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Disorders of carbohydrate metabolism: Pathology review
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A 5 day old newborn infant girl named Emily is brought to the emergency department due to vomiting, diarrhea, and poor feeding. Her mother also mentions that Emily seems to be tired and sleepy all day long. Physical examination reveals bilateral clouding of the lens, along with yellowing of the sclera. Upon palpation of the abdomen, Emily’s liver appears enlarged. Emily’s mother mentions that she lives in a remote area and gave birth at home. You decide to run a urine dipstick test, which comes back negative for sugars, followed by a nonspecific urine test, which shows increased levels of reducing sugars. Some days later, you see a 21 year old man of Asian descent named Chris, who’s complaining of repeated episodes of bloating, abdominal cramps, and excessive flatulence that are often associated with watery, frothy stool. He has noticed that his symptoms tend to occur when he eats cheese or ice cream. Upon further questioning, he denies any concomitant diseases or recent gastrointestinal infections. Physical examination is unremarkable. Okay, based on the history and initial presentation, both Emily and Chris seem to have some form of disorder of carbohydrate metabolism. Carbohydrates are our main source of energy, and can be classified as simple or complex. Simple carbohydrates include monosaccharides, which contain one sugar molecule, like glucose, fructose, and galactose; and disaccharides, where two sugar molecules are linked together. Disaccharides include lactose, which is made up of glucose and galactose, and sucrose, which is formed when glucose links up with fructose. On the other hand, complex carbohydrates include oligosaccharides and polysaccharides, which are respectively short and long chains made up of more than two sugar molecules. All right, now, for your exams, the most high yield disorders of carbohydrate metabolism include pyruvate dehydrogenase complex deficiency, galactosemia, disorders of fructose metabolism, and lactose intolerance. Let’s begin with pyruvate dehydrogenase complex deficiency or PDC deficiency. This is mainly caused by mutations in the PDHA1 gene, which is located on the X chromosome. So, PDC deficiency is an X-linked recessive disorder, which means that all carrier males develop the disease, because they only have one X chromosome and thus one PDHA1 gene available. On the other hand, females have two X chromosomes, so having a single mutation makes them a carrier, and two mutations are needed to have the disease. Now, the PDHA1 gene codes for one of the enzymes of the pyruvate dehydrogenase complex or PDC for short. Normally, after a meal, glucose is broken down into pyruvate in the cytoplasm through a process called glycolysis. Pyruvate can then enter the mitochondria, where a complex of three mitochondrial enzymes, called the pyruvate dehydrogenase complex, converts pyruvate into acetyl-CoA. Acetyl-CoA can then be used in the Krebs cycle, also known as the tricarboxylic acid or TCA cycle, to produce energy in the form of ATP. For your exams, remember that the pyruvate dehydrogenase complex requires a set of 5 cofactors to work properly. To help you remember these 5 cofactors, think of the mnemonic “The Lovely Coenzymes For Nerds”, which stands for thiamine pyrophosphate, which is a derivative of thiamine or vitamin B1; lipoic acid; CoA; which is a derivative of pantothenic acid or vitamin B5; FAD, which is a riboflavin or vitamin B2 derivative; and NAD+, which is a niacin or vitamin B3 derivative.
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