<P> Since inhibitors modulate the function of enzymes they are often used as drugs . Many such drugs are reversible competitive inhibitors that resemble the enzyme's native substrate, similar to methotrexate above; other well - known examples include statins used to treat high cholesterol, and protease inhibitors used to treat retroviral infections such as HIV . A common example of an irreversible inhibitor that is used as a drug is aspirin, which inhibits the COX - 1 and COX - 2 enzymes that produce the inflammation messenger prostaglandin . Other enzyme inhibitors are poisons . For example, the poison cyanide is an irreversible enzyme inhibitor that combines with the copper and iron in the active site of the enzyme cytochrome c oxidase and blocks cellular respiration . </P> <P> Enzymes serve a wide variety of functions inside living organisms . They are indispensable for signal transduction and cell regulation, often via kinases and phosphatases . They also generate movement, with myosin hydrolyzing ATP to generate muscle contraction, and also transport cargo around the cell as part of the cytoskeleton . Other ATPases in the cell membrane are ion pumps involved in active transport . Enzymes are also involved in more exotic functions, such as luciferase generating light in fireflies . Viruses can also contain enzymes for infecting cells, such as the HIV integrase and reverse transcriptase, or for viral release from cells, like the influenza virus neuraminidase . </P> <P> An important function of enzymes is in the digestive systems of animals . Enzymes such as amylases and proteases break down large molecules (starch or proteins, respectively) into smaller ones, so they can be absorbed by the intestines . Starch molecules, for example, are too large to be absorbed from the intestine, but enzymes hydrolyze the starch chains into smaller molecules such as maltose and eventually glucose, which can then be absorbed . Different enzymes digest different food substances . In ruminants, which have herbivorous diets, microorganisms in the gut produce another enzyme, cellulase, to break down the cellulose cell walls of plant fiber . </P> <P> Several enzymes can work together in a specific order, creating metabolic pathways . In a metabolic pathway, one enzyme takes the product of another enzyme as a substrate . After the catalytic reaction, the product is then passed on to another enzyme . Sometimes more than one enzyme can catalyze the same reaction in parallel; this can allow more complex regulation: with, for example, a low constant activity provided by one enzyme but an inducible high activity from a second enzyme . </P>

What type of chemical reactions do digestive enzymes catalyze
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