Question

Describe enzyme catalysis as a series of steps, with \(E\) symbolizing the enzyme and \(S\) symbolizing the substrate.

Short answer

Enzyme catalysis consists of three main steps: (1) Formation of enzyme-substrate complex: the enzyme (E) and substrate (S) interact to form an enzyme-substrate complex (ES), represented as \(E + S \rightleftharpoons ES\). (2) Catalytic reaction: the enzyme converts the substrate into the product (P) via a transition state (TS), represented as \(ES \rightarrow ETS \rightarrow EP\). (3) Release of the product: the enzyme-product complex (EP) dissociates, releasing the product and freeing the enzyme for another catalytic cycle, represented as \(EP \rightleftharpoons E + P\). The entire process can be summarized as \(E + S \rightleftharpoons ES \rightarrow ETS \rightarrow EP \rightleftharpoons E + P\).

Step-by-step solution

Formation of enzyme-substrate complex

The first step in enzyme catalysis is the binding of the substrate (S) to the enzyme's active site, forming an enzyme-substrate complex (ES). This can be represented by the equation: \[E + S \rightleftharpoons ES\] In this process, the enzyme (E) interacts with the substrate (S) through non-covalent interactions, such as hydrogen bonding, hydrophobic interactions, and ionic bonds. This forms a temporary complex called the enzyme-substrate complex (ES).

Catalytic reaction

Once the enzyme-substrate complex is formed, the enzyme can perform its catalytic function and convert the substrate (S) into the product (P). This step involves a transition state (TS), in which the substrate is in an intermediate state between being fully bound to the enzyme and being converted into the product. This step can be represented as: \[ES  \rightarrow  ETS  \rightarrow  EP\] The enzyme stabilizes the transition state (ETS) and lowers the activation energy for the reaction, allowing it to proceed more efficiently.

Release of the product

Once the catalytic reaction is complete, and the product (P) is formed, the enzyme-product complex (EP) dissociates. This allows the enzyme to return to its original state and be available to bind to another substrate molecule. This step can be represented by the equation: \[EP \rightleftharpoons E + P\] The product (P) dissociates from the enzyme (E), leaving the enzyme free to participate in another catalytic cycle. Overall, the entire enzyme catalysis process can be summarized in the following equation: \[E + S \rightleftharpoons ES \rightarrow ETS \rightarrow EP \rightleftharpoons E + P\]

Key concepts

Enzyme-Substrate Complex

The enzyme-substrate complex is the very first step in the process of enzyme catalysis. Consider an enzyme as a key and a substrate as a lock. The enzyme's active site fits the substrate perfectly, just like a key fits into a lock. This action forms what we call the enzyme-substrate complex. It's a bit like getting ready before a magic trick; everything is set up perfectly for the chemical reaction to occur.


While hooking up, the enzyme binds to the substrate using non-covalent interactions. These can be hydrogen bonds, hydrophobic interactions, or ionic bonds. Even though these interactions are not as strong as covalent bonds, they are essential for holding the substrate in place inside the enzyme’s active site. This ensures that the enzyme is ready to transform the substrate into a product.

• Provides a place for the reaction to occur.
• Keeps the substrate locked in place, ready for the next step.
• Relies on weak, temporary bonds to stabilize the complex.

Transition State

Once the enzyme-substrate complex is formed, we move on to an intriguing phase called the transition state. Think of a roller coaster at its very peak right before it zooms downward; it's a thrilling, tense moment while the ride changes directions. In an enzyme reaction, this high-energy phase is called the transition state.


It’s the point where the substrate is neither bound fully to the enzyme nor converted entirely into the product; it's hovering between these states. This is the enzyme’s moment to shine by stabilizing the transition state and preparing it to move to the next step.

• The substrate is in between being a reactant and a product.
• This state is very unstable, and hence quick to change.
• The enzyme plays a crucial role here, helping push the reaction forward.

Activation Energy

Activation energy is like the hurdle that a chemical reaction needs to overcome before it can proceed. Think of it as the initial energy required to push a boulder over a hill. With enzyme catalysis, the enzyme acts like a helpful assistant that lowers this hurdle, making it easier for the reaction to occur.


In the enzyme-substrate complex, enzymes work their magic by finding the path of least resistance, effectively lowering the activation energy. This helps accelerate the reaction, guiding the substrate to successfully transform into the product.

• Lowers the energy needed for the reaction to proceed.
• Enables the reaction to occur more rapidly.
• Influences the rate of the catalytic reaction.

Catalytic Reaction

Following the transition state, the enzyme engages in the catalytic reaction. In this step, the substrate is transformed into the product. Imagine this as the final artwork after a series of thoughtful brush strokes aided by a skillful artist.


The enzyme works as a craftsman, first stabilizing that tricky transition state and then smoothing it into a beautiful product. This catalytic reaction is the culmination of all previous phases, resulting in a brand new product ready to be released. Afterward, the enzyme lets go of the product, resetting itself for another round of reactions.

• Transforms substrate into the product efficiently.
• The enzyme is not used up in the process and can be reused.
• Releases the product, leaving the enzyme ready for another cycle.