In a zero-order reaction, what prevents an increase in velocity?

In a zero-order reaction, the velocity of the reaction is constant and independent of the substrate concentration. This means that the reaction rate remains the same no matter how much substrate is added, as long as the concentration of substrate is above a certain level.

The scenario where all enzyme molecules are bound to substrate is critical because, in this case, all available active sites on the enzyme are occupied. When this saturation occurs, the enzyme cannot process additional substrate molecules, leading to a situation where the reaction rate (velocity) does not increase, irrespective of further increases in substrate concentration. The enzyme is effectively working at its maximum capacity, thus maintaining a constant reaction velocity characteristic of zero-order kinetics.

Other scenarios would not have the same effect. Low substrate concentration would typically lead to a faster increase in reaction rate if more substrate were available to bind to the enzyme. A high enzyme concentration could enhance the reaction rate assuming there is substrate available. The presence of inhibitors might lower the overall reaction rate, but it is the saturation of enzyme with substrate that directly causes the zero-order kinetics in the context of this question.