It has been shown that the denaturation of phosphoglycerate kinase (PGK) can be observed not only when the solution is heated above 30 degrees C, but also when it is cooled below this temperature. The disruption of the native PGK structure upon cooling and the subsequent formation of this structure upon heating both proceed in two distinct stages which correspond to the independent disruption or reformation of each of its domains. In contrast, the heat denaturation of PGK proceeds in one stage, showing that the two domains of the molecule are associated into a single complex which figures in the denaturation process as a cooperative unit. It follows that, at elevated temperature, there is a positive interaction between the domains, which disappears at lower temperatures. This might be due to hydrophobic interactions, which are known to be temperature dependent. The temperature decrease leads to a decrease in inter- and intradomain interactions, which results in an increase of the independence of the domains and a decrease in their stability.