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How Enzymes Work


An enzyme is an organic substance that initiates and accelerates a chemical reaction. Enzymes are vital for the body’s chemistry. Without them, many of the chemical reactions on which life depends would not occur.

The life of the cell in the human body depends on the production of energy. The chemical reactions that release this energy normally require temperatures in excess of 90- degree C. Enzymes allow these reactions to occur-so life exists at normal body temperatures.

Complex Proteins

The majority of enzymes are complex proteins-which are strings of amino acids, which in turn, are made up of carbon, hydrogen, oxygen and nitrogen atoms, and in many cases sulphur atoms as well. Like all proteins, they are produced by the cell using DNA as a template. A few of them, however, are made up of RNA (ribonucleic acid, which along with DNA is part of the genetic code), in which case they are known as ribozymes.

What Enzymes Do

Enzymes act as catalysts to chemical reactions, speeding them up and reducing the amount of energy that they require. They may be either ‘catabolic’-meaning involved in breaking complex substances down to their simpler components- or ‘anabolic’, when they help reactions that build up materials by putting their components together. Other enzymes are involved in helping chemicals cross the membrane that encloses each cell.

Examples of Enzymes

An enzyme called sucrase, for example, is catabolic: meaning that it helps break sucrose (sugar) down into glucose and fructose, forms in which it can be more easily digested. An enzyme called carbonic anhydrase is anabolic: meaning it helps water to combine with carbon dioxide (CO2), a by-product of the energy-producing process within the cells, to make carbonic acid. Once in this form, it is transported in the blood to the lungs, to be expelled from the body as CO2. An enzyme called glucose permease (the names of most enzymes end in ‘-ase’) helps transport glucose across cell membranes so that it can be used to produce energy.

The ‘Lock & Key’ Theory

Every enzyme in the body has a specific task, and various theories have been put forward to explain how each enzyme fulfills this one task only.

The first theory is known as the ‘lock and key’ hypothesis. This theory works on the principle that there is an area or ‘active site’ on the surface of the enzyme molecule into which a molecule of a chemical, known as the ‘substrate’, fits and is then held in place by electrical attraction. As the reaction proceeds, the substrate is turned into another chemical (the ‘product’) that has different electrical properties so the electrical attraction disappears and the product molecule abandons the site. The whole process repeats itself with new substrate molecules many thousands of times in a fraction of a second.

Unfortunately, the lock and key theory does not completely fit the facts. For one thing, enzyme activity can be inhibited by many factors such as change in temperature or pH, which would not be the case if the fit was a purely physical matter. Also, molecules other than the substrate can lock on to the site.

The ‘Induced Fit’ Theory

Another concept, known as the ‘induced fit’ theory, satisfies these objections. It holds that the active site has elastic properties and expands and contracts as necessary to accommodate the substrate-rather as a glove changes shape to accommodate the hand.

Enzymes and Energy

Enzymes reduce the amount of energy required for a chemical reaction. However, being proteins, they are sensitive to changes in their surroundings.

Whenever a chemical reaction takes place, energy is used. For a reaction to take place, bonds between atoms must be broken for new ones to form. The energy required to break these bonds is called ‘activation energy’. For instance, many substances will burn, but only after energy has been supplied (such as heat from a lighted match). Enzymes work by lowering this activation energy, allowing reactions to proceed at lower temperatures, without chemically becoming involved in the reaction.

What Effects Enzyme Activity?

The three main factors that affect enzyme activity are temperature, pH and the presence of other chemicals that either occupy the active site or distort its shape. Depending on their action, such chemicals are known as competitive and non-competitive inhibitors.


For each enzyme in the body, there is a temperature range within which it works at maximum efficiency. Outside this range, the bonds that hold together the complex protein structures of which most enzymes are made, start to break down. As a result, the shape of the enzyme’s active site changes making it impossible for the substrate to lock on to the enzyme. This why body systems start to shut down when the temperature is too high (hyperthermia), or too low (hypothermia).


As with temperature, enzymes have an optimum range of pH values within which they work most effectively. Outside this range, the action of an enzyme is inhibited, and at extremes it does not work at all. (pH is a value that indicates the concentration of hydrogen ions in a solution and so how acidic or alkaline a solution is; distilled water, for example, has a pH of 7, while bleach has a pH of 12 and orange juice has a pH of 2).

The optimum pH range varies from enzyme to enzyme, but is not normally a problem because it is matched to the enzyme’s environment, and a buffering system (which compensates for small changes in pH) helps keep pH in different areas of the body relatively constant. Stomach enzymes, such as pepsin and chymotrypsin, work best at a low, acidic pH, which is brought about by the acidic conditions in the stomach.

Competitive and Non-Competitive Inhibition

Enzyme activity can be inhibited by other chemicals. In competitive inhibition, the active site is taken up by a competing molecule. Competitive inhibition accounts for the ability of various chemicals-including cyanide, to shut down enzyme activity and cause death. Other poisons, such as lead and mercury, have the same effect, but are non-competitive inhibitors, since they do not compete for occupation of the active site. Instead, they fix themselves onto the enzyme, thereby distorting its shape.

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