How Enzymes Speed Up the Reactions of Life

A chemical reaction needs particles to collide with enough energy and in a useful arrangement. The minimum energy needed to begin is called activation energy. Enzymes speed reactions by providing an alternative pathway with a lower activation energy, so more collisions can lead to change.

Most enzymes are proteins, although a few RNA molecules can also act as catalysts. An enzyme does not add energy to the reaction or change the final balance between reactants and products. It helps the system reach that result faster, which is useful when a cell has deadlines imposed by breathing, moving and remaining alive.

The substances an enzyme acts on are called substrates. They bind to a region called the active site. The enzyme holds them in a position that makes bonds easier to break or form. Products are released, and the enzyme can usually begin the cycle again.

The active site has a particular three-dimensional shape and chemical environment. A matching substrate can bind through attractions between the two. The fit is not always perfectly rigid. Many enzymes shift shape slightly as the substrate arrives, creating an induced fit that supports the reaction.

This selectivity lets cells control many reactions at once. One enzyme may help digest starch, while another copies DNA or joins amino acids. If every enzyme accepted every substrate, the cell would become a chemical jumble with the organisational quality of a drawer full of mystery cables.

Cells also regulate enzymes. Some molecules activate them, while others bind and reduce their activity. Feedback inhibition can slow an early step when enough final product has accumulated. This prevents the cell from wasting materials and energy.

Temperature usually increases reaction rate at first because particles move faster. If the temperature becomes too high, bonds holding the enzyme's shape can break. The active site changes, and the enzyme becomes denatured. Very low temperatures normally slow activity without permanently changing the enzyme.

Each enzyme also works best within a certain pH range. A major pH change can alter charges and bonds in the protein, changing the active site. Enzymes in the acidic stomach therefore have different preferred conditions from enzymes working in the small intestine.

Remember the main pattern:

• •Substrates bind at an active site.
• •The enzyme lowers activation energy.
• •Products leave and the enzyme can work again.
• •High heat or extreme pH can change enzyme shape.
• •Cells regulate enzymes to control reaction pathways.

Biological washing powders contain enzymes chosen to break down particular stains. Proteases attack protein stains such as blood and egg, lipases target fats and oils, and amylases break down starch. Because enzymes speed up these reactions, the powder can clean effectively at lower washing temperatures than a non-biological detergent might need, which also saves energy. The enzymes used are often sourced from microorganisms and adjusted to stay stable in the alkaline, soapy conditions inside a washing machine.

Enzymes also explain lactose intolerance. Digesting lactose, the main sugar in milk, requires an enzyme called lactase. Most mammals produce plenty of lactase as infants, then make much less after weaning. In many human adults the same thing happens. Some populations, however, carry genetic variants that keep lactase production going into adulthood, a trait linked to long histories of dairy farming. When lactase is low, undigested lactose passes to the large intestine, where bacteria ferment it and can cause bloating and discomfort.

For much of the nineteenth century, scientists argued about fermentation, the process in which yeast turns sugar into alcohol and carbon dioxide. Louis Pasteur showed that living yeast cells were responsible, and many researchers believed fermentation could only happen inside an intact living organism, as if life itself supplied some special force. Others suspected that cells simply contained chemical agents that could, in principle, work on their own. Digestive juices that broke down food outside the body hinted at the second idea, but the question for yeast stayed open.

In 1897 the German chemist Eduard Buchner ground yeast cells with sand, pressed out the juice and filtered away the cells. When he added sugar to this cell-free liquid, fermentation still happened. The active substances inside the cells, not life itself, were doing the chemistry, and Buchner later received a Nobel Prize for the work. A further question remained: what were enzymes made of? In 1926 the American biochemist James Sumner crystallised the enzyme urease and showed it was a protein. His claim was doubted for years before other enzymes were crystallised and confirmed it.

Because enzymes control so many reactions, stopping one of them can change how the body behaves, and a large share of medicines work this way. Some drugs are competitive inhibitors. Their shape resembles the normal substrate closely enough to sit in the active site, so the real substrate is crowded out. Raising the amount of substrate can partly overcome this kind of blocking, because the two molecules are competing for the same space. Statins, taken to lower cholesterol, work like this, inhibiting an enzyme in the liver's pathway for making cholesterol.

Other drugs bind permanently. Aspirin attaches a small chemical group to cyclooxygenase enzymes, which make signalling molecules involved in pain, fever and inflammation, and each affected enzyme stays switched off until the cell replaces it. Penicillin takes a similar approach against bacteria. It binds to the enzymes that link together the building blocks of the bacterial cell wall, so growing bacteria are left with weak walls that can burst. Human cells have no cell wall and none of these enzymes, which is why penicillin can harm bacteria while leaving your own cells largely alone.

Many enzymes cannot work with their protein chain alone. They need a non-protein helper called a cofactor, which often sits in the active site itself. Some cofactors are metal ions. Carbonic anhydrase, which helps red blood cells handle carbon dioxide, depends on a zinc ion, and many enzymes that handle ATP need magnesium. Other helpers are organic molecules called coenzymes, which often carry electrons or small chemical groups from one reaction to the next. Many coenzymes are built from vitamins, which is a large part of why your diet needs small amounts of them. Niacin, for example, is used to make NAD, an electron carrier in respiration.

Scurvy shows what happens when a helper goes missing. Vitamin C is needed by enzymes that finish building collagen, the protein that gives strength to skin, gums, blood vessels and healing wounds. These enzymes add small chemical groups to collagen chains so that they can twist together firmly. Without enough vitamin C, the enzymes stall, the collagen comes out weak, gums bleed and cuts heal slowly. Sailors on long voyages living on preserved food suffered badly. In 1747 the naval surgeon James Lind compared several treatments on sick sailors and found that citrus fruit worked, although the enzyme explanation came only in the twentieth century.

Enzymes make biological reactions fast enough for cells to function by lowering activation energy. Their active sites give them selectivity, while temperature, pH and regulatory molecules affect their activity. Follow substrate binding, reaction and product release, and enzymes become practical tools rather than mysterious protein names.