Why Can't Fat Produce Glucose?
Tousief Irshad Ahmed Sirwal Author has 77 answers and 106.2k answer views Acetyl CoA is NOT a substrate for gluconeogenesis in animals 1. Pyruvate dehydrogenase reaction is irreversible. So, acetyl CoA cannot be converted back to pyruvate. 2. 2C Acetyl CoA enters the TCA cycle by condensing with 4C oxaloacetate. 2 molecules of CO2 are released & the oxaloacetate is regenerated. There is no NET production of oxaloacetate. Animals cannot convert fat into glucose with minimal exceptions 1. Propionyl CoA derived from odd chain fatty acids are converted to Succinyl CoA Glucogenic 2. Glycerol derived from triglycerides are glucogenic. Answered Mar 26, 2017 Author has 942 answers and 259.1k answer views Yijia Xiong pointed out that the glycerol portion of triglycerides (fats) can indeed be converted to glucose. It is not so energy-inefficient that it is avoided by our bodies. If nutritionally, we are in a gluconeogenesis mode (building up glucose stores rather than consuming them), glycerol would be a perfectly acceptable precursor. However, I think the original question had more to do with the vast bulk of the triglycerides that are not glycerol, but are fatty acids. And it is true that we cant produce glucose from fatty acids. The reason is that the catabolic reactions of fatty acids break off two carbon atoms at a time as Acetyl-CoA. But our metabolic suite of pathways has no way to convert a two-carbon fragment to glucose. The end product of glycolysis is pyruvate, a three-carbon compound. Pyruvate can be back-synthesized into glucose. But the committing reaction for the Krebs cycle is the pyruvate dehydrogenase step, forming acetyl-CoA. That reaction is not reversible. Once pyruvate loses a carbon atom, it cant go back. The three main macronutrients are carbohydrates, pr Continue reading >>
Can Amino Acids Be Used By The Body To Make Glucose & Fatty Acids?
Amino acids are nitrogen-containing molecules that are the building blocks of all proteins in food and in the body. They can be used as energy, yielding about 4 calories per gram, but their primary purpose is the synthesis and maintenance of body proteins including, but not limited to, muscle mass. Video of the Day During normal protein metabolism, a certain number of amino acids are pushed aside each day. When these amino acids are disproportionate to other amino acids for the synthesis of new protein, your liver and kidneys dispose of the nitrogen as urea, and the rest of the molecule is used as energy in a variety of ways. Then certain amino acids -- minus their nitrogen -- can enter the citric acid cycle -- the biochemical pathway that converts food into energy. Others can be converted to glucose or fat. This process may be enhanced when you take in more protein than you need. Your body relies on a continuous supply of glucose and fatty acids for energy for physical activity and cellular needs during rest. When you exercise, your body relies still more on glucose because fat is slower to metabolize. The higher your exercise intensity is, the more your body requires quicker-burning glucose. Some glucose is stored as glycogen in the liver and muscles and can be recruited when blood glucose is used up. When glycogen becomes depleted, the process of gluconeogenesis can take over -- the creation of new glucose from another source. The usual source for gluconeogenesis is amino acids. Healthy people store adequate body fat to cover their energy needs. Although certain amino acids can be converted to fatty acids, there should be no need for this to occur in order to supply energy. But if a very high protein intake adds substantially more calories, theoretically those extra Continue reading >>
Connections Of Carbohydrate, Protein, And Lipid Metabolic Pathways
Connecting Other Sugars to Glucose Metabolism Sugars, such as galactose, fructose, and glycogen, are catabolized into new products in order to enter the glycolytic pathway. Learning Objectives Identify the types of sugars involved in glucose metabolism Key Takeaways When blood sugar levels drop, glycogen is broken down into glucose -1-phosphate, which is then converted to glucose-6-phosphate and enters glycolysis for ATP production. In the liver, galactose is converted to glucose-6-phosphate in order to enter the glycolytic pathway. Fructose is converted into glycogen in the liver and then follows the same pathway as glycogen to enter glycolysis. Sucrose is broken down into glucose and fructose; glucose enters the pathway directly while fructose is converted to glycogen. disaccharide: A sugar, such as sucrose, maltose, or lactose, consisting of two monosaccharides combined together. glycogen: A polysaccharide that is the main form of carbohydrate storage in animals; converted to glucose as needed. monosaccharide: A simple sugar such as glucose, fructose, or deoxyribose that has a single ring. You have learned about the catabolism of glucose, which provides energy to living cells. But living things consume more than glucose for food. How does a turkey sandwich end up as ATP in your cells? This happens because all of the catabolic pathways for carbohydrates, proteins, and lipids eventually connect into glycolysis and the citric acid cycle pathways. Metabolic pathways should be thought of as porous; that is, substances enter from other pathways, and intermediates leave for other pathways. These pathways are not closed systems. Many of the substrates, intermediates, and products in a particular pathway are reactants in other pathways. Like sugars and amino acids, the catabo Continue reading >>
Evolving Health: Why Can't We Convert Fat To Glucose?
