Which type of substances serves as a catalyst for reactions that break large food molecules into smaller useful molecules

Enzymes.

Explanation:

Enzymes are the substances which play the role of catalysts in bio-chemical reactions. These enzymes are proteins and play a very important role in glycolysis stage of cellular respiration in which large food molecule like glucose is broken down into smaller useful substance like pyruvate. Some enzymes which participate in glycolysis are phosphofructokinase, enolase, pyruvate kinase etc. This pyruvate is further converted into even smaller molecules like Acetyl-CoA which enters the next stage of cellular respiration known as TCA cycle.

TCA cycle produces many intermediates (molecules) which are useful for other bio-synthetic pathways in the body. For example: Alpha-ketoglutarate which is an intermediate of TCA cycle is used in the synthesis of many amino acids like glutamine, proline, and arginine.

Enzymes serve as catalyst that breakdown large substance into smaller molecules. Enzymes are proteins that function as biological catalysts to speed up chemical reactions in an organism.