First step of an enzyme - catalyzed reaction is

Options

A. Enzyme breaks the chemical bond of substrate
B. Enzyme releases the products of the reaction
C. The substrate binds to the active site of the enzyme , fitting into the active site
D. Active site reacts with substrate forming enzyme product complex

Answer

To solve the question regarding the first step of an enzyme-catalyzed reaction, we can break it down into a series of logical steps:

### Step-by-Step Solution:

1. **Understanding Enzymes**: Enzymes are biological catalysts that speed up chemical reactions in living organisms. They are crucial for various biochemical processes.

2. **Enzyme-Substrate Interaction**: The first step in an enzyme-catalyzed reaction involves the interaction between the enzyme and its substrate. This is a critical initial phase where the enzyme recognizes and binds to the substrate.

3. **Binding to Active Site**: The substrate specifically binds to the active site of the enzyme. The active site is a unique region on the enzyme that is complementary in shape to the substrate, allowing for a precise fit.

4. **Formation of Enzyme-Substrate Complex**: Once the substrate binds to the active site, an enzyme-substrate complex is formed. This complex is essential for the subsequent steps of the reaction.

5. **Breaking of Bonds**: After the enzyme-substrate complex is formed, the enzyme facilitates the breaking of bonds within the substrate, leading to the formation of products.

6. **Release of Products**: Finally, the products of the reaction are released, and the enzyme is free to catalyze another reaction.

### Conclusion:
The first step of an enzyme-catalyzed reaction is when the substrate binds to the active site of the enzyme.

### Answer:
The correct answer is: **The substrate binds to the active site of the enzyme.**

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