Which external factor is NOT a determinant for enzyme activity?

Amino acid structure is not considered an external factor influencing enzyme activity; rather, it pertains to the intrinsic properties of the enzyme itself. Enzymes are proteins made up of amino acids, and their specific sequence and arrangement determine their shape and function.

In contrast, factors like pH, temperature, and substrate concentration are external conditions that can affect how well an enzyme operates. For example, changes in pH can alter the charge of the enzyme or substrate, potentially affecting binding and reactivity. Similarly, temperature can influence the kinetic energy of molecules, which can enhance or hinder enzyme activity based on optimal conditions. Substrate concentration plays a critical role as well; as substrate levels change, it can directly impact the rate at which the enzyme catalyzes a reaction, up to a saturation point.

Thus, while pH, temperature, and substrate concentration are crucial to enzyme activity, amino acid structure is intrinsic and not influenced by external factors.