A compound with almost similar structure to the substrate can act as a ………….

Explanation

**Step-by-Step Solution:**

1. **Understanding the Context**: The question asks about a compound that has a structure similar to a substrate and how it acts in relation to enzymes.

2. **Define Enzymes**: Enzymes are biological catalysts that speed up chemical reactions in living organisms. They are typically proteins and have specific regions known as active sites where substrates bind.

3. **Role of Substrates**: A substrate is the molecule upon which an enzyme acts. When a substrate binds to the active site of an enzyme, it undergoes a transformation to form products.

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