A1. Explain how enzymes act as catalysts in biochemical processes (e.g., fructose metabolism, glycolysis); A2. Explain how a deficiency in aldolase B can be responsible for hereditary fructose intolerance; A4. Discuss the specific substrate acted on by aldolase B during the breakdown of fructose; A5. Explain the role of aldolase B in the breakdown of fructose by discussing the products of the reaction.
Enzymes are proteins that work as catalysts in reactions. A catalyst is something that is neither created nor destroyed during a reaction. Instead, an enzyme is shaped such that when the substrate reacts with the enzyme the substrate is put into a form that reduces the amount of energy needed for the reaction to occur. For example, if a bond is to be broken within a reaction, the enzyme-substrate complex (Figure 1, Wolfe, 2000b) puts the bond into a form that requires less energy for the bond to be broken than without the enzyme (Figure 2, Wolfe, 2000a). This is called reducing the activation energy of the reaction. After the reaction occurs, the enzyme is unchanged. It can then create another complex with another substrate molecule so catalyze a further reaction.
Hereditary fructose intolerance is caused by inheriting a mutated gene for the aldolase B protein (Hereditary fructose intolerance, 2011). Aldolase B (enzyme) is a protein that, through its shape, reduces the activation energy needed to catalyze the breakdown of fructose-1-phosphate (A-B) into two products, glyceraldehyde (A) and dihydroxyacetone phosphate (B). The reaction catalyzed by aldolase B is the second step in the breakdown of fructose, a carbohydrate, to make energy, a group of reactions called fructose metabolism (Hereditary fructose intolerance, 2011). Because the mutated enzyme has an altered shape, it does not function well so the activation energy needed for the reaction remains high, and this causes this second step of fructose metabolism to not occur (Hereditary fructose intolerance, 2011).
Two resulting issues cause the symptoms of hereditary fructose intolerance. First, there is an accumulation of the substrate fructose-1-phosphate that is toxic to the body’s cells. This is bad for the cells, particularly over time. Second, the lack of one of the reaction products, specifically dihydroxyacetone phosphate, reduces the energy reserve available to the cells as the phosphate from this product is used in later metabolism steps, specifically oxidative phosphorylation to store energy by converting adenosine diphosphate (ADP) into adenosine triphosphate (ATP). Because of the lack of available phosphate, less energy is stored so less energy is available to the cells to perform cellular functions (Hereditary fructose intolerance, 2011). The damage of hereditary fructose intolerance caused by these two problems is seen physically primarily in organ damage to the liver and kidney. Symptoms of liver damage include yellowing of the skin and whites of the eyes, irreversible liver damage, and an enlarged liver (Hereditary fructose intolerance, 2011). It is so serious that continued exposure to fructose can cause “seizures, coma, and ultimately death” from the combined organ damage (Hereditary fructose intolerance, 2011).
B1. Explain what would hypothetically happen to the amount of ATP available to a cell if the entire Cori cycle occurred and remained within that single cell (i.e., a muscle cell).
The Cori cycle describes a pathway in carbohydrate metabolism that the body uses to utilize a product of anaerobic (without oxygen) metabolism produced by the muscles, called lactate, to make more energy (Boyer, 2002). Anaerobic glycolysis within the muscle produces two ATP molecules and two molecules of lactate per molecule of glucose. The lactate can be used by cells to make more energy, but it requires rebuilding the molecule back into glucose, a process called gluconeogenesis. Gluconeogenesis requires six ATP molecules for energy. Thus, if this cycle occurred in one cell, it would be what is called a “futile cycle” because there would be a net loss of four ATP per molecule of glucose used (Boyer, 2002). This would not be an effective way of producing energy for the cells of the body. But by separating the different stages of the Cori cycle into cells that do not have ready access to oxygen (the muscles during intense exercise) and cells that do have ready access to oxygen (the liver during recovery), it is a process that is useful to the body overall to keep ATP and glycogen stores at levels needed within the muscles when contraction is necessary during exercise (Boyer, 2002).
B2. Create an original dynamic diagram to show why the citric acid cycle is central to aerobic metabolism and how it leads to ATP production.
