Enzyme action

Where it happens

Enzyme action takes place at the active site of an enzyme, a region with a specific shape into which only a matching substrate can fit. Enzymes act inside cells, such as in respiration, and outside cells, such as digestive enzymes in the alimentary canal.

Intracellular enzymes work within the cytoplasm and organelles: the enzymes of glycolysis in the cytoplasm, the enzymes of the Krebs cycle in the mitochondrial matrix, and the enzymes of photosynthesis in the chloroplast stroma. Extracellular enzymes are made inside a cell, packaged by the Golgi apparatus into vesicles, and released by exocytosis to act outside, salivary amylase in the mouth, pepsin in the stomach, and pancreatic lipase, amylase and trypsin in the duodenum.

The site matters for exam answers because each location has its own optimum pH: pepsin works in the acidic stomach at about pH 2, while trypsin works in the alkaline duodenum at about pH 8.

Inputs and outputs

• Input: the substrate, the molecule the enzyme acts on, such as starch for amylase, protein for pepsin, or hydrogen peroxide for catalase.
• Input: the enzyme, a globular protein with a precisely shaped active site made from a few amino acids folded together.
• Input: suitable conditions, a temperature near the optimum, a pH near the optimum, and water for hydrolysis reactions.
• Output: the product or products, maltose from starch, peptides from protein, water and oxygen from hydrogen peroxide.
• Output: the enzyme, released unchanged and immediately able to bind another substrate molecule.
• Not an output: energy in the usual sense, the enzyme lowers the activation energy of the reaction so it proceeds faster at body temperature, but the enzyme does not supply energy.

The steps

1. Random movement brings a substrate molecule close to the enzyme; frequent collisions are more likely when substrate concentration and temperature are higher.
2. The substrate fits into the active site because the two shapes are complementary, like a key entering a lock; a substrate with the wrong shape cannot bind.
3. Weak bonds form between substrate and active site, holding the substrate in position and straining the bonds within it.
4. An enzyme-substrate complex now exists; the activation energy needed for the reaction has been lowered.
5. The reaction takes place: in a breakdown reaction the substrate splits into two or more products, in a building reaction two substrates are joined.
6. The products have a different shape from the substrate and no longer fit the active site, so they are released.
7. The enzyme is unchanged in shape and chemical composition; its active site is empty and ready for the next substrate molecule.
8. The cycle repeats thousands of times per second until the substrate is used up, the products are removed, or conditions change.

Why it matters and how it is controlled

Enzymes speed up reactions that would otherwise happen too slowly to sustain life, without being used up themselves. Because each active site has a specific shape, an enzyme is specific and usually acts on only one type of substrate.

Enzyme activity is affected by temperature, pH and substrate concentration, and extreme temperature or pH can denature the enzyme by changing the shape of its active site.

Temperature controls enzyme action in two opposite ways. As temperature rises towards the optimum, about 37 °C for human enzymes, molecules gain kinetic energy, collide more often, and the rate of reaction increases.

Above the optimum, vibration breaks the weak hydrogen bonds that hold the protein in its folded shape, the active site loses its specific shape, and the substrate can no longer bind: the enzyme is denatured, permanently. Cold does not denature; it only slows the reaction, which is why enzymes recover when a chilled tube is warmed again.

pH controls enzyme action through the charges on the amino acids of the active site. Each enzyme has an optimum pH at which those charges hold the correct shape; a move away from it reduces activity, and an extreme change denatures the enzyme.

Substrate concentration controls the rate up to a point: more substrate means more collisions and a faster rate, until every active site is occupied all the time and the enzyme is saturated, after which adding substrate makes no difference. Enzyme concentration has the same effect from the other side, more enzyme molecules mean more active sites available, so the rate rises as long as there is enough substrate.

Factors affecting the rate of an enzyme reaction

How it is examined

You may be asked to describe the lock-and-key model using a labelled diagram, to explain why a change in temperature or pH affects the rate of an enzyme-controlled reaction, or to explain what denaturation means and why a denatured enzyme cannot function.

Graph questions are the most common: you are shown rate of reaction against temperature or pH and asked to describe and explain the shape. The mark scheme separates description (rate increases up to 37 °C then decreases) from explanation (more kinetic energy and collisions; denaturation of the active site above the optimum), so write both.

A second type gives a practical set-up, starch and amylase with iodine tests at set intervals, or hydrogen peroxide and catalase from potato measuring the oxygen produced, and asks you to identify variables, predict results and explain them. Comparison questions may contrast the lock-and-key model with the induced-fit model, or ask why one enzyme cannot digest a different substrate.

Common misconceptions

Worked exam-style question

Question. A student placed equal volumes of starch solution and amylase solution in test tubes kept in water baths at 10 °C, 25 °C, 37 °C, 50 °C and 70 °C. Every minute, a drop from each tube was tested with iodine solution.

The time taken for the iodine to stop turning blue-black was recorded as 18, 9, 4, 12 and 'no change after 30' minutes respectively. (a) Explain what the iodine test result shows.

(b) Explain why the reaction was fastest at 37 °C. (c) Explain the result at 70 °C.

(d) The student then cooled the 70 °C tube to 37 °C and tested again. Predict the result and explain your prediction.

Model answer. (a) Iodine turns blue-black in the presence of starch; when it stays brown, the starch has been completely hydrolysed to maltose by amylase, so the time taken measures the rate of enzyme action. (b) 37 °C is the optimum temperature of amylase: molecules have high kinetic energy, so collisions between substrate and active site are frequent and enzyme-substrate complexes form quickly.

(c) At 70 °C the enzyme is denatured: heat breaks the bonds holding the protein's shape, the active site changes shape, the substrate no longer fits, so starch is not broken down and the iodine stays blue-black. (d) The iodine will still turn blue-black.

Denaturation is permanent; cooling does not restore the shape of the active site, so no starch is digested. In contrast, the 10 °C tube was slow but not denatured, because low temperature only reduces kinetic energy.

Source:SRC-DSKP-EN

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