Individual enzymes are named by adding ase to the name of the substrate with which they react. The enzyme that controls urea decomposition is called urease; those that control protein hydrolyses are known as proteinases. Some enzymes, such as the proteinases trypsin and pepsin, retain the names used before this nomenclature was adopted.
* They are all proteins
* They are specific in their action, one enzyme speeding up one reaction only.
* They work within only a narrow range of acidity or alkalinity.
* Heat alters enzymes so they work at different rates. At certain high temperatures enzymes denature.
* They can be used over and over again.
* They all require water before being able to function.
* Living cells are the only producers of enzymes.
* The enzyme molecule is only temporarily changed during its action and can be used repeatedly. Large quantities of enzymes are not necessary.
* They work within a narrow range of temperature.
Enzymes are typical catalysts: they are capable of increasing the rate of reaction without being consumed in the process.
Some enzymes, such as pepsin and trypsin, which bring about the digestion of meat, control many different reactions, whereas others, such as urease, are extremely specific and may accelerate only one reaction. Still others release energy to make the heart beat and the lungs expand and contract. Many facilitate the conversion of sugar and foods into the various substances the body requires for tissue-building, the replacement of blood cells, and the release of chemical energy to move muscles.
Pepsin, trypsin, and some other enzymes possess, in addition, the peculiar property known as autocatalysis, which permits them to cause their own formation from an inert precursor called zymogen. As a consequence, these enzymes may be reproduced in a test tube.
Source: Encarta 1998
Enzymes are extraordinarily efficient. Minute quantities of an enzyme can accomplish at low temperatures what would require violent reagents and high temperatures by ordinary chemical means. About 30 g of pure crystalline pepsin, for example, would be capable of digesting nearly 2 metric tons of egg white in a few hours.
The kinetics of enzyme reactions differ from those of simple inorganic reactions. Each enzyme is selectively specific for the substance in which it causes a reaction and is most effective at a temperature peculiar to it. Although an increase in temperature may accelerate a reaction, enzymes are unstable when heated. Many enzymes require the presence of another ion or a molecule called a cofactor, in order to function.
As a rule, enzymes do not attack living cells. As soon as a cell dies it is rapidly digested by enzymes that break down protein. The resistance of the living cell is due to the enzyme’s inability to pass through the membrane of the cell as long as the cell lives. When the cell dies, its membrane becomes permeable, and the enzyme can then enter the cell and destroy the protein within it. Some cells also contain enzyme inhibitors, known as antienzymes, which prevent the action of an enzyme upon a substrate
Enzyme Controlled Reactions
Enzymes are usually given a similar name to that of the substrate but ending in ‘ase’. The reaction has two arrows because the reaction is reversible. If there is more maltose than glucose, the reaction will go from left to right and the other way round.
How do enzymes work?
Molecules are constantly moving around and bumping into each other. When a substrate molecule bumps into a molecule of the correct enzyme, it fits into a depression on the surface of the enzyme molecule. The depression is called the active site. The reaction then takes place and the molecules of product leave the site, freeing it for another substrate molecule.
The active site of a particular molecule has a specific shape into which only one kind of substrate will fit. The substrate fits into the active site rather like a key to a lock. This is why enzymes are specific in their action. When an enzyme is altered by heat, the shape of the active site is changed so that the substrate no longer fits. A change in pH has a similar effect.
We carried out a preliminary test on a smaller scale of the final experiment. We put hydrogen peroxide and liver together and measured the time taken for 10cm3 of oxygen to be given off. Liver was used because it is a source of catalase that is the catalyst we needed to speed this reaction up. We used the same apparatus as this final experiment and placed both reactants into the water bath. Once the reactants had achieved the desired temperature we then mixed them together, while keeping them in the bath. We then started the timer and inserted the bung immediately after mixing the two reactants. Once 10cm3 of oxygen had been given off we stopped the timer and took a reading.