The enzyme that begins the breakdown of protein is Pepsin.

Here's why:

* Location: Pepsin is found in the stomach, the first major site of protein digestion.

* Function: Pepsin breaks down large protein molecules into smaller peptides, which are then further digested in the small intestine.

* Activation: Pepsin is produced in an inactive form called pepsinogen. It is activated by the acidic environment of the stomach.

While other enzymes are involved in protein digestion later in the digestive process, pepsin is the key player that initiates the breakdown of proteins.