Shifts in the optimum pH of Rhizopus glucoamylase depending on the reaction temperatures

There seems a general concept that the optimum pH of enzyme reaction is little affected by environmental conditions, because very few papers have so far been published on factors, especially temperature, affecting the optimum pH of enzymes. Rhizopus glucoamylase digests uncooked cereal starch and thus, it has been used to process uncooked starch for alcohol fermentation. The optimum pH of the glucoamylase is about 5.0. We have found an interesting phenomenon that the alcohol fermentation of uncooked raw corn starch or uncooked polished rice grain by yeast in the presence of Rhizopus glucoamylase proceeds significantly faster at pH 4 than at pH 5 at low temperatures. This was observed more clearly as the temperature was further lowered. In other words, the optimum pH of Rhizopus glucoamylase changes depending on the temperature.