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Carbohydrates

Carbohydrates. Chapter 5. Carbohydrate Structure. Presence of an aldose or a ketose Presence of multiple hydroxyl groups, which provide sugars with many reactive OH groups These characteristics lead to the possibility of many different spatial arrangements. Carbohydrate Structure.

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Carbohydrates

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  1. Carbohydrates Chapter 5

  2. Carbohydrate Structure • Presence of an aldose or a ketose • Presence of multiple hydroxyl groups, which provide sugars with many reactive OH groups • These characteristics lead to the possibility of many different spatial arrangements

  3. Carbohydrate Structure Key point: glucose major fuel source. But for galactose to be used in the same manner it has to be converted to glucose via enzyme reaction • The orientation of the hydroxyl group around any given carbon atom can differ • Leads to the formation of optical isomers • Have the same chemical formula • Do not have the same 3D structure • Are not mirror images By convention the carbons are numbered consecutively, starting with the end nearest the carbonyl group.

  4. Carbohydrate Structure • In aqueous solutions carbohydrates exist in a chemical equilibrium that heavily favors ring formation

  5. Chemical Evolution • A wide variety and diversity of monosaccharides existed under conditions that mimic the conditions of early earth • When formaldehyde CH2O molecules are heated they react to form almost all pentoses & hexoses

  6. Chemical Evolution • A 3 carbon ketose, along with a wide array of compounds closely related to sugars was found on a meteorite • Once again minerals may have been involved • Minerals could have catalyzed the synthesis of sugars, leading to an accumulation in early oceans • How does polymerization occur? • Much like building a peptide or phosphodiester bond

  7. Glycosidic Bond Linkage

  8. Chemical Evolution • However, linking these sugars together spontaneously (without enzyme help) is unlikely • Polymerization • Disaccharides • Oligosaccharides • Polysaccharides • Polymerization reactions: • Occur between intermolecular hydroxyl groups • Result in a covalent glycosidic bond linkage

  9. α 1, 4 & β 1, 4 Bond Geometry • Bond angles can originate between 0º to 360º • At the lowest energy configuration the α 1, 4 bond angle is 60º • Therefore, the most stable 3D structure for α 1, 4 bonds is a helix • At the lowest energy configuration the β 1, 4 bond angle is 180º • Therefore, the most stable 3D structure for β 1, 4 bonds is a linear sheet

  10. Starch • α glucose monomers • Broken down by amylases • Amylose • Unbranced helix α 1, 4 • Amylopection • Branched helix α 1, 6 • Main storage for plant carbohydrates • Energy • We eat rice, wheat, potatoes

  11. Glycogen • Animal carbohydrate storage • Liver & Muscles • Regulated by two hormones • Insulin & glucagon • Broken down by glycogen phosphorylase • No long term energy storage • Requires water for storage • Limit to how much liver can store excess glucose is converted and stored as anhydrous fat • Highly branched • α 1, 6 occurs 1 out of 10 glucose units

  12. Starch

  13. Cellulose • Geometry of the β 1, 4 bond is such that each glucose monomer is flipped in relation to the adjacent molecule • Extensive hydrogen bonding between parallel strands

  14. Cellulose • Most organisms have the enzymes required to break the α 1, 4 bonds or α 1, 6 bonds • Only a few organisms have the enzymes with active sites with the correct geometry to break apart β 1, 4 bonds • