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Flashcards · 9700 · AS Topic 3

Enzymes (AS Level) flashcards

14 cards on AS Topic 3, subtopics 3.1 to 3.2. Pick a subtopic or study them all.

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3.1 Mode of action of enzymes

Intracellular vs extracellular enzyme, with examples
Decided by where the enzyme acts. Intracellular: inside the cell that made it (catalase, DNA polymerase). Extracellular: secreted to act outside (amylase).
Active site
The region of the enzyme where the substrate binds. Its shape and R groups are complementary (not "the same") to the substrate.
How does an enzyme speed up a reaction?
The substrate binds to the active site, forming an enzyme–substrate complex. This lowers the activation energy. The overall energy change of the reaction is unchanged, and the enzyme is not used up.
Lock-and-key vs induced fit
Lock-and-key: a rigid active site exactly complementary to the substrate. Induced fit: the active site changes shape slightly as the substrate binds, moulding around it and straining its bonds.
How is catalase activity followed, and how is amylase activity followed?
Catalase: rate of product formation, the volume of O₂ collected (gas syringe). Amylase: rate of substrate disappearance, the time until iodine stays orange-brown (no starch).
Initial rate vs mean rate
Initial rate: the gradient of a tangent to the progress curve at t = 0. Mean rate: change ÷ time over an interval.
Using a colorimeter for an enzyme reaction
Use a filter of the complementary colour, zero with a blank, make a calibration curve of absorbance against known concentrations, then sample the reaction at set times and convert absorbance to concentration.

3.2 Factors that affect enzyme action

Explain the effect of increasing temperature on enzyme activity
Up to the optimum: more kinetic energy, more collisions, more E–S complexes. Above it: hydrogen and ionic bonds break, the active site changes shape and is no longer complementary; the enzyme is denatured.
Why does pH affect enzyme activity?
H⁺ concentration changes the charges on R groups, disrupting ionic and hydrogen bonds, so the shape and charges of the active site change. Buffers keep pH constant.
Why does rate level off at high substrate concentration?
All active sites are occupied (saturated), so enzyme concentration is limiting. The enzyme is not used up. The maximum rate is Vmax.
Km: definition and meaning
The substrate concentration at which rate = ½Vmax. A low Km means high affinity for the substrate; a high Km means low affinity. Km is a concentration, not a rate.
Competitive vs non-competitive inhibitor: effect on Vmax and Km
Competitive: binds the active site; Vmax unchanged, Km increased; overcome by more substrate. Non-competitive: binds elsewhere, changing the active site shape; Vmax lowered, Km unchanged.
How are enzymes immobilised in alginate?
Mix enzyme with sodium alginate solution and drip it from a syringe into calcium chloride solution. Calcium alginate beads form with the enzyme trapped inside; rinse with distilled water.
Advantages of immobilised enzymes
Product not contaminated with enzyme; enzyme recovered and reused (so cheaper); more stable over a wider range of temperature and pH; allows a continuous process in a column.

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