Report on Case 1: Diagnosis of Hereditary Fructose Intolerance
Enzymes act as biological catalysts, they speed up biological reactions and are also capable of regulation. Enzymatic reactions can be anabolic – where a new substance is built from smaller ones; or catabolic – where a substance is broken down to yield simpler molecules. In the example of fructose, enzymes catabolize this simple sugar to substances that are intermediates of the glycolytic pathway, allowing fructose, a 6-carbon sugar, to be used by cells to obtain energy in a way similar to glucose (also a 6-varbon sugar). Aldolase B is one of the enzymes involved in the catabolism of fructose, and its deficiency results in an inability of the body to utilize fructose. The buildup of ingested fructose creates metabolic complications, leading to hereditary fructose intolerance (Bender and Mayes, 2011).
The diagram above shows how the enzyme and its substrate molecule have exactly complementary shapes, allowing the substrate, and no other substance, to fit into the enzyme’s active site. The enzyme active site acts as a ‘lock’ and the substrate, with the perfectly complementary shape, is like the key. During the enzymatic reaction, the substrate enters the active site, the enzyme-substrate complex is formed, the reaction takes place, and the products formed no longer fit the active site, and therefore are released from the active site as reaction products. A further demonstration of how this ‘lock and key’ model of the enzyme/substrate complex undergoes the enzymatic activity reaction is shown below:
When an enzyme catalyzes a reaction, it essentially reduces the amount of energy that was required for the reaction to occur spontaneously. This is because the enzyme brings the substrate/s into a structural and chemical position that favors the reaction. The diagram below (Kaiser, 2002) shows how the amount of energy of activation is reduced in the presence of the enzyme.
With regards to fructose, the first step in fructose metabolism is conversion of fructose to fructose 1-phosphate by fructokinase enzyme. This reaction has low activation energy, it occurs easily whenever plenty of fructose is present in the cells. Fructose 1-phosphate is fructose with a phosphate group attached to its first carbon atom. It is then split by aldolase B enzyme, found in the liver, to 2 smaller 3-carbon molecules: D-glyceraldehyde and dihydroxyacetone phosphate (DHAP). The energy of activation for this reaction is very high, therefore this reaction cannot occur in the body in the absence of aldolase B. Further, although there are other aldolases such as aldolase A in the body, they cannot use fructose 1-phosphate as the substrate due to significant differences in substrate shape. DHAP can then continue directly into glycolysis. D-Glyceraldehyde undergoes phosphorylation to glyceraldehyde 3-phosphate catalyzed by triokinase, or isomeric conversion to DHAP, to also enter glycolysis (Bender and Mayes, 2011).
A genetic deficiency in aldolase B leads to the accumulation of fructose 1-phosphate, its substrate, in the liver cells whenever fructose is ingested and brought to the liver. Fructose is abundant in fruits, its levels rise on taking fruit juice. As it builds up in liver cells, it sequesters all the phosphate supply of the cell, leaving little phosphate for other metabolic processes such as glycogenolysis and gluconeogenesis. Therefore, liver cells are unable to synthesize any glucose in the presence of fructose. This results in hereditary fructose intolerance: a condition where profound hypoglycemia and vomiting develop upon the consumption of fructose. It usually occurs in babies when they are first started on a diet of food containing fructose.
Report on Case 2: Mitochondrial Disease
The Cori cycle involves glucose metabolism during exercise. Rapidly working muscles start anaerobic metabolism to generate ATP from glycolysis, generating pyruvate. This pyruvate is converted to lactate in the muscle by the enzyme lactate dehydrogenase, in a reaction that requires NAHD (reduced NAD). Lactate is released into the blood, and transported to the liver. The liver takes up the lactate, uses energy during aerobic conditions to convert this lactate back to pyruvate and glucose, and in this way it eliminates the lactate from the blood. The glucose produced by the liver is released and transported in the blood to the muscle, which can take it up and store it as glycogen, for rapid anaerobic metabolism during the next burst of activity.
The Cori cycle also takes place between the red blood cells and liver cells. Red blood cells, which can carry out anaerobic respiration only, rely on glycolysis for producing ATP. Pyruvate produced in the red blood cell is converted into lactate, and released in the blood so that the liver can convert it back into glucose (Diwan, 2007)..
If the interconversions of the Cori cycle took place but the products remained within the cell, this would mean that the liver would accumulate the glucose and start converting it to fatty acids rather than releasing it into the blood, leading to hypoglycemia. Without glucose release into the blood, the muscle cells and red cells would rapidly deplete their glucose stores. If the lactate produced from the Cori cycle interconversions stayed inside the muscle cells, the cell’s ATP and reduced NAD would be rapidly used up, and no reduced NAD would be available to carry out aerobic respiration with oxidative phosphorylation. The red blood cell would not be able to make ATP in the absence of glucose release by the liver, resulting in depletion of ATP.
