Biochemistry : Rate-Limiting Step

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Rate Limiting Step

According to Michaelis-Menton kinetics, what is a characteristic of the rate limiting step in enzyme kinetics?

Possible Answers:

Enzyme is not present

The enzyme-substrate complex is formed

It is dependent on activation energy from catalysis

It dissociates the enzyme-substrate complex into enzyme + substrate

Does not involve a catalyst

Correct answer:

It is dependent on activation energy from catalysis

Explanation:

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.

Example Question #2 : Rate Limiting Step

Which of the following best describes the rate-limiting step in a chemical reaction?

Possible Answers:

It is always an anabolic reaction

It is the step that consumes the most amount of energy in the overall reaction

It is the fastest step in the overall reaction

It is the slowest step of the overall reaction

It is the step that liberates the most amount of energy in the overall reaction

Correct answer:

It is the slowest step of the overall reaction

Explanation:

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.

Example Question #3 : Rate Limiting Step

Which of the following is true regarding the rate-limiting reaction?

Possible Answers:

It has the highest activation energy

It has the lowest activation energy

It is always endothermic

More than one of these are true

Correct answer:

It has the highest activation energy

Explanation:

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.

Example Question #2 : Rate Limiting Step

Consider the following series of reactions.

Reaction 1:  (fast)

Reaction 2:  (slow)

Reaction 3:  (fast)

What is the rate law for the overall reaction? ( and  are arbitrary reaction orders)

Possible Answers:

None of these are correct

Correct answer:

Explanation:

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.

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