The cold-active alpha-amylase from the Antarctic bacterium Pseudoalteromonas haloplanktis (AHA) is the largest known multidomain enzyme that displays reversible thermal unfolding (around 30 degrees C) according to a two-state mechanism. Transverse urea gradient gel electrophoresis (TUG-GE) from 0 to 6.64 M was performed under various conditions of temperature (3 degrees C to 70 degrees C) and pH (7.5 to 10.4) in the absence or presence of Ca2+ and/or Tris (competitive inhibitor) to identify possible low-stability domains. Contrary to previous observations by strict thermal unfolding, two transitions were found at low temperature (12 degrees C). Within the duration of the TUG-GE, the structures undergoing the first transition showed slow interconversions between different conformations. By comparing the properties of the native enzyme and the N12R mutant, the active site was shown to be part of the least stable structure in the enzyme. The stability data supported a model of cooperative unfolding of structures forming the active site and independent unfolding of the other more stable protein domains. In light of these findings for AHA, it will be valuable to determine if active-site instability is a general feature of heat-labile enzymes from psychrophiles. Interestingly, the enzyme was also found to refold and rapidly regain activity after being heated at 70 degrees C for 1 h in 6.5 M urea. The study has identified fundamental new properties of AHA and extended our understanding of structure/stability relationships of cold-adapted enzymes.