As evident by many sugar-laden soda pop "potbellies" of North America, lipogenesis can obviously occur from drinking and eating too much sugar (1). Wouldnt it be just grand to reverse the process and be able to lose all that fat via gluconeogenesis? Unfortunately mammals do not have the ability to synthesize glucose from fats (1). The fact is that once glucose is converted to acetyl coA there is no method of getting back to glucose. The pyruvate dehydrogenase reaction that converts pyruvate to acetyl CoA is not reversible (1p252). Because lipid metabolism produces acetyl CoA via beta-oxidation, there can be no conversion to pyruvate or oxaloacetate that may have been used for gluconeogenesis (1p252). Further, the two carbons in the acetyl CoA molecule are lost upon entering the citric acid cycle (1p252). Thus, the acetyl CoA is used for energy (1p252). There are some fatty acids that have an odd number of carbon atoms that can be converted to glucose, but these are not common in the diet (1p253). Maybe they should be made more common. Do they taste good? 1. Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism. Belmont, CA: Thomson Wadsworth, 2009. Continue reading >>
Leaf: How Do Cells Obtain Energy From Molecules Other Than Glucose?
The immediate source of energy for most cells is the sugar glucose. However, cells can use other molecules for energy too. In fact, under certain conditions, some cells actually prefer other molecules for energy. They can use: Other carbohydrates (both simple and complex) Fats and fatty acids Proteins and amino acids Fortunately, cells do not need to maintain separate metabolic systems for each of these molecules. Instead, cells can convert them into one of the intermediate compounds that participate in glycolysis or the citrate cycle. After that, the molecule can be broken down and its energy extracted just like glucose. When cells use fats as fuel, the fat molecules must be broken down first. From the Leaf on lipids, you should know that fats (also called triglycerides) have 3 fatty acid tails, and a glycerol backbone holding them together. To burn fats: The 3 fatty acid tails are removed from the glycerol. The glycerol portion of the molecule is converted into PGAL, one of the intermediate molecules used in glycolysis. After conversion, glycerol molecules can be broken down to produce ATP just like PGAL molecules obtained from glucose. Each fatty acid molecule is converted into several molecules of acetyl-CoA The molecules of acetyl-CoA enter the citrate cycle in mitochondria, where their energy is extracted to make ATP. Like glycerol converted to PGAL, cells do not distinguish between acetyl-CoA coming from glucose and acetyl-CoA coming from fats. Overview of how fats and proteins are broken down via glycolysis and the Krebs cycle. The molecules that are intermediates in the main pathway from glucose to acetyl-CoA then the Krebs cycle are marked in gold. Fats are broken into component molecules (blue boxes) then sent into the pathway. Proteins are broken down to ami Continue reading >>
Metabolism: Transformations and Interactions Chemical Reactions in the Body Plants use the sunâ€™s energy to make carbohydrate from carbon dioxide and water. This is called photosynthesis. Humans and animals eat the plants and use the carbohydrate as fuel for their bodies. During digestion, the energy-yielding nutrients are broken down to monosaccharides, fatty acids, glycerol, and amino acids. After absorption, enzymes and coenzymes can build more complex compounds. In metabolism they are broken down further into energy (ATP), water and carbon dioxide. Chemical Reactions in the Body Metabolic reactions take place inside of cells, especially liver cells. Anabolism is the building up of body compounds and requires energy. Catabolism is the breakdown of body compounds and releases energy. Chemical Reactions in the Body Enzymes and coenzymes are helpers in reactions. Enzymes are