The optimal pH levels for digestive enzymes vary between $1.5$ and $8.5$, depending on the specific enzyme and its location within the digestive system.

Digestive enzymes are specialized proteins that accelerate chemical reactions in the body, primarily those involved in breaking down food into essential nutrients. Each enzyme operates most effectively at certain pH levels, which differ based on both the type of enzyme and its specific location in the digestive tract.

In the stomach, the main digestive enzyme is pepsin, which is most active in a highly acidic environment, with a pH ranging from approximately $1.5$ to $3.5$. This acidity is achieved through the secretion of hydrochloric acid by the stomach lining. The low pH not only enhances pepsin’s activity but also plays a crucial role in eliminating bacteria present in food.

Moving to the small intestine, the environment becomes less acidic, with a pH level between $6$ and $8.5$. In this region, several key enzymes are active, including amylase, lipase, and proteases such as trypsin and chymotrypsin. These enzymes, secreted by the pancreas, function optimally in this slightly alkaline setting. Amylase is responsible for breaking down carbohydrates, lipase aids in the digestion of fats, and proteases facilitate the breakdown of proteins into amino acids.

In the mouth, the process of digestion begins with salivary amylase, which operates best at a neutral pH of around $7$. This enzyme initiates the breakdown of complex carbohydrates into simpler sugars.

In conclusion, the optimal pH for digestive enzymes is not a fixed value; rather, it is a variable range that changes depending on the specific enzyme and its location within the digestive system. This range, spanning from highly acidic conditions in the stomach to slightly alkaline conditions in the small intestine, promotes the effective breakdown of various food components into nutrients that the body can readily utilize.