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PSL425 Lecture 1.docx

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University of Toronto St. George

PSL425 Lecture 1 Carbohydrates on of the most important circulating carbohydrates in our body is D-glucose (OH on carbon 5) important! b/c L0glucose not something we have in our body. Physiological one in our body is D-glucose, most enzymes specific for D-forms - 6C molecule; C=O aldehyde bond (glucose is a aldehyde) - Linear and cyclical forms Aldehyde in water (becomes polarized) b/c water is polar (proton and OH- in water); the proton adds to this O here and makes an OH group, OH adds to carbon here, and instead of double bond her, you have 2 single bonds and 2 -OH - aldehyde to hemiacetal - alcohol also have dissociable proton similar to water, proton can add to O here, forming a new OH group and rest (R group) adds here in a very similar way as the oxygen of water added have a hemiacetal - think about R being of the rest of glucose molecule - think about alcohol group being this group (proton dissociates and add to this oxygen) and OH adds here, and rest of the molecule adds to this carbon - by cyclizing: the new OH group can go on the right or left totally interchangeable - anomers Sugar tautemerization - carbon 5 OH onto C2 OH can add on either on the left or right of C2: forms 2 anomers (a- and b-forms) - intramolecular reaction (i.e. within the molecule) Glucose and Galactose - very similar 6C sugar is galactose (found in milk) - galactose also found in lactose (diasaccharide) - diff on Carbon 4 (epimers) OH, glucose down, galactose up epimer of glucose - alpha and beta glucose anomers Glucose and Fructose - fructose found bound to glucose in sucrose, also in fruit - fructose has 6C but cyclizes as pentamer instead of hexamer - double bond of fructose is on C2 i.e. it is a ketone: why cyclizes as a pentamer - we usually eat diassacharides or polysaccharides Glycogen or starch - polysaccharides; glycogen from meat (muscles), starch (starchy vegetables) - Chains of glucose - Bonds between them is formed by hydrolysis b/t 2 adjacent carbons of neighboring glucose molecules b/t carbon 1 (a-glucose) and 4 to form 1-4 linear chain - branched is between carbon 1 and 6: 1-6 linkage Lactose - D-saccharide of galactose; anomers (OH can be up or down) - Alpha-glucose with oxygen of carbon 4, but bond with beta-galactose (beta bond) - Enzyme in our body: E.g. if you have beta-bond with exact same molecular structure as glycogen: it gives you cellulose; cellulose is not digestible by our body, b/c all our intestinal enzymes are enzymes that work (most of them) with only alpha families - We have amylase that can digest starch and glycogen, but NOT cellulose (little in saliva and mainly in pancreas) - Pancreas produces most of digestive enzymes: e.g. pancreatic amylase important in digestion of glycogen and starch o It breaks them down into little piece so they can be digested by disaccharidases (produced in the brush border of microvilli of small intestine) e.g. maltase (maltose into 2 glucoses) in intestinal cells absorbed by sodium-glucose transporters (using energy of sodium moving down its electrochem gradient to carry out glucose transport) - Lactose has beta bond (however most of our digestive enzymes have specificity for alpha bonds), we do have one disaccaridase that can digest beta-bond called lactase; but this enzyme is not sufficient? o Lots of people of lactose-intolerance (genetic lactase deficiency); large molecules do not get absorbed, have osmotic effect - Large molecules in large intestine (non-digestable) = diarrhea - You can have acquired lactase deficiency: E.g. after you have intestinal infections, IBD, other bowl disease, etc. tend to produce less of this enzyme lactose ingested can cause problems due the difficulty in breaking down this beta bond in lactose Sucrose (a little different) - i.e. table sugar, not regular 1-4 alpha bond - bond between C1 of glucose to C2 of fructose (1-2 bond) - We have sucrase in our body (sucrose have alpha kind of bond) that can digest this - we dont have problems digesting this SGLTs secondary active transport using the energy form the Na+ going down its [gradient] Note: Active transport occurs on brush border; apical side (SGLTs) basal side has the GLUT transporters that are found in any cell for glucose transport At the level of the portal blood: they are absorbed into portal circulation Veins of intestine go to portal vein, in portal blood (have glucose, galactose, fructose) to the liver (1 thing they encounter) At liver: galactose and fructose into glucose by enzymes here; big chemical factory of our body; after big carb meal, part of glucose we absorbed is stored as glycogen, rest goes out into circulation to other tissue why after a meal, there is a physiological elevation of blood glucose but not that much, as glucose is very well controlled in our body Most toxic for the body acutely: low levels of glucose brain (utilizes most of the glucose); glucose is tightly regulated b/c it cant get too low, b/c otherwise you get hypoxia in the brain, by it cannot go too high b/c of toxic (complications in diabetes) and osmotic effects (prolonged) Therefore glucose [ ] very tightly controlled after a meal it can go up but never over 10mmol = almost diabetic at this point nd 2 : Peripheral tissues (relative to liver) does not see fructose nor galactose; only glucose circulating glucose goes into the tissues via GLUT transporters NOT the ones found in brush border of the intestine, these are the usual glucose transporters of the tissues (many GLUT transporters)The most important ones here are GLUT1 = ubiquitous in tissues, GLUT2 in some tissues such as the liver and the pancreas Muscles and fat have GLUT4 (NOT the liver, NOT brain)- only one regulated by insulin i.e. insulin stimulates glucose transport Insulin: affects translocation of glucose transporters (carriers) from inside the membrane to the plasma mmbrn. Vesicles contain transporters, where they are stuck, and these vesicles gets incorporated into the plasma mmbrn under insulin influence. So, insulin favors trafficking of vesicles containing GLUT4 to mmbrn, mmbrn enriched with these transporters Glycogen equilibrium Liver: totality of the carbohydrate pathways occurring Glucose enter liver via 2 transporters (GLUT2) GLUT2 are coupled with enzyme glucokinase; whereas GLUT4 coupled with hexokinase In muscles: system that picks up as much glucose as it can happy to get anything that provides energy - As soon as glucose passes through transporter, it gets phosphorylated by the enzyme, as enzyme has great affinity for glucose it doesnt make sense for liver to do this, as liver is a regulator of glucose In liver: has to store glucose, but also have to give glucose back to the blood; glucokinase, does not have much affinity for glucose, and works according to concentration, not working always at maximum; intracellular [glucose] in hepatocytes exactly same as [glucose] in circulation, b/c liver has to sense glucose levels and regulate it If circulating glucose too much take up and convert and store as glycogen; glucose too less break down of glycogen to release more g
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