protein catalysts that cause chemical reactions. Coenzymes are organic molecules that function as enzyme helpers. Cofactors are organic or inorganic substances that facilitate enzyme action. Breaking Down Nutrients for Energy The breakdown of glucose to energy starts with glycolysis to pyruvate. Pyruvate may be converted to lactic acid anaerobically (without oxygen) and acetyl CoA aerobically (with oxygen). Eventually, all energy-yielding nutrients enter the TCA cycle or tricarboxylic acid cycle (or Krebâ€™s cycle) and the electron transport chain. Breaking Down Nutrients for Energy Glucose Glucose-to-pyruvate is called glycolysis or glucose splitting. Pyruvateâ€™s Options Anaerobic â€“ lactic acid Aerobic â€“ acetyl CoA Pyruvate-to-Lactate Oxygen is not available or cells lack sufficient mitochondria Lactate is formed when hydrogen is added to pyruvate. Liver cells recycle Continue reading >>
Connections Between Cellular Respiration And Other Pathways
So far, we’ve spent a lot of time describing the pathways used to break down glucose. When you sit down for lunch, you might have a turkey sandwich, a veggie burger, or a salad, but you’re probably not going to dig in to a bowl of pure glucose. How, then, are the other components of food – such as proteins, lipids, and non-glucose carbohydrates – broken down to generate ATP? As it turns out, the cellular respiration pathways we’ve already seen are central to the extraction of energy from all these different molecules. Amino acids, lipids, and other carbohydrates can be converted to various intermediates of glycolysis and the citric acid cycle, allowing them to slip into the cellular respiration pathway through a multitude of side doors. Once these molecules enter the pathway, it makes no difference where they came from: they’ll simply go through the remaining steps, yielding NADH, FADH, and ATP. Simplified image of cellular respiration pathways, showing the different stages at which various types of molecules can enter. Glycolysis: Sugars, glycerol from fats, and some types of amino acids can enter cellular respiration during glycolysis. Pyruvate oxidation: Some types of amino acids can enter as pyruvate. Citric acid cycle: Fatty acids from fats and certain types of amino acids can enter as acetyl CoA, and other types of amino acids can enter as citric acid cycle intermediates. In addition, not every molecule that enters cellular respiration will complete the entire pathway. Just as various types of molecules can feed into cellular respiration through different intermediates, so intermediates of glycolysis and the citric acid cycle may be removed at various stages and used to make other molecules. For instance, many intermediates of glycolysis and the cit Continue reading >>
Why Can Fatty Acids Not Be Converted Into Glucose? : Mcat
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on on Fats (or triglycerides) within the body are ingested as food or synthesized by adipocytes or hepatocytes from carbohydrate precursors ([link]). Lipid metabolism entails the oxidation of fatty acids to either generate energy or synthesize new lipids from smaller constituent molecules. Lipid metabolism is associated with carbohydrate metabolism, as products of glucose (such as acetyl CoA) can be converted into lipids. Lipid metabolism begins in the intestine where ingested triglycerides are broken down into smaller chain fatty acids and subsequently into monoglyceride molecules (see [link]b) by pancreatic lipases, enzymes that break down fats after they are emulsified by bile salts. When food reaches the small intestine in the form of chyme, a digestive hormone called cholecystokinin (CCK) is released by intestinal cells in the intestinal mucosa. CCK stimulates the release of pancreatic lipase from the pancreas and stimulates the contraction of the gallbladder to release stored bile salts into the intestine. CCK also travels to the brain, where it can act as a