Carbohydrate metabolism -
Sucrose table sugar is made of two simpler sugars called glucose and fructose. Lactose milk sugar is made of glucose and galactose.
Both sucrose and lactose must be broken down into their component sugars by enzymes before the body can absorb and use them. The carbohydrates in bread, pasta, rice, and other carbohydrate-containing foods are long chains of simple sugar molecules. These longer molecules must also be broken down by the body.
If an enzyme that is needed to process a certain sugar is missing, that sugar can accumulate in the body, causing problems. Galactosemia Galactosemia Galactosemia a high blood level of galactose is a carbohydrate metabolism disorder that is caused by a lack of one of the enzymes necessary for metabolizing galactose, a sugar that is part Glycogen storage diseases Glycogen Storage Diseases Glycogen storage diseases are carbohydrate metabolism disorders that occur when there is a defect in the enzymes that are involved in the metabolism of glycogen, often resulting in growth abnormalities Hereditary fructose intolerance Hereditary Fructose Intolerance Hereditary fructose intolerance is a carbohydrate metabolism disorder that is caused by a lack of the enzyme needed to metabolize fructose.
Very small amounts of fructose cause low blood sugar Pyruvate metabolism disorders Pyruvate Metabolism Disorders Pyruvate metabolism disorders are carbohydrate metabolism disorders that are caused by a lack of the ability to metabolize a substance called pyruvate. These disorders cause a buildup of lactic The following are some English-language resources that may be useful.
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In: Janson LW, Tischler ME. Janson L. Lee W. Janson, and Marc E. The Big Picture: Medical Biochemistry. McGraw-Hill Education; Accessed February 14, APA Citation Carbohydrate metabolism.
Janson LW, Tischler ME. McGraw-Hill Education. Download citation file: RIS Zotero. Reference Manager. Autosuggest Results. Sections View Full Chapter Figures Tables Videos Annotate. Figure Glycolysis can be divided into two phases: energy consuming also called chemical priming and energy yielding.
The first phase is the energy-consuming phase , so it requires two ATP molecules to start the reaction for each molecule of glucose. However, the end of the reaction produces four ATPs, resulting in a net gain of two ATP energy molecules.
The NADH that is produced in this process will be used later to produce ATP in the mitochondria. Importantly, by the end of this process, one glucose molecule generates two pyruvate molecules, two high-energy ATP molecules, and two electron-carrying NADH molecules.
The following discussions of glycolysis include the enzymes responsible for the reactions. When glucose enters a cell, the enzyme hexokinase or glucokinase, in the liver rapidly adds a phosphate to convert it into glucosephosphate.
A kinase is a type of enzyme that adds a phosphate molecule to a substrate in this case, glucose, but it can be true of other molecules also. This conversion step requires one ATP and essentially traps the glucose in the cell, preventing it from passing back through the plasma membrane, thus allowing glycolysis to proceed.
It also functions to maintain a concentration gradient with higher glucose levels in the blood than in the tissues. By establishing this concentration gradient, the glucose in the blood will be able to flow from an area of high concentration the blood into an area of low concentration the tissues to be either used or stored.
Hexokinase is found in nearly every tissue in the body. Glucokinase , on the other hand, is expressed in tissues that are active when blood glucose levels are high, such as the liver.
Hexokinase has a higher affinity for glucose than glucokinase and therefore is able to convert glucose at a faster rate than glucokinase. This is important when levels of glucose are very low in the body, as it allows glucose to travel preferentially to those tissues that require it more.
In the next step of the first phase of glycolysis, the enzyme glucosephosphate isomerase converts glucosephosphate into fructosephosphate. Like glucose, fructose is also a six carbon-containing sugar. The enzyme phosphofructokinase-1 then adds one more phosphate to convert fructosephosphate into fructosebisphosphate, another six-carbon sugar, using another ATP molecule.
Aldolase then breaks down this fructosebisphosphate into two three-carbon molecules, glyceraldehydephosphate and dihydroxyacetone phosphate. The triosephosphate isomerase enzyme then converts dihydroxyacetone phosphate into a second glyceraldehydephosphate molecule.
