Imagine a workshop tool that speeds up a job enormously, does the work without being used up, and then is ready to do the same job again on the next piece — but which only works within a narrow range of conditions and is ruined if handled too roughly. That captures the character of an enzyme. Enzymes are the catalysts of living cells, and they have a distinctive set of properties worth knowing as a group.
Most fundamentally, almost every enzyme is a protein, folded into a precise three-dimensional shape. There is one important exception: certain nucleic acids also act as enzymes, and these are called ribozymes. Because a typical enzyme is a folded protein, it carries a special pocket on its surface called the active site, into which its substrate (the molecule it acts on) fits so the reaction can be catalysed.
The properties that make enzymes remarkable, and also delicate, include:
- Enormous catalytic power. Enzymes lower the activation energy of a reaction — the energy hump that has to be climbed before substrate can become product — so reactions run vastly faster. Carbonic anhydrase, for example, speeds its reaction up by around ten million times.
- They emerge unchanged. After releasing the product, the enzyme is regenerated exactly as it was and can immediately act on a fresh substrate molecule, running the same cycle again and again.
- Sensitivity to temperature. Each enzyme has an optimum temperature at which it is most active. Low temperature merely slows it into a temporary, recoverable inactivity, but high temperature (usually above about forty degrees Celsius) denatures the protein, unravelling its fold and destroying its activity for good. Enzymes from heat-loving (thermophilic) organisms are unusual in staying stable and active up to around eighty to ninety degrees — a quality called thermal stability.
- Sensitivity to pH. Likewise, each enzyme has an optimum pH, and activity falls away on either side of it.
- Saturation. As more substrate is supplied, the reaction speeds up only until every enzyme molecule is occupied. Beyond that point the enzymes are saturated and the rate levels off at a ceiling called the maximum velocity. …