CIE IGCSE Biology: Enzymes

Fundamental Characteristics and Functions of Enzymes

Enzymes are specialized proteins that act as biological catalysts. The term "biological" refers to the fact that they are produced by and function within living cells, while "catalyst" refers to their ability to speed up the rate of a chemical reaction without being consumed or chemically altered by the process. Because they remain unchanged, a single enzyme molecule can be used repeatedly to facilitate the same reaction multiple times. These molecules are vital to every living organism because they maintain the speed of all metabolic reactions—defined as the totality of reactions that keep an organism alive—at a rate necessary to sustain life.

To illustrate the critical importance of enzymes, one can consider the process of human digestion. Without the presence of digestive enzymes, it would take approximately $2-3$ weeks to digest a single meal. In contrast, with the assistance of enzymes, this same process is completed in roughly $4$ hours.

The Mechanism of Enzyme Action and the Lock and Key Hypothesis

Enzymes exhibit a high degree of specificity, meaning each enzyme is specific to one particular substrate or a set of molecules that are either joined together or broken down during a reaction. This specificity is dictated by the enzyme's active site, which is the specific region where the substrate attaches. Enzymes are proteins and thus possess a complex, specific three-dimensional (3-D) shape. The active site has a shape that is exactly complementary to the shape of its specific substrate. This relationship is commonly explained through the "lock and key" hypothesis, where the enzyme acts as the lock and the substrate acts as the key that fits perfectly into it.

The process occurs in a series of steps. First, enzymes and substrates move about randomly in a solution. When a successful collision occurs between an enzyme and its complementary substrate, the substrate fits into the active site, forming what is known as an enzyme-substrate complex. At this stage, the chemical reaction takes place. Once the reaction is complete, the resulting product (or products) is released from the active site. Because the product no longer fits the shape of the active site, it is expelled, leaving the enzyme unchanged and free to take up another substrate molecule to repeat the process.

The Impact of Temperature on Enzyme Activity and Denaturation

Enzymes are held together by specific chemical bonds that maintain their 3-D structure. This shape is particularly crucial at the active site, as it ensures the substrate can fit to allow the reaction to proceed. Enzymes have an "optimum temperature" at which they work fastest; for enzymes in the human body, this is typically $37^\circ\text{C}$. As the temperature increases from $0^\circ\text{C}$ toward the optimum, the rate of reaction increases. This happens because the molecules gain more kinetic energy, move faster, and collide with substrate molecules more frequently.

However, if the temperature rises beyond the optimum point, the heat energy becomes sufficient to break the bonds holding the enzyme's protein structure together. When these bonds break, the enzyme loses its specific 3-D shape, a process known as denaturation. Once an enzyme is denatured, the substrate can no longer fit into the active site because the site's shape has been lost. Denaturation is irreversible; once the shape is gone, the enzyme cannot regain its activity, and the reaction stops. Conversely, very low temperatures do not denature enzymes; they simply cause the enzymes to work very slowly due to a lack of kinetic energy and fewer successful collisions.

The Influence of pH on Enzyme Function

Just as with temperature, enzymes have an optimum pH at which they function most efficiently. Most enzymes have an optimum pH of $7$, which is neutral. However, enzymes produced in specific environments are adapted to different conditions. For example, enzymes in the stomach operate best in acidic conditions with a low optimum of pH $2$. In contrast, enzymes in the duodenum (part of the small intestine) operate best in alkaline conditions with a higher optimum of pH $8$ or pH $9$.

If the pH moves too far away from the enzyme's optimum—either becoming too acidic or too alkaline—the bonds holding the amino acid chains together in the protein can be destroyed. This destruction changes the shape of the active site, preventing the substrate from fitting and thereby reducing or stopping the rate of activity. Extreme shifts in pH will result in the denaturation of the enzyme.

Experimental Investigations into Enzyme Activity

Practical investigations often utilize the enzyme amylase, which catalyzes the breakdown of starch (a polysaccharide of glucose) into maltose (a disaccharide of glucose). Starch can be easily detected using iodine solution, which turns from orange-brown to blue-black in the presence of starch.

To investigate the effect of temperature on amylase, starch solution is heated to a specific temperature and then mixed with amylase. Samples of the mixture are added to wells of a spotting tile containing iodine at one-minute intervals. The process continues until the iodine no longer turns blue-black, indicating all starch has been digested. The time taken is recorded, and the experiment is repeated at various temperatures to determine the relationship between temperature and reaction rate; a shorter completion time indicates a faster enzyme reaction.

A similar procedure is used to investigate the effect of pH. Amylase ($2\,\text{cm}^3$) is mixed with a buffer solution ($1\,\text{cm}^3$) of a specific pH. Then, $2\,\text{cm}^3$ of starch solution is added, and a stopwatch is started. Every $10$ seconds, a drop of the mixture is tested with iodine. The experiment is repeated using different buffer solutions to observe how quickly the iodine remains orange-brown across various pH levels.

Questions & Discussion

Question 1: Which of the following best describes what an enzyme is? Response: D. A protein that functions as a biological catalyst. (Options A and B were incorrect because they claimed enzymes are used up, and C was incorrect because it claimed they are carbohydrates).

Question 2: What is a feature of all catalysts? Response: B. They are not changed by the reaction. (Option A is incorrect because not all catalysts are proteins; only biological enzymes are. Options C and D are incorrect because catalysts are not broken down or altered).

Question 3: Which graph shows the correct effect of temperature on the activity of an enzyme? Response: The graph should show a gradual increase in activity as temperature rises toward the optimum, followed by a sharp decline toward zero activity as the enzyme denatures at high temperatures.

Question 4: Which represents the enzyme responsible for catalysing a reaction where specific shaped molecules are joined? Response: The correct representation is the structure with an active site that is a complementary match to the shape of the substrate molecules shown in the reaction diagram.

Question 5: What can be said to be true of all enzymes? Response: They are made from amino acids and they move about randomly in a fluid. It is false that all enzymes have an optimum pH of $7$, as some require acidic or alkaline environments. (Correct Answer: B).

Exam Tip: When answering questions about experimental results, for higher marks ($7$, $8$, or $9$), you must explain your observations. If the temperature is high, relate the drop in activity to changes in the active site shape due to denaturation. If the temperature is low, relate the slow activity to the reduced kinetic energy of the molecules.