Main course page
Topic 11: Enzymes-Part II
TLO 1: Describes how an enzyme works.
Enzymes lower the activation energy of a chemical reaction by decreasing the “randomness” of the collisions between molecules. They also help bring the substrates together in the proper orientation so that the reaction can occur. The process includes:
- The substrates contact the active site on the surface of the enzyme molecule, forming a temporary intermediate compound called the enzyme–substrate complex. In this reaction the two substrate molecules are sucrose (a disaccharide) and water.
- The substrate molecules are transformed by the rearrangement of existing atoms, the breakdown of the substrate molecule, or the combination of several substrate molecules into the products of the reaction. Here the products are two monosaccharides: glucose and fructose.
- After the reaction is completed and the reaction products move away from the enzyme, the unchanged enzyme is free to attach to other substrate molecules.
Figure 1 Illustration showing enzyme action (source: Tortora Textbook)
TLO 2: Describe the factors regulating enzyme activity
As we have concluded before, there are several factors that can enhance or inhibit the enzyme action.
Effect of substrate concentration
For a given quantity of enzyme, the velocity of the reaction increases as the concentration of the substrate is increased. Increasing substrate concentration cause increase rate of reaction until the active site of enzymes are used.
Figure 2 The effect of substrate concentration on enzyme action (source: //www.geeksforgeeks.org)
Effect of Enzyme Concentration
The velocity of a reaction is directly proportional to the amount of enzyme present as long as the amount of substrate is not limiting. The substrate must be present at a concentration sufficient to ensure that all of the enzyme molecules have substrate bound to their active site.
Figure 3 The relationship between enzyme concentration and rate of reaction
Effect of pH (concentration of hydrogen ions)
Each enzyme has an optimum pH, i.e. a pH at which the enzyme activity is maximum. Below or above this pH, enzyme activity is decreased. The optimum pH differs from enzyme to enzyme, for example optimum pH for pepsin is 1.2 (acidic) while for trypsin is 8.0 (alkaline).
Figure 4 Effects of pH on enzyme action
Effects of temperature
Enzyme catalysed reactions show an increase in rate with increasing temperature only within a relatively small and low temperature range. Each enzyme shows the highest activity at a particular temperature called optimum temperature. The activity progressively declines both above and below this temperature. Increase in velocity is due to the increase in the kinetic energy. Further elevation of the temperature results in a decrease in reaction velocity due to denaturation of the enzyme protein.
Figure 5 Effects of temperature on enzyme action
Effect of end-products
Accumulation of products of the reaction causes the inhibition of enzyme activity for some enzymatic reactions, this form of control will limit the rate of formation of the product when the product is under used. In biological systems, however, the product is usually removed as it becomes a substrate for a succeeding enzyme in a metabolic pathway.
Effect of activators and co-enzymes
The activity of many enzymes is dependent on the activators (metallic ions) like Mg2+, Mn2+, Zn2+, Ca2+, Co2+, Cu2+, etc. and coenzymes for their optimum activity. In absence of these activators and coenzymes, enzymes become functionally inactive.
Effect of time
Under optimum conditions of pH and temperature, time required for an enzyme reaction is less. The time required for the completion of an enzyme reaction increases with changes in temperature and pH from its optimum.
Effect of physical agent
Physical agent like light rays can inhibit or accelerate certain enzyme reactions. For example, the activity of salivary amylase is increased by red and blue light. On the other hand, it is inhibited by ultraviolet rays.
Effect of inhibitors
The substances which stop the enzymatic reaction are called inhibitors. Presence of these substances in reaction medium decreases the rate of enzyme reaction. Any substance that can diminish the velocity of an enzyme catalysed reaction is called inhibitor. Two general classes of inhibitors are recognized according to whether the inhibitor action is reversible or irreversible:
- Reversible inhibitor
- Irreversible inhibitor
Figure 6 Classification of enzyme inhibitors
Reversible inhibitor
Reversible inhibitors bind to enzymes through noncovalent bonds and the activity of the enzyme is restored fully when the inhibitor is removed from the system.
Figure 7 Mechanism of enzyme inhibitors
Different types of reversible inhibitors are:
- Competitive inhibitor
A competitive inhibitor is usually a structurally similar of the substrate. But because it is not identical with the substrate, breakdown into products does not take place. When both the substrate and this type of inhibitor are present, they compete for the same binding site on the enzyme.One of the examples of drug that inhibit enzymes as their mechanism of action is sulphonamide. Sulphonamide is an analogue of P-aminobenzoic acid (PABA) and inhibits pteroid synthetase enzyme required for the synthesis of folic acid in microorganisms. Many drugs which act as competitive inhibitors are given below:
2. Non-competitive inhibitorAs the name implies, in this type of inhibition no competition occurs between substrate and inhibitor. Inhibitor is usually structurally different from the substrate. It binds at a site on the enzyme molecule other than the substrate-binding site and thus there is no competition between inhibitor and substrate. Examples of non-competitive inhibitors are:
- Ethanol or certain narcotic drugs are non-competitive inhibitor of acid phosphatase.
- Trypsin inhibitors occur in soybean and raw egg white, inhibit activity of trypsin noncompetitively.
- As well as Ascaris parasites (worm) contain pepsin and trypsin inhibitors, which inhibit noncompetitively action of pepsin and trypsin, that is why ascaris worm is not digested in human intestine.
Irreversible inhibitors
An irreversible inhibitor binds with an enzyme tightly covalently and forms a stable complex. An irreversible inhibitor cannot be released by dilution or dialysis or simply by increasing the concentration of substrate. Irreversible inhibitors can be further subdividing into three:
- Group Specific Inhibitor
These inhibitors react with specific R-groups (side chain) of amino acid residues in the active site of enzyme. Examples of medication that works through this mechanism is:
Di-isopropylphosphofluoride (DIPF): DIPF can inhibit an enzyme acetylcholine esterase by covalently reacting with hydroxyl group of a serine residue present at the active site of the enzyme. DIPF is a parasympathomimetic drug, irreversibleanti-cholinesterase inhibitors and has been used locally in the oily eye drops in glaucoma treatment.
2. Substrates Analogue Irreversible Inhibitor
Substrate analogues or affinity labels are molecules that are structurally similar to the substrate. These substrate analogues possess a highly reactive group which is not present in the natural substrate. The reactive group of substrate analogues covalently reacts with amino acid residues of the active site of the enzyme and permanently block the active site of the enzyme.
3. Suicide Inhibitor
These compounds are relatively unreactive until they bind to the active site of a specific enzyme. On binding to the active site of the enzyme they carry out the first few catalytic activities of the normal enzyme reaction. Instead of being transformed into a normal product, however, the inhibitor is converted to a very reactive compound that combines irreversibly with the enzyme leading to its irreversible inhibition. The enzyme literally commits suicide. Some drugs that works through this mechanism are:
Penicillin: Penicillin irreversibly inactivates an essential bacterial enzyme glycopeptidyl transpeptidase involved in the formation of bacterial cell wall.
Aspirin: Aspirin inactivates an enzyme cyclo-oxygenase which catalyzes the first reaction in the biosynthesis of prostaglandins from arachidonic acid.