Acetyl CoA is the common end-product of metabolism of carbohydrates, fatty acids and amino acids, which enters the citric acid cycle. The citric acid cycle is essential to aerobic metabolism as it involves using acetyl CoA in a series of reactions that result in reducing molecules of NAD and FAD to NADH and FADH2, respectively. The electrons given to these molecules are high in energy, and the products - NADH and FADH2 - are then used in oxidative respiration to obtain the electrons’ energy and use it to synthesize ATP, which is the energy currency of the cell. Thus, without the citric acid cycle, the substrates for aerobic respiration cannot be made (Bender and Mayes, 2011). These reactions of the citric acid cycle are shown in the diagram below.
A hypothetical defect could occur in the citric acid cycle at the red arrow shown. This step is normally carried out by the enzyme isocitrate dehydrogenase that also is the first step in the cycle to reduce NAD to NADH. An interruption of the cycle at this step by a defect in isocitrate dehydrogenase would mean that the cycle would not be able to produce reduced NAD. Lack of NAD would mean that ATP from ADP cannot be made by oxidative phosphorylation. In addition, isocitrate dehydrogenase is also the major control regulator enzyme of the cycle. When there is an abundance of ADP in the cell, this molecule activates isocitrate dehydrogenase and increases its activity so that more NADH can be generated for oxidative phosphorylation. Conversely, when a large amount of ATP has been made, it inhibits isocitrate dehydrogenase and slows down the cycle to reduce further NADH production, until more ATP is needed. When this enzyme becomes defected, the cell can no longer detect the levels of ATP and ADP and is unable to regulate the cycle, resulting in inadequate amounts of NADH, FADH and ATP being made.
The process of oxidative phosphorylation uses the products of the citric acid cycle – NADH and FADH – to make ATP, utilizing oxygen and making carbon dioxide in the process. The process occurs on the inner mitochondrial membrane, which contains a series of enzymes collectively called the ‘electron transport chain’. These enzymes include Complex I (NADH dehydrogenase), Complex II (succinate dehydrogenase), Coenzyme Q10 (Ubiquinone), Complex III (Cytochrome b/c1), Complex IV (Cytochrome a/a3), and ATP synthase. NADH gives its electrons to complex I, which are shuttled down the chain to Complex IV. Each enzyme, as it transfers the electron, uses some of its energy to transfer a H+ ion into the inter-membrane mitochondrial space. This builds up an H+ gradient in the space. Complex IV gives the electrons to oxygen, synthesizing water.
ATP synthase, the last enzyme in the chain, allows H+ ions to flow down the diffusion gradient, using the electrochemical energy of the diffusion gradient to drive the reaction making ATP from ADP. Thus the electrochemical energy of the electrons, which came from the acetyl CoA entering the citric acid cycle, is transferred to ATP in oxidative phosphorylation via NADH and FADH2 as intermediate carriers.
Coenzyme Q10, also called ubiquinone, is a lipid molecule that acts as an electron carrier in the electron transport chain. It takes electrons from Complexes I and II and transfers them to Complex III, which cannot be transferred directly. It also transfers H+ ions across the membrane with the electron transfer, thus facilitating ATP synthesis. The importance of coenzyme Q10 in the electron transport chain is evident by its link to certain diseases. For example, its levels are found to be low in the mitochondria of patients with Parkinson’s disease. Also, its supplementation slows the neurologic decline of these patients (Rakel, 2012).
References
Bender D.A., Mayes P.A. (2011). Chapter 21. The Pentose Phosphate Pathway & Other Pathways of Hexose Metabolism. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds),Harper's Illustrated Biochemistry, 29e. Retrieved October 17, 2012 from http://www.accessmedicine.com/content.aspx?aID=55882947.
Bender D.A., Mayes P.A. (2011). Chapter 17. The Citric Acid Cycle: The Catabolism of Acetyl-CoA. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e. Retrieved October 17, 2012 from http://www.accessmedicine.com/content.aspx?aID=55882678.
Diwan J.J. (2007). Gluconeogenesis. Retrieved October 17, 2012 from http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/gluconeo.htm
Rakel D. (2010). Chapter 13. Parkinson Disease. In Rakel: Integrative Medicine, 3rd ed. Retrieved October 17, 2012 from ww.mdconsult.com
