The enzyme-substrate complex dissociates into enzyme + product. The rate limiting step is providing the activation energy to get to the transition state, which is greatly decreased by an enzyme.

Although chemical reactions are typically displayed in the form of an equation, with reactants on the left and products on the right, these reactions are not a simple one step conversion. Often, there are several individual steps that the reactants go through on their way to becoming products. This is shown by the mechanism for that particular reaction.

Furthermore, when talking about chemical reactions, it is very important to distinguish between two concepts that are sometimes confused with one another. The first concerns the kinetics of the reaction, while the second concerns the thermodynamics.

Chemical kinetics is concerned with time. If a chemical reaction is occurring, kinetics answers the question of how fast the reaction is going. Thermodynamics, on the other hand, is not concerned with time. It doesn't care how fast or how slow a reaction goes. All it cares about is whether a chemical reaction is spontaneous or nonspontaneous. To answer this, thermodynamics considers the energetics of a reaction.

When looking at the answer choices, we can immediately eliminate three of them based on this information. The rate-limiting step of a chemical reaction is not concerned with how much energy is liberated or consumed. Instead, the rate-limiting step is defined as the slowest step out of all the steps that occur for a given chemical reaction. In other words, a reaction can only proceed as fast as its slowest step, just like a chain is only as strong as its weakest link. Further, the rate-limiting step in a reaction may be anabolic or catabolic.

It is important to note, however, that there is one component of energy that does affect the rate of a reaction. This energy is called the activation energy, and it represents how much energy needs to be invested into a reaction in order for that reaction to proceed. The reason why this is distinct from thermodynamics, however, is because thermodynamics cares only about initial and final energy states; it doesn't care about how a reaction goes from initial to final, whereas kinetics does. Even though the activation energy for a reaction can change (via enzymes, for instance), this will not affect the initial and final energy levels.

Rate-limiting reaction is the slowest reaction in a series of reactions. It is used to calculate the rate law of the overall reaction. Recall from thermochemistry that slow reactions tend to have higher activation energy (energy “hill’). Slower reactions have to climb a higher energy hill to produce transition states and, subsequently, products; therefore, rate-limiting reaction will have the highest activation energy of all reactions in the series. Enthalpy (exothermic or endothermic) does not determine the speed of a reaction; therefore, a rate-limiting reaction could be exothermic or endothermic.

The overall reaction is as follows.

The rest of the molecules are intermediates (meaning they are produced and consumed in the reaction). The overall reaction rate always depends on the slowest step, which is also called the rate-determining step. Reaction 2 is the rate-determining step in this series of reactions; therefore, the rate law for the overall reaction is:

where  is the reaction order. Since it is an intermediate, molecule C is not part of the final reaction and, therefore, not part of the rate law. We cannot determine the reaction order from the given information.