Enzymes
Most of the food we eat is complex carbohydrateFood belonging to the food group consisting of sugars, starch and cellulose. Carbohydrates are vital for energy in humans and are stored as fat if eaten in excess. In plants, carbohydrates are important for photosynthesis., proteinOrganic compound made up of amino acid molecules. One of the three main food groups, proteins are needed by the body for cell growth and repair. and lipidFat or oils, composed of fatty acids and glycerol.. These must be broken down to be absorbed into the body.
The chemical reactions required to break them down would be too slow without enzymeA protein which catalyses or speeds up a chemical reaction..
Enzymes are biological catalystA substance that changes the rate of a chemical reaction without being changed by the reaction itself. 鈥 they speed up chemical reactions.
Enzymes are required for most of the chemical reactions that occur in organismLiving entity, eg animals, plants or microorganisms.. These reactions occur in the breakdown of chemical molecules, which we see in the digestive systemOrgan system involved in breaking food down so that it can be absorbed into the bloodstream..
Enzymes are also involved in the building up of chemical molecules elsewhere in the body.
Enzymes are proteins that have a complex 3D shape. Each enzyme has a region called an active siteThe part of the enzyme to which a specific substrate can attach or fit on to..
The substrateA substance on which enzymes act. 鈥 the molecule or molecules taking part in the chemical reaction 鈥 fits into the active site. Once bound to the active site, the chemical reaction takes place .
In an organism, the active site of each enzyme is a different shape. It is a perfect match to the shape of the substrate molecule, or molecules. This is essential to the enzyme being able to work. One enzyme is therefore specific to one substrate's chemical reaction, or type of chemical reaction.
This theory for the way in which enzymes work is called the lock and key hypothesis Model which compares the specificity of enzymes with a key and its lock..
Factors affecting enzyme action
Physical factors affect enzyme activity.
Temperature
At low temperatures, the number of successful collisions between the enzyme and substrate is reduced because their molecular movementThe movement of molecules resulting from their kinetic energy. decreases. The reaction is slow.
The human body is maintained at 37掳C as this is the temperature at which the enzymes in our body work best. This is not true of the enzymes in all organisms.
Higher temperatures disrupt the shape of the active site, which will reduce its activity, or prevent it from working. The enzyme will have been denatureTo change the shape of an enzyme's active site, for example because of high temperatures or extremes of pH. Denatured enzymes no longer work..
Enzymes therefore work best at a particular temperature.
Enzyme action
Proteins are chains of amino acidThe building blocks that make up a protein molecule. joined end to end. This chain is not straight 鈥 it twists and folds as different amino acids in the chain are attracted to, or repel each other.
Each enzyme is made from proteins made of these twisting and folding amino acids, and therefore the enzyme has a unique shape. This structure is held together by weak forces between the amino acid molecules in the chain.
High temperatures will break these forces. The enzyme, including its active site, will change shape and the substrate will no longer fit. The rate of reaction will be affected, or the reaction will stop.
The effect of pH
Enzymes are also sensitive to pHScale of acidity or alkalinity. A pH (power of hydrogen) value below 7 is acidic, a pH value above 7 is alkaline.. Changing the pH of its surroundings will also change the shape of the active site of an enzyme.
Many amino acids in an enzyme molecule carry a chargeProperty of matter that causes a force when near another charge. Charge comes in two forms, positive and negative. For example, a negative charge causes a repulsive force on a neighbouring negative charge.. Within the enzyme molecule, positively and negatively charged amino acids will attract each other. This contributes to the folding of the enzyme molecule, its shape, and the shape of the active site.
Changing the pH will affect the charges on the amino acid molecules. Amino acids that attracted each other may no longer attract each other. Again, the shape of the enzyme, along with its active site, will change.
Extremes of pH also denature enzymes. The changes are usually permanent.
Enzymes work inside and outside cells, for instance in the digestive system where cell pH is kept at 7.0 to 7.4. Cellular enzymes will work best within this pH range.
Different parts of the digestive system produce different enzymes. These have different optimumThe best or most appropriate - for instance, the conditions under which an enzyme works best (eg temperature and pH). pHs.
The optimum pH in the stomach is produced by the secretion of hydrochloric acid.
The optimum pH in the duodenum is produced by the secretion of sodium hydrogencarbonate.
The following table gives examples of how some of the enzymes in the digestive system have different optimum pHs:
Enzyme | Optimum pH |
Salivary amylase | 6.8 |
Stomach protease (pepsin) | 1.5 - 2.0 |
Pancreatic protease (trypsin) | 7.5 - 8.0 |
Enzyme | Salivary amylase |
---|---|
Optimum pH | 6.8 |
Enzyme | Stomach protease (pepsin) |
---|---|
Optimum pH | 1.5 - 2.0 |
Enzyme | Pancreatic protease (trypsin) |
---|---|
Optimum pH | 7.5 - 8.0 |
Question
Suggest an enzyme that would produce a trend as shown in the graph above.
Pancreatic protease (trypsin).