Mode of action of enzymes

Biology AS · Enzymes. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Enzymes are globular proteins that act as catalysts. They speed up a reaction and are not used up. Intracellular enzymes work inside cells. Extracellular enzymes are secreted and work outside cells.

Each enzyme has an active site with a shape complementary to its substrate. The substrate binds to form an enzyme-substrate complex, the products form and leave, and the enzyme is free to be used again. This is why an enzyme is specific to one substrate.

Enzymes work by lowering the activation energy, the energy needed to start the reaction. To follow a reaction you measure either product appearing or substrate disappearing over time.

Rules to remember

Lock-and-key hypothesis: the active site has a fixed shape that fits the substrate exactly
Induced-fit hypothesis: the active site changes shape slightly as the substrate binds, giving a closer fit
Catalase: 2H₂O₂ → 2H₂O + O₂; follow the reaction by the volume of oxygen collected per unit time (product formed)
Amylase: starch → maltose; follow the reaction by testing samples with iodine solution until it stays orange-brown (substrate gone)
rate = change in amount ÷ time taken; compare initial rates, taken from the start of the reaction when the curve is steepest
Colorimeter: measures absorbance of light by a coloured solution; a deeper colour gives a higher absorbance; set it to zero first with a blank

Common mistake

Writing that the active site and the substrate have the same shape. They have complementary shapes, like a key and the lock it fits.

Worked example

Catalase is added to hydrogen peroxide. 24 cm³ of oxygen is collected in the first 30 s and a total of 30 cm³ after 60 s. What is the initial rate, and why is the reaction slower in the second 30 s?

initial rate = volume of oxygen ÷ time = 24 ÷ 30 = 0.8 cm³ s⁻¹.
In the second 30 s only 30 − 24 = 6 cm³ is made: 6 ÷ 30 = 0.2 cm³ s⁻¹.
Substrate has been used up, so its concentration is lower and fewer enzyme-substrate complexes form each second.

Answer: The initial rate is 0.8 cm³ s⁻¹; the rate falls because the substrate concentration falls.

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