hunger suppressant. Together, the pancreatic lipases and bile salts break down triglycerides into free fatty acids. These fatty acids can be transported across the intestinal membrane. However, once they cross the membrane, they are recombined to again form triglyceride molecules. Within the intestinal cells, these triglycerides are packaged along with cholesterol molecules in phospholipid vesicles called chylomicrons ([link]). The chylomicrons enable fats and cholesterol to move within the aqueous environment of your lymphatic and circulatory systems. Chylomicrons leave the enterocytes by exocytosis and enter the lymphatic system via lacteals in the villi of the intestine. From the lymphatic system, the chylo Continue reading >>
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Not to be confused with Glycogenesis or Glyceroneogenesis. Simplified Gluconeogenesis Pathway Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. From breakdown of proteins, these substrates include glucogenic amino acids (although not ketogenic amino acids); from breakdown of lipids (such as triglycerides), they include glycerol (although not fatty acids); and from other steps in metabolism they include pyruvate and lactate. Gluconeogenesis is one of several main mechanisms used by humans and many other animals to maintain blood glucose levels, avoiding low levels (hypoglycemia). Other means include the degradation of glycogen (glycogenolysis) and fatty acid catabolism. Gluconeogenesis is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis takes place mainly in the liver and, to a lesser extent, in the cortex of the kidneys. In ruminants, this tends to be a continuous process. In many other animals, the process occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise. The process is highly endergonic until it is coupled to the hydrolysis of ATP or GTP, effectively making the process exergonic. For example, the pathway leading from pyruvate to glucose-6-phosphate requires 4 molecules of ATP and 2 molecules of GTP to proceed spontaneously. Gluconeogenesis is often associated with ketosis. Gluconeogenesis is also a target of therapy for type 2 diabetes, such as the antidiabetic drug, metformin, which inhibits glucose formation and stimulates glucose uptake by cells. In ruminants, because dietary carbohydrates tend to be metabolized by rumen organisms, gluconeogenesis occurs Continue reading >>
We Really Can Make Glucose From Fatty Acids After All! O Textbook, How Thy Biochemistry Hast Deceived Me!
Biochemistry textbooks generally tell us that we can’t turn fatty acids into glucose. For example, on page 634 of the 2006 and 2008 editions of Biochemistry by Berg, Tymoczko, and Stryer, we find the following: Animals Cannot Convert Fatty Acids to Glucose It is important to note that animals are unable to effect the net synthesis of glucose from fatty acids. Specficially, acetyl CoA cannot be converted into pyruvate or oxaloacetate in animals. In fact this is so important that it should be written in italics and have its own bold heading! But it’s not quite right. Making glucose from fatty acids is low-paying work. It’s not the type of alchemy that would allow us to build imperial palaces out of sugar cubes or offer hourly sweet sacrifices upon the altar of the glorious god of glucose (God forbid!). But it can be done, and it’ll help pay the bills when times are tight. All Aboard the Acetyl CoA! When we’re running primarily on fatty acids, our livers break the bulk of these fatty acids down into two-carbon units called acetate. When acetate hangs out all by its lonesome like it does in a bottle of vinegar, it’s called acetic acid and it gives vinegar its characteristic smell. Our livers aren’t bottles of vinegar, however, and they do things a bit differently. They have a little shuttle called coenzyme A, or “CoA” for short, that carries acetate wherever it needs to go. When the acetate passenger is loaded onto the CoA shuttle, we refer to the whole shebang as acetyl CoA. As acetyl CoA moves its caboose along the biochemical railway, it eventually reaches a crossroads where it has to decide whether to enter the Land of Ketogenesis or traverse the TCA cycle. The Land of Ketogenesis is a quite magical place to which we’ll return in a few moments, but n Continue reading >>
