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Carbohydrates
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Carbohydrates Carbohydrates are one of the four major classes of biomolecules. Carbohydrates contain an aldehyde or ketone group and have multiple hydroxyl groups. Carbohydrates comprise most of the organic materials on Earth. In human, carbohydrates serve as a fuel source, energy stores, and metabolic intermediates. They are also a part of the nucleic acids. When they are linked to proteins and lipids, they mediate cellular interactions.
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Carbohydrate forms Monosaccharides -- a single sugar
glucose, galactose, fructose, mannose Disaccharides -- two monosaccharides linked together sucrose, lactose, maltose, trehalose Oligosaccharides -- three to ten monosaccharides linked together Polysaccharides -- polymers of more than ten monosaccharides glycogen, starch Heteropolysaccharides -- polysaccharides with atoms other than C, O and H chitin, hyaluronic acid
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Metabolic fates of carbohydrates
Monosaccharides (e.g., Glucose) Glycolipids & glycoproteins Pentose phosphate pathway Glycolysis Glycogen Amino acids Acetyl CoA TCA cycle Fatty acid synthesis Cholesterol synthesis
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Aldoses vs. ketoses Aldoses have a carbonyl group on C-1
2 2 2 3 3 3 aldotriose ketotriose sugar alcohol Aldoses have a carbonyl group on C-1 Ketoses have a carbonyl group on C-2
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Monosaccharides have varying number of
carbon atoms Aldose Ketose 3 C Triose Glyceraldehyde Dihydroxyacetone 4 C Tetrose D-Threose D-Erythrulose 5 C Pentose D-Ribose D-Ribulose 6 C Hexose D-Glucose D-Fructose D-Galactose D-Mannose 7 C Heptose Sedoheptulose
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ul designates a keto sugar
Aldoses D-Glucose D-Arabinose D-Threose D-Glyceraldehyde Ketoses ul designates a keto sugar D-Fructose D-Ribulose D-Erythrulose Dihydroxyacetone
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Monosaccharides can form optical isomers (D vs. L stereoisomers)
Plane of Symmetry Fischer Projection Aldehyde * * Hydroxyl * * * * L D Asymmetric carbon farthest away from aldehyde or ketone * * Penultimate Carbon L-Glucose D-Glucose
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Open-chain glucose can cyclize to form a pyranose
Haworth Fischer 5 Anomeric carbon 1 1 * 5 5 * Anomeric carbon 1 Biochemistry 5th Ed. 2007 W.H. Freeman and Company
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Open-chain fructose can cyclize to form a furanose
2 5 A.C. 2 * 5 -D-fructofuranose can also be formed Biochemistry 5th Ed. 2007 W.H. Freeman and Company
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Most aldohexoses are pyranoses Most ketohexoses are furanoses
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-D-Glucose Hemiacetal oxygen Anomeric carbon Fischer Haworth Chair
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Glucose ring can form free aldehyde
-D-Glucose -D-Glucose D-Glucose Free aldehyde group In solution, reducing sugars, such as glucose, reversibly form a free aldehyde at the number 1 carbon. The aldehyde group also irreversibly forms advanced glycation end products with proteins (AGEs). This transient and reversible opening and closing of the ring will lead to the formation of the -anomer of glucose from -anomer through mutarotation.
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Monosaccharides can be modified by the addition of functional groups
Modified sugars are often found on cell surfaces. Biochemistry 5th Ed. 2007 W.H. Freeman and Company
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Structures of A, B, and O oligosaccharides
for ABO blood groups Biochemistry 5th Ed. 2007 W.H. Freeman and Company
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Dextrose & Levulose Dextrose is a synonym for glucose.
So named dextrose, because it rotates plane-polarized light to the right (dextrorotatory). Levulose is a synonym for fructose. So named levulose, because it rotates plane-polarized light to the left (levorotatory).
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Disaccharides Disaccharides are made of two monosaccharide
molecules joined together via an O-glycosidic linkage. Biochemistry 5th Ed. 2007 W.H. Freeman and Company
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Major disaccharides Sucrose (Table sugar) glucose-fructose
Lactose (Milk sugar) glucose-galactose Maltose (Malt sugar) glucose-glucose Isomaltose
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Sucrose Table sugar Hydrolyzed by invertase (sucrase) in brush-border of small intestine. Major sources: sugar cane and sugar beets. If injected, it is not hydrolyzed, since invertase is present only in the small intestine.
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Sucralose Sucralose is an artificial sweetener and is a chlorinated derivative of sucrose. It is 600 times sweeter than sucrose, and it passes through the GI tract unchanged and unabsorbed. Splenda contains sucralose and some other components.
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Lactose Abundant in all milk products, but less in yogurt.
