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Enzyme Activity - Q&A Review
1. What are catalysts? Catalysts are substances that reduce the activation energy of a chemical reaction, facilitating it or making it energetically viable. The catalyst increases the speed of the chemical reaction. 2. What amount of catalyst is consumed in the reaction it catalyzes? Catalysts are not consumed in the reactions they catalyze. 3. Is there a difference between the initial and the final energy levels in catalyzed and non-catalyzed reactions? The catalysis does not alter the energetic state of reagents and products of a chemical reaction. Only the energy necessary for the reaction to occur, i.e., the activation energy, is altered. Enzyme Activity: activation energy graphic 4. What are enzymes? What is the importance of enzymes for living beings? Enzymes are proteins that are catalysts of chemical reactions. From Chemistry it is known that catalysts are non-consumable substances that reduce the activation energy necessary for a chemical reaction to occur. Enzymes are highly specific to the reactions they catalyze. They are of vital importance for life because most chemical reactions of the cells and tissues are catalyzed by enzymes. Without enzymatic action those reactions would not occur or would not happen in the required speed for the biological processes in which they participate. 5. What is meant by substrates of enzymatic reactions? Substrates are reagent molecules upon which enzymes act. The enzyme has spatial binding sites for the attachment of its substrate. These sites are called activation centers of the enzyme. Substrates bind to theses centers forming the enzyme-substrate complex. Enzyme Activity: enzyme-substrate complex 6. What are the main theoretical models that try to explain the formation of the enzyme-substrate complex? There are two main models that explain the formation of the enzyme-substrate complex: the lock and key model and the induced fit model. In the lock and key model the enzyme has a region with specific spatial conformation for the binding of the substrate. In the induced fit model the binding of the substrate induces a change in the spatial configuration of the enzyme for the substrate to fit. Enzyme Activity: lock and key model induced fit model 7. How does the formation of the enzyme-substrate complex explain the reduction of the activation energy of chemical reactions? The enzyme possibly works as a test tube within which reagents meet to form products. With the facilitation of the meeting provided by enzymes it is easier for collisions between reagents to occur and thus the activation energy of the chemical reaction is reduced. This is one of the possible hypotheses. 8. On what structural level of the enzyme (primary, secondary, tertiary or quaternary) does the enzyme-substrate interaction depend? The substrate binds to the enzyme in the activation centers. These are specific three-dimensional sites and thus they depend on the protein tertiary and quaternary structures. The primary and secondary structures, however, condition the other structures and so they are equally important. 9. What is the activation center of an enzyme? Is it the key or the lock of the lock and key model? The activation center is a region of the enzyme produced by its spatial conformation to which the substrate binds. In the lock and key model the activation center is the lock and the substrate is the key. 10. Why can it be said that the enzymatic action is highly specific? The enzymatic action is highly specific because only specific substrates of one enzyme bind to the activation center of that enzyme. Each enzyme generally catalyzes only a specific chemical reaction. 11. What happens to a denatured enzyme regarding its functionality? How can that result be explained with the help of the lock and key model? According to the lock and key model the enzyme functionality depends entirely on the integrity of the activation center, a molecular region with specific spatial characteristics. After the denaturation the spatial conformation of the protein is modified, the activation center is destroyed and the enzyme loses its catalytic activity. 12. What are the main factors that alter the speed of enzymatic reactions? The main factors that change the speed of enzymatic reactions are temperature, pH and substrate concentration (quantity). 13. How does the substrate concentration affect the speed of enzymatic reactions? Initially as substrate concentration increases, the speed of the reaction increases; this happens because free activation centers of the enzyme bind to free substrates. Once all activation centers of the available enzymes become bound to their substrates new increments of the substrate concentration will have no effect on the speed of the reaction. 14. How does temperature affect the action of enzymes upon their substrates? There are defined temperature ranges under which enzymes operate and there is a specific temperature level (optimum temperature) in which enzymes have maximum efficiency. Therefore temperature variations affect enzymatic activity and the speed of the reactions they catalyze. In addition, as proteins, enzymes can be denatured under extreme temperatures. 15. Concerning enzymatic reactions, how different are the graphic curve of the variation of the speed of a reaction as function of substrate concentration and the curve of variation of the speed of a reaction as function of temperature? The curve of variation of speed of the enzymatic reaction as a function of growing substrate concentration is a growing curve until the point where it stabilizes due to the saturation of the activation centers of the enzymes. The curve of variation of speed of the enzymatic reaction as a function of growing temperature has a crescent portion and reaches a peak (the optimum temperature) then it decreases and reaches zero in the point of inactivity of the enzymes by denaturation. 