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All Of The Information In These Notes
Cellular respiration is the enzymatic breakdown of glucose (C6H12O6) in the presence of oxygen (O2) to produce cellular energy (ATP): 1. Glycolysis: (Fig. 18-2) a ten-step process that occurs in the cytoplasm converts each molecule of glucose to two molecules of pyruvic acid (a 3-carbon molecule) an anaerobic process - proceeds whether or not O2 is present ; O2 is not required net yield of 2 ATP per glucose molecule net yield of 2 NADH per glucose (NADH is nicotine adenine dinucleotide, a co-enzyme that serves as a carrier for H+ ions liberated as glucose is oxidized.) The pyruvic acid diffuses into the inner compartment of the mitochondrion where a transition reaction (Fig. 18-3) occurs that serves to prepare pyruvic acid for entry into the next stage of respiration: (a) pyruvic acid ® acetic acid + CO2 (a waste product of cell metabolism) + NADH+ (b) acetic acid + co-enzyme A ® acetyl CoA 2. Citric Acid or TCA Cycle:(Fig. 18-3) occurs in the inner mitochondrial matrix the acetyl group detaches from the co-enzyme A and enters the reaction cycle an aerobic process; will proceed only in the presence of O2 net yield of 2 ATP per glucose molecule (per 2 acetyl CoA) net yield of 6 NADH and 2 FADH2 (FAD serves the same purpose as NAD) in this stage of cellular respiration, the oxidation of glucose to CO2 is completed 3. Electron Transport System: consists of a series of enzymes on the inner mitochondrial membrane electrons are released from NADH and from FADH2 and as they are passed along the series of enzymes, they give up energy which is used to fuel a process called chemiosmosis by which H+ ions are actively transported across the inner mitochondrial membrane into the outer mitochondrial compartment. The H+ ions then flow back through special pores in the membrane, a pr Continue reading >>
Glucose Can Be Synthesized From Noncarbohydrate Precursors - Biochemistry - Ncbi Bookshelf
Glucose is formed by hydrolysis of glucose 6-phosphate in a reaction catalyzed by glucose 6-phosphatase. We will examine each of these steps in turn. 16.3.2. The Conversion of Pyruvate into Phosphoenolpyruvate Begins with the Formation of Oxaloacetate The first step in gluconeogenesis is the carboxylation of pyruvate to form oxaloacetate at the expense of a molecule of ATP . Then, oxaloacetate is decarboxylated and phosphorylated to yield phosphoenolpyruvate, at the expense of the high phosphoryl-transfer potential of GTP . Both of these reactions take place inside the mitochondria. The first reaction is catalyzed by pyruvate carboxylase and the second by phosphoenolpyruvate carboxykinase. The sum of these reactions is: Pyruvate carboxylase is of special interest because of its structural, catalytic, and allosteric properties. The N-terminal 300 to 350 amino acids form an ATP -grasp domain ( Figure 16.25 ), which is a widely used ATP-activating domain to be discussed in more detail when we investigate nucleotide biosynthesis ( Section 25.1.1 ). The C -terminal 80 amino acids constitute a biotin-binding domain ( Figure 16.26 ) that we will see again in fatty acid synthesis ( Section 22.4.1 ). Biotin is a covalently attached prosthetic group, which serves as a carrier of activated CO2. The carboxylate group of biotin is linked to the -amino group of a specific lysine residue by an amide bond ( Figure 16.27 ). Note that biotin is attached to pyruvate carboxylase by a long, flexible chain. The carboxylation of pyruvate takes place in three stages: Recall that, in aqueous solutions, CO2 exists as HCO3- with the aid of carbonic anhydrase (Section 9.2). The HCO3- is activated to carboxyphosphate. This activated CO2 is subsequently bonded to the N-1 atom of the biotin ring to Continue reading >>
