act as catalysts
The most important difference between synthetic and naturally occurring polymers is that the former usually contain very few different monomers, whereas biological polymers can have as many as 20 different kinds of amino acid residues arranged in many different orders. Chains with less than about 50 amino acid residues are calledpeptides, whereas those with more than about 50 amino acid residues are calledproteins. Many proteins are enzymes, which are catalysts that increase the rate of a biological reaction.