Final answer:
Enzymes, which are sensitive to changes in pH, are activated at a low pH due to the alteration in ionization of amino acid residues, affecting their substrate binding and catalytic activity. Acid-base catalysis is enhanced as the presence of hydrogen ions can lower the activation energy required for the reaction.
Step-by-step explanation:
Expansions are activated at a low pH primarily due to the structure and function of enzymes, which are sensitive to pH changes. These biological catalysts have an optimum pH at which they function most effectively, and deviations from this pH can alter their activity. At a low pH, the increased concentration of hydrogen ions can change the ionization of amino acid residues in the enzyme, specifically those in the active site. This alteration in charge can affect the enzyme's ability to bind to its substrate, which often leads to increased catalytic activity for certain reactions such as those involved in acid-base catalysis. In these reactions, the presence of excess hydrogen ions can lower the activation energy by prompting proton movements critical in the catalytic process.
It's also worth noting that some microorganisms have adapted to acidic environments and have enzymes that are optimized to work at low pH levels. In such environments, the high acidity can control microbial growth and confer specific flavours in foods like yogurt and pickles. Similarly, enzymes that function in our stomach, which has a highly acidic environment, are expressly designed to work at low pH values.