Cold lability of the mutant forms of Escherichia coli inorganic pyrophosphatase

The variants of Escherichia coli pyrophosphatase carrying the substitutions Glu20 → Asp, His136 → Gln or His140 → Gln are inactivated, in contrast to the wild-type enzyme, at temperatures below 25°C: their activity measured at 25°C decreases with decreasing the temperature of the stock enzyme solution. The inactivation is completely reversible and is explained by cold-induced dissociation of these hexameric enzymes into less active trimers.