Final answer:
The correct statement is that the rate of an enzyme-catalyzed reaction is directly proportional to enzyme concentration under specific conditions, until a maximum rate is achieved where all enzyme active sites are occupied.
Step-by-step explanation:
Among the given statements about enzyme-catalyzed reactions, the true one is that under given conditions of substrate concentration, pH, and temperature, the rate of product formation varies directly with enzyme concentration up to a maximum, at which point the rate cannot be increased further. This is because enzymes can become saturated with the substrate, meaning once all of the active sites on the enzyme molecules are occupied by the substrate, adding more substrate will not further increase the rate of the reaction.
It is also important to consider that the rate of reaction for an enzyme-catalyzed process is dependent on temperature. Reactions typically increase in rate with a rise in temperature until an optimal temperature is reached, after which the enzyme may denature and the rate will decline. Similarly for pH, each enzyme has an optimal pH; deviations from this pH can reduce enzyme activity. Unlike the statement suggests, enzymes are not universally most active at pH 7, since some enzymes, like pepsin, work best in highly acidic conditions.
Lastly, the claim that reaction rates are independent of substrate concentration is incorrect for enzyme-catalyzed reactions, as initially the reaction rate will increase with substrate concentration until the point of saturation.