Which statement accurately describes a substrate in the presence of an enzyme operating at V_max? Only when the enzyme operates at V_max is the substrate:

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11ecba2d_0ede_312f_a3bf_3175d754f4ec_MD0008_00 The maximum possible reaction rate V_max of an enzymatic reaction is limited by the number of enzyme active sites present in solution. A Michaelis-Menten graph depicts the rate of an enzymatic reaction V₀ as a function of substrate concentration [S] when the concentration of enzyme [E] is fixed. As [S] increases, the number of active sites bound by substrate also increases, leading to a faster reaction rate. When [S] is sufficiently high, all enzyme active sites are bound (saturating conditions) and V_max is achieved. In contrast, when [S] is not saturating, some enzyme active sites are not bound by substrate and the reaction proceeds at a rate that is slower than V_max. Accordingly, only when the enzyme is operating at V_max does substrate occupy every active site in solution. (Choice B) In a Michaelis-Menten graph, [S] is significantly higher than [E] at every substrate concentration measured, not just under saturating conditions. Having [S] that is significantly higher than [E] does not necessarily mean that all active sites are occupied, so the fact that [S] is significantly higher than [E] does not necessarily mean that V_max is achieved. (Choice C) Enzymes catalyze reactions by stabilizing the transition state between substrate and product. This occurs regardless of whether or not sufficient substrate is present to achieve V_max. (Choice D) In first-order reactions, the reaction rate increases linearly with substrate concentration. In Michaelis-Menten kinetics, reactions are approximately first-order at low substrate concentrations. However, under the saturating conditions required to achieve V_max, the reaction follows approximately zero-order kinetics, in which the rate does not increase with increasing substrate concentration. Educational objective: V_max is the maximum possible rate of an enzymatic reaction when a given concentration of enzyme is present. In a Michaelis-Menten graph, [S] is significantly larger than [E] at every [S] measured. However, V_max occurs only when the concentration of substrate [S] is high enough that all enzyme active sites are bound.

Which Statement Accurately Describes a Substrate in the Presence of an Enzyme