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Gluconeogenesis
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Need help preparing for the Bio/Bio Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Gluconeogenesis of Metabolic Pathways which is a key component of the biochemistry section of the MCAT. Watch this video to get all the mcat study tips you need to do well on this section of the exam! • Aside from cellular respiration, other metabolic pathways are essential to understand for the MCAT exam. One is gluconeogenesis. Gluconeogenesis is the synthesis of glucose from pyruvate and other related compounds. In many ways, gluconeogenesis is the opposite of glycolysis. Glycolysis takes a molecule of glucose and breaks it down into two pyruvate molecules, whereas gluconeogenesis takes two pyruvate molecules and makes glucose. In animals, the process of gluconeogenesis occurs primarily in the liver. Gluconeogenesis and glycolysis are not complete opposites of each other, however. • Notice that seven of the ten steps of glycolysis are reversed in gluconeogenesis. Reversing the process means that seven steps employ the same enzymes in both pathways. However, there are three steps in glycolysis that are irreversible because they are exergonic processes and cannot be reversed using the same enzymes. Those three steps require a separate set of enzymes to proceed. • Furthermore, gluconeogenesis is an exergonic process. There are six molecules with phosphate bonds that are broken. Hydrolyzing phosphate bonds releases energy. In this way, gluconeogenesis uses reaction coupling of ATP or GTP hydrolysis, and it becomes an irreversible exergonic process. • Irreversible Steps • There are three irreversible steps in gluconeogenesis. The first step is the conversion of pyruvate to phosphoenolpyruvate. This process requires two steps. In the first step, pyruvate is converted to oxaloacetate by pyruvate carboxylase, and this requires a molecule of ATP. This reaction is stimulated by increased levels of acetyl-CoA produced in the liver. It is also inhibited by increased ADP and glucose levels. • In the second step, oxaloacetate is decarboxylated and phosphorylated to form phosphoenolpyruvate. This reaction is catalyzed by the enzyme phosphoenolpyruvate carboxykinase. One GTP molecule is hydrolyzed to GDP during this reaction, and this step expends energy in the form of ATP and GTP. Also note that in reality, two pyruvate molecules go through this process, so the net energy expenditure in this step is two ATP and two GTP. • The next irreversible step is the conversion of fructose 1,6-bisphosphate to fructose-6-phosphate. This step is the reverse step of glycolysis. In this case, one of the phosphate groups from fructose 1,6-bisphosphate will be hydrolyzed to form fructose-6-phosphate. This reaction is achieved by the enzyme fructose 1,6-bisphosphatase, and it will use one water molecule and release one phosphate. Moreover, this is the rate-limiting step in gluconeogenesis. • The last irreversible step is the conversion of glucose-6-phosphate to glucose. Here, the phosphate group of glucose-6-phosphate is hydrolyzed to produce glucose and inorganic phosphate. Moreover, this reaction is catalyzed by glucose-6-phosphatase. This step occurs in the lumen of the endoplasmic reticulum, and glucose will be shuttled into the cytoplasm of the cell by glucose transporters located on the membrane of the endoplasmic reticulum. • MEDSCHOOLCOACH • To watch more MCAT video tutorials like this and have access to study scheduling, progress tracking, flashcard and question bank, download MCAT Prep by MedSchoolCoach • IOS Link: https://play.google.com/store/apps/de... • Apple Link: https://apps.apple.com/us/app/mcat-pr... • #medschoolcoach #MCATprep #MCATstudytools
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