Can Sugars Be Produced From Fatty Acids? A Test Case For Pathway Analysis Tools
Can sugars be produced from fatty acids? A test case for pathway analysis tools Department of Bioinformatics, 2Bio Systems Analysis Group, Friedrich-Schiller-Universitt Jena, Ernst-Abbe-Platz 2, 07743 Jena, Germany and 3School of Life Sciences, Oxford Brookes University, Headington, Oxford, OX3 0BP, UK *To whom correspondence should be addressed. Search for other works by this author on: Department of Bioinformatics, 2Bio Systems Analysis Group, Friedrich-Schiller-Universitt Jena, Ernst-Abbe-Platz 2, 07743 Jena, Germany and 3School of Life Sciences, Oxford Brookes University, Headington, Oxford, OX3 0BP, UK *To whom correspondence should be addressed. Search for other works by this author on: Department of Bioinformatics, 2Bio Systems Analysis Group, Friedrich-Schiller-Universitt Jena, Ernst-Abbe-Platz 2, 07743 Jena, Germany and 3School of Life Sciences, Oxford Brookes University, Headington, Oxford, OX3 0BP, UK Search for other works by this author on: Department of Bioinformatics, 2Bio Systems Analysis Group, Friedrich-Schiller-Universitt Jena, Ernst-Abbe-Platz 2, 07743 Jena, Germany and 3School of Life Sciences, Oxford Brookes University, Headington, Oxford, OX3 0BP, UK Search for other works by this author on: Bioinformatics, Volume 25, Issue 1, 1 January 2009, Pages 152158, Luis F. de Figueiredo, Stefan Schuster, Christoph Kaleta, David A. Fell; Can sugars be produced from fatty acids? A test case for pathway analysis tools, Bioinformatics, Volume 25, Issue 1, 1 January 2009, Pages 152158, Motivation: In recent years, several methods have been proposed for determining metabolic pathways in an automated way based on network topology. The aim of this work is to analyse these methods by tackling a concrete example relevant in biochemistry. It concerns the question wh Continue reading >>
Chapter 19 : Carbohydrate Biosynthesis
Thus the synthesis of glucose from pyruvate is a relativelycostly process. Much of this high energy cost is necessary toensure that gluconeogenesis is irreversible. Under intracellularconditions, the overall free-energy change of glycolysis is atleast -63 kJ/mol. Under the same conditions the overallfree-energy change of gluconeogenesis from pyruvate is alsohighly negative. Thus glycolysis and gluconeogenesis are bothessentially irreversible processes under intracellularconditions. Citric Acid Cycle Intermediates and Many Amino Acids AreGlucogenic The biosynthetic pathway to glucose described above allows thenet synthesis of glucose not only from pyruvate but also from thecitric acid cycle intermediates citrate, isocitrate,-ketoglutarate, succinate, fumarate, and malate. All may undergooxidation in the citric acid cycle to yield oxaloacetate.However, only three carbon atoms of oxaloacetate are convertedinto glucose; the fourth is released as CO in the conversion ofoxaloacetate to phosphoenolpyruvate by PEP carboxykinase (Fig.19-3). In Chapter 17 we showed that some or all of thecarbon atoms of many of the amino acids derived from proteins areultimately converted by mammals into either pyruvate or certainintermediates of the citric acid cycle. Such amino acids cantherefore undergo net conversion into glucose and are calledglucogenic amino acids (Table 19-3). Alanine and glutamine makeespecially important contributions in that they are the principalmolecules used to transport amino groups from extrahepatictissues to the liver. After removal of their amino groups inliver mitochondria, the carbon skeletons remaining (pyruvate anda-ketoglutarate, respectively) are readily funneled intogluconeogenesis. In contrast, there is no net conversion of even-carbon fattyacids into gl Continue reading >>