Enzymes and Enzyme Reactions

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1Summary 重點摘要

Enzymes are protein catalysts that recognise their substrates through the specific shape of the active site; away from the optimum temperature or pH they denature, inhibitors slow the reaction — and these properties explain enzyme washing powder and meat tenderiser.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: The two directions of metabolismMetabolism covers all the chemical reactions inside a cell and runs in two directions: catabolism breaks large molecules into small ones and releases energy, as respiration oxidises glucose to carbon dioxide and water; anabolism builds large molecules from small ones and consumes energy, as amino acids are joined into proteins. Both run at once, in coordination. For example, after a meal blood glucose rises and the body stores the surplus as glycogen — anabolism; during exercise the glycogen is broken back down to glucose for fuel — catabolism.
2. Step 2: Enzymes — biological catalystsWithout catalysts the cell's reactions would crawl along at body temperature and life could not continue. Enzymes are globular protein biological catalysts: they speed up reactions enormously, are unchanged before and after so they can be reused, work in tiny amounts, and do not change the direction of a reaction — only how fast it goes. For example, salivary amylase breaks the starch in rice down to maltose right in the mouth; without enzymes, starch would barely react at all in water at 37°C.
3. Step 3: Active site and specificityOn an enzyme's surface sits a groove called the active site, whose three-dimensional shape matches the substrate like a lock matches its key: only a substrate of the right shape can fit in. This is enzyme specificity. Once bound, enzyme and substrate form an enzyme-substrate complex, the reaction happens inside it, the products leave, and the enzyme recovers its shape ready for the next substrate. For example, the active site of amylase accepts starch but not protein; breaking protein down is the job of proteases.
4. Step 4: The course and rate of enzyme reactionsAn enzyme-catalysed reaction can be written simply as substrate turning into products with the enzyme's help. The rate depends on substrate concentration and enzyme concentration: more substrate means more active sites occupied and a rising rate, until the enzymes are saturated; more enzyme likewise raises the rate. For example, industry fixes enzymes inside reaction columns through which substrate solution flows, so the reaction runs continuously and the enzyme is reused for a long time — an application of exactly these rules.
5. Step 5: The effect of temperatureAt low temperature molecules have little kinetic energy, collisions between enzyme and substrate are rare, and the reaction is slow — but the enzyme is unharmed. As temperature rises collisions become more frequent and the rate climbs to a peak at the optimum temperature, about 37°C for most human enzymes. Beyond it, heat breaks the bonds holding the enzyme's shape, the active site deforms, the enzyme denatures, the rate plunges, and cooling does not restore it. For example, a fever above 41°C reduces the activity of many human enzymes — one reason high fevers are dangerous.
6. Step 6: The effect of pHEvery enzyme has an optimum pH: away from it, excess hydrogen ions or hydroxide ions interfere with the bonds holding the enzyme's shape, the active site deforms and the rate falls; far from it the enzyme denatures, again irreversibly. Optimum pH values differ widely between enzymes. For example, pepsin works best at about pH 2, matching the strongly acidic stomach; trypsin prefers alkaline conditions in the small intestine — one digestive tract, with enzymes each suited to its own post.
7. Step 7: InhibitorsInhibitors slow enzyme reactions. Competitive inhibitors resemble the substrate and compete for the active site; the more substrate present, the better its chance of grabbing the active site, so the inhibition is weakened. Non-competitive inhibitors bind at a site away from the active site and change its shape so the substrate no longer fits — adding more substrate cannot undo this. For example, some antibiotics are designed as inhibitors of enzymes found only in bacteria, disrupting the pathogen's metabolism without touching human cells.
8. Step 8: Enzymes in everyday lifeEnzymes' specificity and efficiency at ordinary temperatures make them everyday tools: enzyme washing powder carries proteases and lipases that attack protein stains and grease, working best in warm water at 30–40°C, saving electricity and sparing fabrics; meat tenderiser contains papain, a protease that partially digests muscle protein so the meat softens; cheese-making uses rennin to curdle milk, and the juice industry uses pectinase to clarify juice. For example, washing-powder labels warn against hot water — heat denatures the enzymes.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• Metabolism splits into catabolism (breaking down large molecules, releasing energy) and anabolism (building large molecules, consuming energy).
• Enzymes are protein catalysts whose active sites complement their substrates, giving specificity — the lock-and-key model.
• The temperature curve: slow when cold, fastest at the optimum, then an irreversible plunge as heat denatures the enzyme; pH away from the optimum denatures too.
• Competitive inhibition can be weakened by adding substrate; non-competitive inhibition cannot.
• Enzyme washing powder and meat tenderiser exploit exactly the specificity and peak efficiency of enzymes near their optimal conditions.