When substrate concentration is high, what happens to the rate of reaction in a saturated enzyme system?

• A. The rate approaches maximum velocity.
• B. The reaction slows down significantly.
• C. The substrate becomes a limiting factor.
• D. The reaction rate becomes negligible.

Answer: (A)

In a saturated enzyme system, when substrate concentration is high, the enzymes are fully occupied, meaning that there are no available active sites left for additional substrate molecules to bind. As a result, the reaction rate increases until it reaches a point of maximum velocity, often referred to as Vmax. At this stage, adding more substrate does not increase the rate of reaction because the enzyme's active sites are saturated. This relationship is described by the Michaelis-Menten kinetics model, where the rate of reaction depends on substrate concentration until saturation is attained, after which the rate levels off. Thus, the correct answer reflects that when substrate concentration is high, the rate of reaction indeed approaches this maximum velocity.