient α1‐antitrypsin variants. Unlike most α1‐antitrypsin variants, which were unstable, D256V and L41P variants exhibited extremely retarded protein folding as compared with the wild‐type molecule. Once folded, however, the stability and inhibitory activity of these variant proteins were comparable to those of the wild‐type molecule. Retarded protein folding may promote protein aggregation by allowing the accumulation of aggregation‐prone folding intermediates. Repeated observations of retarded protein folding indicate that it is an important mechanism causing α1‐antitrypsin deficiency by variant molecules, which have to fold into the metastable native form to be functional.