.........................catalyse covalent bonding between two molecules to form a large molecule.

Explanation

To answer the question regarding which catalyst catalyzes covalent bonding between two molecules to form a large molecule, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Question**: The question is asking for a type of catalyst that facilitates the formation of a larger molecule through covalent bonding between two smaller molecules.

2. **Identifying the Type of Catalyst**: In biological systems, enzymes serve as catalysts that speed up chemical reactions. Specifically, we are looking for an enzyme that can catalyze the joining of two molecules.

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