Answer the following question given this paragraph: 

Proteins are an essential part of all organisms. They play many roles in living things. Certain proteins provide a scaffolding that maintains the shape of cells (structural proteins). Proteins also make up the majority of muscle tissues. Many proteins are enzymes that speed up chemical reactions in cells. Enzymes interact with the substrates (reactants) of a biochemical reaction, helping the reaction proceed at a much faster rate. Other proteins are antibodies that protect you from pathogens. Antibodies bond to foreign substances in the body and target them for destruction. Still other proteins help carry messages or materials in and out of cells (transport proteins) or around the body. For example, the blood protein hemoglobin (see Figure below ) bonds with oxygen and carries it from the lungs to cells throughout the body.


Q: What type of protein speeds up chemical reactions in cells?


A:
enzyme