Flashcards · 5090 · Topic 5

Enzymes flashcards

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Catalyst: A substance that increases the rate of a chemical reaction and is not changed (not used up) by the reaction.
Enzyme: A protein that functions as a biological catalyst.
How an enzyme works (lock and key): The substrate fits into the active site, whose shape is complementary to it. An enzyme-substrate complex forms, the reaction occurs, and the products are released. The enzyme is unchanged and reused.
Why are enzymes specific?: Each active site has a shape complementary to one particular substrate, so only that substrate can bind and form an enzyme-substrate complex.
Two ways to follow the progress of an enzyme reaction: Measure the disappearance of substrate (e.g. sample amylase and starch into iodine until no blue-black forms) or the appearance of product (e.g. collect oxygen from catalase and hydrogen peroxide in a gas syringe).

Effect of temperature below the optimum: Rate increases with temperature: molecules have more kinetic energy, so there are more frequent successful collisions between enzyme and substrate.
Effect of temperature above the optimum: Rate falls sharply. The active site changes shape, the substrate no longer fits and no enzyme-substrate complexes form: the enzyme is denatured.
Is denaturation reversible? Are enzymes "killed"?: Denaturation is permanent: cooling does not restore activity. Enzymes are denatured, never killed, because they were never alive.
Effect of pH on enzyme activity: Each enzyme has an optimum pH. Either side of it activity falls; extremes of pH denature the enzyme by changing the shape of the active site.
Approximate optimum pH of pepsin, amylase and trypsin: Pepsin: about 2 (stomach). Amylase: about 7 (mouth, small intestine). Trypsin: about 8 (small intestine).