2.6 – Enzymes
2.6.1 – Define enzyme and active site
Enzyme – A biological catalyst made of globular protein
Enzymes speed up the reactions by influencing the stability of bonds in the reactants. They may also provide an alternative reaction pathway, and reduce the energy needed for the reaction.
Active Site – The region of an enzyme molecule surface where the substrate molecule binds and catalysis occurs
The substrate is drawn in to the active site. It has both binding and catalytic regions. The molecules are positioned to promote the reaction.
2.6.2 – Explain enzyme-substrate specificity
A substrate is the starting substance, which is converted to the product.
Enzymes are very specific, and will only catalyse one type of reaction or a very small group of similar reactions. They recognize the substrate as the active site had a precise shape and
distinctive chemical properties. Hence, only particular substrate molecules will be attracted to the active site and fit there. Others cannot fit and will not bind.
Enzymes can have high specificity (when it will only bind to a single type of substrate) or low specificity (when it will bind to a range of related substances). When they bind, the enzyme substrate complex is formed. In the lock and key model, it is suggested that the enzyme and substrate possess specific, complementary shapes that fit exactly into each other.
2.6.3 – Explain the effects of temperature, pH and substrate concentration on enzyme
Temperature -Each enzyme has an optimal temperature for function. When at this temperature, the enzyme will work at its peak, speeding up the reaction. After the temperature reaches its optimum level, the reaction rate abruptly declines. Many enzymes are adversely affected by high temperatures, at which point denaturation occurs. Many enzymes only have a narrow range of conditions under which they operate properly. This is usually at low temperatures for plant and animal enzymes.
pH -Enzymes also have an optimal pH. At this point, it works best and the reaction occurs the fastest, as the enzyme is the most active. There is lower activity above and below the optimum pH (see graph). Extremes in pH will usually result in a complete loss of activity for most enzymes as it leads to a change in shape of the active site. The H+ ions interfere with hydrogen and ionic bonds within the protein structure, which means that the substrate cannot bind. The optimum pH for each enzyme varies greatly. For example, pepsin has an optimum pH of 1.5, but lipase has an optimum pH of 8.0.
Substrate Concentration – If the amount of the enzyme is kept constant and the substrate concentration is increased, the reaction velocity will increase until it hits its maximum. After that, the velocity plateaus. At this point, all of the enzymes have formed complexes with the substrates.
Enzyme Concentration – The rate of reaction, so long as there is excess substrate, will continue to increase as the concentration of the enzyme increases
2.6.4 – Define denaturation
Denaturation is a structural change in a protein that alters its shape and results in a loss of biological properties. This can be caused by pH or temperature. This is when the protein loses its three-dimensional structure, usually along with function. It is often permanent. The bonds in the secondary and tertiary structure are altered, although the sequence is unchanged.
This can result from strong acids and alkalis, which disrupt ionic bonds, resulting in coagulation. Long exposure will eventually break down the primary structure. Heavy metals also disrupt ionic bonds, and form bonds with the carboxyl groups of the R group, reducing the charge of the protein. This generally causes the protein to precipitate. Heat and radiation (such as UV rays) disrupt the bonds because of the increased energy provided to the atoms. Detergents and solvents form bonds with the non-polar groups in the protein, which disrupts hydrogen bonding.
2.6.5 – Explain the use of lactase in the production of lactose-free milk
The production of lactose-free milk is an example of industrial use of biotechnology, which is of huge and increasing economic importance. People who cannot digest lactose are lactose intolerant and do not produce lactase. They must instead drink lactose-free milk, which is made by using lactase from bacteria.
This used to be done through whole-cell preparations. This is not efficient, however, and inappropriate for a food like liquid milk. Cell-free preparation is also used, although the enzymes cannot be re-used, and removal can be expensive.
Instead, immobilized enzymes are used. The advantages of this method are:
- The enzyme preparation can be re-used
- The product received is enzyme-free
- The enzyme may be more stable and long lasting due to protection by the inert matrix
Today, lactose free milk is produced by passing milk over lactase enzyme, bound to an inert carrier. The enzyme is obtained from bacteria, purified, and enclosed in capsules. Once the molecule is cleaved, there are no lactose ill-effects. Alternatively, a harmless bacterium may be added (such as L. Acidophilus), which affects the lactose in milk and yoghurt.