16. How is the cooling of organs and tissues for medical transplants associated with the effect of temperature upon enzymatic reactions? The molecular degradation during the decomposition of organs and tissues is catalyzed by enzymes. The cooling to adequate temperatures of some organs and tissues destined for transplantation reduces that enzyme activity and thus lessens the natural decomposition process. By the same rationale, the cooling reduces the metabolic work of cells and prevents degradation of their own structures to obtain energy. Elevation of temperature later reverts denaturation of enzymes and the organs and tissues also preserved by other specific techniques may be grafted into the receptors. 17. Does pH affect the enzyme activity? The concentration of hydrogen ions in solution affects the enzyme activity. Each enzyme has maximal efficiency under an optimum pH. Since pH is one of the factors for the denaturation of proteins, if an enzyme is submitted to a pH level under which it is denatured there will be no enzymatic activity. 18. Do enzymes act better under acid or basic pH? Most enzymes act in pH between 6 and 8, a range that corresponds to the general acidic level of cells and blood. There are enzymes, however, that act only under very acid or very basic pH. So enzyme activity depends on pH interval. In the stomach, for example, the gastric juice has a very low pH, around 2, and there the enzyme pepsin acts to intensively digest proteins. In the duodenum, pancreatic secretions increase the pH of the enteric juice for the action of other digestive enzymes, for example, trypsin. 19. Since pepsin is a gastric enzyme does it have an acid or basic optimum pH? What happens to pepsin when it passes into the duodenum? Pepsin acts within the stomach so its optimum pH is around 2, an acid pH. When the enzyme passes into the duodenum it meets a higher pH and its enzyme activity ends. 20. What are enzyme cofactors? Some enzymes need other associated molecules to work. These molecules are called enzyme cofactors and they can be, for example, organic ions like mineral salts, or organic molecules. Inactive enzymes which are not bound to their cofactors are called apoenzymes. Active enzymes bound to their cofactors are called holoenzymes. 21. What is the relationship between vitamins and enzyme cofactors? Many vitamins are enzyme cofactors that cannot be synthesized by the organism and must be obtained from the diet. 22. For the enzymatic reaction what is the effect of a substance with the same spatial conformation as an enzymatic substrate? How is this type of substance known? Substances that “simulate” substrates can bind to the activation center of enzymes thus blocking the true substrates to bind to these enzymes and paralyzing the enzymatic reaction. Such “fake substrates” are called enzyme inhibitors. The binding of enzyme inhibitors to enzymes can be reversible or irreversible. Many medical drugs, for example, some antibiotics, antivirals, antineoplastics, antihypertensives and even sildenafil (trade name Viagra), are enzyme inhibitors that block enzyme activity. 23. What is the action mechanism of the antibiotic penicillin? Penicillin, discovered by the Scottish doctor Alexander Fleming in 1928, is a drug that inhibits enzymes necessary for the synthesis of peptidoglycans, a constituent of the bacterial cell wall. With the inhibition the bacterial population stops growing because there is no new cell wall formation. Fleming won the Nobel prize in Medicine for the discovery of penicillin. 24. What is the action mechanism of the antiretroviral drugs called protease inhibitors which are used against __________ infection? Protease inhibitors are some of the antiretroviral drugs used to treat __________ infection. Protease is an enzyme necessary for the assembling of __________ after the synthesis of its proteins within the host cell. The protease inhibitor binds to the activation center of the enzyme blocking the formation of the enzyme-substrate complex and the enzyme activity thus impairing the viral replication. 25. What are allosteric enzymes? Allosteric enzymes are those that have more than one activation center and to which other substances, called allosteric regulators, bind. Allosteric regulators can be allosteric inhibitors or allosteric activators. The interaction between an allosteric enzyme and the allosteric inhibitor disallows the binding of the substrate to the enzyme. The interaction between the allosteric enzyme and the allosteric activator allows the binding of the substrate to the enzyme and sometimes increases the affinity of the enzyme for the substrate. This regulatory phenomenon of the enzyme activity is called allosterism. Enzyme Activity: allosteric enzymes 26. What are zymogens? Zymogens, or proenzymes, are enzymes secreted in inactive form. Under certain conditions a zymogen shifts to the active form of the enzyme. Zymogen secretions in general happen because the enzyme activity can harm the secretory tissue. For example, the pepsinogen secreted by the stomach becomes active under acid pH turning into the enzyme pepsin. Other well-known zymogens are trypsinogen and chymotrypsinogen, enzymes that are secreted by the exocrine pancreas and which become trypsin and chymotrypsin respectively. |