Hydrolyzed by lactase in brush border of small intestine. •Lactase is not expressed in many individuals especially those of Asian and African descent and older individuals of all ethnic backgrounds (lactose intolerance). Of the disaccharidases, lactase is the most sensitive to disruption. Temporary lactase deficiency: caused by intestinal infection or inflammation temporary loss of lactase activity accumulation of undigested lactose cramping, bloating, gas, and diarrhea
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Maltose is a product of starch digestion
Glycosidic Linkage Maltose is hydrolyzed to two molecules of glucose by maltase in the brush border of small intestine. Glc (14) Glc
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Isomaltose is a product of starch digestion at the branch point
Isomaltose is hydrolyzed by isomaltase in the brush border of the small intestine. 1 Glc (16) Glc 6
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Enzymes in the microvilli of the small intestine can hydrolyze carbohydrates
Pancreatic -amylase in the small intestine breaks starch into di-, tri-, and oligosaccharides. Microvilli lining the small intestine contain disaccharidases: maltase isomaltase sucrase lactase that convert disaccharides into monosaccharides which are then absorbed by the intestinal epithelial cells.
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Polysaccharides Polymers of monosaccharides
Starch, glycogen, and cellulose are polymers of glucose. Glycosaminoglycans (GAGs) are comprised of repeating disaccharide units of amino sugars, e.g., heparin, hyaluronan, etc. GAGs can link to proteins to form proteoglycans, e.g., keratin, elastin, etc. Chitin is a polymer of GlcNAc (the source of glucosamine in OTC arthritis supplements).
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Starch Found in virtually all plants as a storage form of glucose.
Two major components: Amylose - unbranched linear polymer of glucose units linked 14 Amylopectin - branched polymer of glucose units two types of linkages -14 and -16
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Amylose portion of starch
(14) glycosidic linkage 1 4 Amylose is the unbranched portion of starch made of linear chain of glucose molecules linked to one another via -1,4 glycosidic linkage.
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Amylopectin portion of starch
1 6 1 4 Amylopectin is the branched form of starch. The branches are linked to another via -1,6 glycosidic linkage.
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Glucose residues in cellulose are joined by
-1,4 linkages Cellulose consists of long linear chains of glucose units linked together with b-1,4 glycosidic bonds. The -configuration allows cellulose to form straight chains, and parallel chain can form compact fibrils. Cellulose is not digested because mammals do not express the enzyme that cleaves b-1,4 glycosidic linkages.
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Glycogen Biochemistry 5th Ed. 2007 W.H. Freeman and Company Glycogen resembles the amylopectin portion of starch. However, glycogen contains more branches. Glycogen is synthesized by glycogen synthase from UDP-glucose. Breakdown of glycogen by glycogen phosphorylase will yield glucose 1-phosphate.
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Blood glucose regulates glycogen metabolism
Glycogen synthase and phosphorylase are reciprocally regulated. Elevation in blood glucose level stimulates glycogen synthesis and inhibits glycogen breakdown. Liver glycogen metabolism is closely controlled by blood glucose level. After a meal rich in carbohydrates, blood glucose level will rise. This rise in blood glucose will inhibit glycogen phosphorylase to prevent glycogen breakdown. At the same time, this elevated level of glucose will stimulate glycogen synthase activity to synthesize glycogen. Biochemistry 5th Ed. 2007 W.H. Freeman and Company
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Glycosaminoglycans contain repeating
disaccharide units of amino sugars Biochemistry 5th Ed. 2007 W.H. Freeman and Company Glycosaminoglycans are generally associated with proteins found on cell surface and in the extracellular matrix.
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Glucose homeostasis is important
Unregulated glucose homeostasis can lead to diabetes. This leads to rapid rises and prolonged elevation in glucose level. High glucose results in increased glycation and free radical production. High carbohydrates intake can lead to obesity: -High glucose induces hyperinsulinism. -Insulin is ANABOLIC and promotes fat synthesis. -Obesity increases demand for insulin. -Pancreas cannot supply enough insulin. -Type 2 Diabetes may result.
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Glycation of cellular components
•High blood glucose can lead to non-enzymatic glycation of proteins, nucleotides, and basic phospholipids by saccharide derivatives, resulting in the formation of AGE products (abnormal glycosylation end products). •Glycation results in cellular dysfunction and induces diabetic complications, macrovascular disease, Alzheimer’s disease, uremia, and aging. •Hemoglobin A1C level can be used as a measure of the variation in glucose level in diabetic patients.
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Hemoglobin A1C and blood glucose
Normal HbA1C is 5% or below. OTC kits are available for measuring HbA1C.
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Is high-fructose syrup really bad for you?
You have probably heard how high fructose syrup is bad for and you should totally avoid it. Well here is a video on it and you can be your own judge.
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Take home message on sugar
We need sugar to maintain biological functions. However, all people,whether diabetic or not, should limit the intake of simple sugars, because Excessive sugars can glycate and denature proteins by forming AGEs. Chronically high glucose can cause hyperinsulinemia, which stimulates fat production, obesity, and could potentially lead to diabetes.
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