Figure 3 (Ahern and Rajagoapal, 2012, p. 142) illustrates why the citric acid cycle is central to aerobic metabolism and shows at each point within the cycle where reduced electron carriers (NADH, GTP, and FADH2) is made. Sugars, amino acids, and fats can all feed into the citric acid cycle. The important aspect of the citric acid cycle in relation to the production of ATP is that it produces reduced electron carriers. Reduced electron carriers are ultimately used to produce ATP through their donation of electrons to the respiratory chain (also known as the electron transport chain). The energy released by the electron transport from one complex to another within the electron transport chain is used to create a proton gradient across the mitochondria inner membrane that the ATP synthase uses to drive the reaction of ADP and phosphate to produce ATP.
B3. Explain where in the citric acid cycle a hypothetical defect of an enzyme could occur that would decrease the overall ATP production, including the consequences of the defect.
If the citric acid cycle had a defect in any one of its enzymes at any step, it would mean that less ATP is produced because each step must occur at approximately the same amount to be an effective cycle and produce reduced electron carriers (Citric Acid Cycle, n.d.). This is the result of the inter-relationship between the substrates and products of each step in the inter-connected reactions within the cycle. A defect in an enzyme in the cycle would cause the following things to occur: increase in the substrate of the reaction that is being blocked and a decrease in all the reaction products, both side products and products used directly in the cycle such as the substrates that follow the blocked reaction within the cycle. Importantly for ATP production, this includes the reduced electron carriers. Because it is a cycle, eventually there would be less substrate needed to start the cycle as well, and this would continue to depress the cycle’s electron carrier production. This would also impact the later part of metabolism, the electron transport chain, as the produced reduced electron carriers are used to make ATP, so ATP production would be much reduced (Citric Acid Cycle, n.d.). This is how a defect in the citric acid cycle would decrease ATP production even though ATP is not directly made in the citric acid cycle itself. If the enzyme defect was serious enough, eventually it would shut metabolism down completely, no energy would be produced, and the cell would die.
B4. Explain the role of coenzyme Q10 in ATP synthesis as part of the electron transport chain.
Coenzyme Q10 is a molecule that is located in the inner membrane of mitrocondria that accepts electrons from Complex I and Complex II of the electron transport chain (Electron Transport Chain, n.d.). Complex I receives the electrons from NADH while Complex II receives electrons from FADH2, so all the electrons that enter the electron transport chain eventually pass through coenzyme Q10. Coenzyme Q10 transfers the electrons to Complex III. Each time electrons are transferred to the various complexes within the electron transport chain, the energy is used to pump protons from the matrix space of the mitochondria to the intermembrane space of the mitocondria. The energy stored within the proton gradient (high proton concentration in the intermembrane space/low proton concentration in matrix) is used to produce ATP using ATP synthase. This final step is known as oxidative phosphorylation. As electron donation is necessary for producing the proton gradient, and the proton gradient is necessary for making ATP, and making ATP is the ultimate goal of metabolism, coenzyme Q10 is an important component of the electron transport chain (respiratory chain) stage of the metabolic process (Electron Transport Chain, n.d.).
Ahern, K. and Rajagopal, I. (2012). Biochemistry: Free & Easy. Retrieved from
Boyer, R. (2002). Interactive concepts in biochemistry. Interactive animations. Retrieved from http://www.wiley.com/college/boyer/0470003790/animations/cori_cycle/cori_cycle.htm
Citric Acid Cycle (n.d.). Aerobic Respiration. [video]. Retrieved from
Electron Transport Chain (n.d.). The Electron Transport Chain and Oxidative Phosphorylation. [video]. Retrieved from
Hereditary fructose intolerance (2011). Genetics Home Reference. U.S. National Library of Medicine. Retrieved from
Wolfe, G. (2000a). Activation Energy. Thinkwell Biochemistry. Retrieved from
Wolfe, G. (2000b). Enzyme Action. Thinkwell Biochemistry. Retrieved from
Figure 1. A3a. Provide a clearly labeled diagram that demonstrates the following: a. The lock and key model or induced fit model of enzymatic activity
Figure 2. A3b. Provide a clearly labeled diagram that demonstrates the following: The effect of enzymes on activation energy
Figure 3. B2. Create an original dynamic diagram to show why the citric acid cycle is central to aerobic metabolism and how it leads to ATP production.