Therefore, by the end of this chemical-priming or energy-consuming phase, one glucose molecule is broken down into two glyceraldehydephosphate molecules. The second phase of glycolysis, the energy-yielding phase , creates the energy that is the product of glycolysis.
Glyceraldehydephosphate dehydrogenase converts each three-carbon glyceraldehydephosphate produced during the energy-consuming phase into 1,3-bisphosphoglycerate. NADH is a high-energy molecule, like ATP, but unlike ATP, it is not used as energy currency by the cell.
Because there are two glyceraldehydephosphate molecules, two NADH molecules are synthesized during this step. Each 1,3-bisphosphoglycerate is subsequently dephosphorylated i. Each phosphate released in this reaction can convert one molecule of ADP into one high-energy ATP molecule, resulting in a gain of two ATP molecules.
The enzyme phosphoglycerate mutase then converts the 3-phosphoglycerate molecules into 2-phosphoglycerate. The enolase enzyme then acts upon the 2-phosphoglycerate molecules to convert them into phosphoenolpyruvate molecules.
The last step of glycolysis involves the dephosphorylation of the two phosphoenolpyruvate molecules by pyruvate kinase to create two pyruvate molecules and two ATP molecules.
In summary, one glucose molecule breaks down into two pyruvate molecules, and creates two net ATP molecules and two NADH molecules by glycolysis. Therefore, glycolysis generates energy for the cell and creates pyruvate molecules that can be processed further through the aerobic Krebs cycle also called the citric acid cycle or tricarboxylic acid cycle ; converted into lactic acid or alcohol in yeast by fermentation; or used later for the synthesis of glucose through gluconeogenesis.
When oxygen is limited or absent, pyruvate enters an anaerobic pathway called fermentation. In these reactions, pyruvate can be converted into lactic acid.
In this reaction, lactic acid replaces oxygen as the final electron acceptor. Anaerobic respiration occurs in most cells of the body when oxygen is limited or mitochondria are absent or nonfunctional.
For example, because erythrocytes red blood cells lack mitochondria, they must produce their ATP from anaerobic respiration. This is an effective pathway of ATP production for short periods of time, ranging from seconds to a few minutes.
The lactic acid produced diffuses into the plasma and is carried to the liver, where it is converted back into pyruvate or glucose via the Cori cycle. Similarly, when a person exercises, muscles use ATP faster than oxygen can be delivered to them.
They depend on glycolysis and lactic acid production for rapid ATP production. The NADH and FADH 2 pass electrons on to the electron transport chain, which uses the transferred energy to produce ATP. As the terminal step in the electron transport chain, oxygen is the terminal electron acceptor and creates water inside the mitochondria.
The pyruvate molecules generated during glycolysis are transported across the mitochondrial membrane into the inner mitochondrial matrix, where they are metabolized by enzymes in a pathway called the Krebs cycle Figure The Krebs cycle is also commonly called the citric acid cycle or the tricarboxylic acid TCA cycle.
During the Krebs cycle, high-energy molecules, including ATP, NADH, and FADH 2 , are created. NADH and FADH 2 then pass electrons through the electron transport chain in the mitochondria to generate more ATP molecules.
Watch this animation to observe the Krebs cycle. The three-carbon pyruvate molecule generated during glycolysis moves from the cytoplasm into the mitochondrial matrix, where it is converted by the enzyme pyruvate dehydrogenase into a two-carbon acetyl coenzyme A acetyl CoA molecule.
This reaction is an oxidative decarboxylation reaction. Acetyl CoA enters the Krebs cycle by combining with a four-carbon molecule, oxaloacetate, to form the six-carbon molecule citrate, or citric acid, at the same time releasing the coenzyme A molecule.
The six-carbon citrate molecule is systematically converted to a five-carbon molecule and then a four-carbon molecule, ending with oxaloacetate, the beginning of the cycle. Along the way, each citrate molecule will produce one ATP, one FADH 2 , and three NADH.
The FADH 2 and NADH will enter the oxidative phosphorylation system located in the inner mitochondrial membrane.
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