How Enzymes Work: Active Sites and Transition States
Enzymes are biological catalysts, mostly proteins, that speed up chemical reactions by lowering the activation energy; each enzyme has an active site into which its specific substrate fits to form an enzyme-substrate complex. In brief: an enzyme is not used up in the reaction and does not change the overall energy released or the equilibrium of the reaction, but it can make it millions of times faster. Two models describe how the substrate binds: the lock-and-key model and the induced-fit model, in which the active site changes shape slightly to fit the substrate.
Without enzymes, most of the reactions needed for life would be far too slow at body temperature. This guide explains activation energy and catalysis, the structure of the active site, the models of enzyme action, why enzymes are specific, compares enzymes with non-biological catalysts, works through a reaction-rate example and corrects common mistakes.
What are biological catalysts?
A catalyst is a substance that increases the rate of a reaction without being used up. Enzymes are the catalysts of living cells. Nearly all are globular proteins, and a few are RNA molecules (ribozymes). The reaction in which an enzyme acts is: substrate → product. For example, the enzyme catalase breaks down hydrogen peroxide, a toxic by-product of metabolism, into water and oxygen: 2H₂O₂ → 2H₂O + O₂.
Enzymes are very efficient. Catalase can break down millions of hydrogen peroxide molecules per second per enzyme molecule. Reactions can be 10⁶ to over 10¹⁷ times faster than without the enzyme. The enzyme works at body temperature, neutral pH and normal pressure, in contrast with industrial catalysts that often need high temperature and pressure. Enzyme names usually end in “-ase” and show the substrate (lipase breaks down lipids) or the reaction (dehydrogenase removes hydrogen).
What is activation energy?
For substrate molecules to react, they must collide with enough energy and in the right orientation, and pass through a high-energy, unstable arrangement called the transition state. The minimum energy needed is the activation energy (Eₐ). Even in reactions that release energy overall, a barrier like this must be overcome. Heating a reaction increases the energy of the molecules, but in a living cell high temperatures would damage the proteins.
Enzymes lower the activation energy by providing an alternative route. They do not change the energy of the substrate or the product, so the overall energy change of the reaction (ΔG) is the same, and the position of the equilibrium does not change. They only help the system to reach equilibrium faster. In an energy profile graph, the enzyme-catalyzed curve has a lower peak than the uncatalyzed one, but the start and end levels are the same.
Enzymes lower the activation energy in several ways:
- They bring the substrate molecules close together in the right orientation.
- They strain bonds in the substrate, so that they break more easily.
- They provide a favorable environment, such as an acidic or non-polar one, in the active site.
- They sometimes take part in the reaction temporarily, with amino acid side chains.
What is the active site?
The active site is a small region of the enzyme, a pocket or groove often made up of only 3 to 12 amino acids, where the substrate binds and the reaction happens. The rest of the protein gives the structure. The shape of the active site and the chemical properties of its amino acids (charges, polarity, hydrophobicity) are determined by the tertiary structure of the protein. They are attracted to the substrate by hydrogen bonds, ionic bonds and hydrophobic interactions.
The sequence is: the substrate binds to the active site to form the enzyme-substrate complex; the substrate is converted to product within the complex (the enzyme-product complex); the products leave, and the enzyme is free to bind another substrate molecule. The enzyme is unchanged at the end.
How do the lock-and-key and induced-fit models differ?
- Lock-and-key model (Emil Fischer, 1894): the active site has a fixed shape complementary to the substrate, as a key fits a lock. This explains specificity, but it suggests a rigid enzyme.
- Induced-fit model (Daniel Koshland, 1958): the active site is flexible. When the substrate binds, the enzyme changes shape slightly, and wraps around the substrate, to give the best fit. This change helps to strain the substrate bonds and lower the activation energy. This model fits most observations, such as the fact that enzymes bind to the transition state more strongly than to the substrate.
Why are enzymes specific?
An enzyme usually acts on only one substrate, or a group of closely related substrates, because only these have the correct shape and chemical properties to fit the active site. For example, sucrase digests sucrose but not lactose, and lactase digests lactose but not sucrose. This is why thousands of different enzymes are needed. Specificity allows the cell to control each reaction separately. A change in the amino acid sequence by mutation may change the shape of the active site and so the function of the enzyme.
Enzymes and chemical catalysts compared
| Feature | Enzyme | Inorganic catalyst (e.g. platinum) |
|---|---|---|
| Composition | Protein (some RNA) | Metal or inorganic compound |
| Specificity | Very high | Often low |
| Conditions | Mild: body temperature, near-neutral pH | Often high temperature and pressure |
| Sensitivity to heat | Denatured by high temperature | Usually stable |
| Regulation | Controlled by inhibitors, feedback | Not regulated |
Worked example: speed-up by catalase
Hydrogen peroxide decomposes slowly by itself. Compare the rates in an experiment in which 10 cm³ of 3 percent hydrogen peroxide is mixed with different materials, and the volume of oxygen collected in 60 seconds is measured.
| Added to the peroxide | Oxygen (cm³) | Rate (cm³/s) |
|---|---|---|
| Nothing | 0.2 | 0.0033 |
| Manganese(IV) oxide (inorganic catalyst) | 12 | 0.20 |
| Fresh liver (contains catalase) | 30 | 0.50 |
| Boiled liver | 0.3 | 0.0050 |
- Rate: volume ÷ time. For the liver: 30 ÷ 60 = 0.50 cm³/s. For nothing: 0.2 ÷ 60 = 0.0033 cm³/s.
- Speed-up by catalase: 0.50 ÷ 0.0033 ≈ 150 times faster than the uncatalyzed reaction, and 0.50 ÷ 0.20 = 2.5 times faster than with manganese oxide in this setup.
- Boiled liver: rate 0.0050 cm³/s, close to the uncatalyzed rate. Heat has denatured the enzyme, and the active site has lost its shape.
The result shows that enzymes are catalysts, and that they are destroyed by heat. The control with boiled liver shows that the effect depends on the intact enzyme, not on other substances in the liver. The real speed-up by catalase is much greater than 150-fold, because the amount of enzyme in a piece of liver is small in this experiment and the oxygen collected is limited by the substrate and by the diffusion of peroxide into the tissue. The numbers are invented for the illustration.
How were enzymes discovered?
Anselme Payen and Jean-François Persoz isolated diastase (amylase) in 1833. Eduard Buchner showed in 1897 that a cell-free extract of yeast can ferment sugar, which proved that enzymes can work outside living cells, and received the Nobel Prize in Chemistry in 1907. James Sumner crystallized urease in 1926 and showed that it is a protein. Emil Fischer proposed the lock-and-key model in 1894. Thomas Cech and Sidney Altman discovered catalytic RNA in the 1980s and shared the 1989 Nobel Prize in Chemistry.
Which mistakes are common?
Mistake 1: Saying that enzymes provide energy for reactions
They lower the activation energy. They do not supply energy.
Mistake 2: Thinking that enzymes are used up
An enzyme is not changed by the reaction, and can be used many times, although it may eventually be broken down by the cell.
Mistake 3: Saying that heat “kills” enzymes
Enzymes are not alive. Heat denatures them, which changes the shape of the active site.
Mistake 4: Believing that enzymes change the products or the equilibrium
They speed up both directions equally. They do not change the equilibrium position.