Most catalases exist as tetramers of 60 or 75 kDa, each subunit containing an active site haem group buried deep within the structure, but which is accessible from the surface through hydrophobic channels. The very rigid, stable structure of catalases is resistant to unfolding, which makes them uniquely stable enzymes that are more resistant to pH, thermal denaturation and proteolysis than most other enzymes. Their stability and resistance to proteolysis is an evolutionary advantage, especially since they are produced during the stationary phase of cell growth when levels of proteases are high and there is a rapid rate of protein turnover.