Answer: Catalyst
Explanation: The specialized protein that acts as catalyst are enzymes. They increase the rate of the reaction by lowering the activation energy.
The activation energy is the minimum energy which is required to start a reaction.Enzymes do not take part in the reaction but it enhances the rate of reaction and enzyme is available again for another set of reaction.
The enzyme forms complex with the substrate and then substrate is converted into product and enzyme is made available for another reaction.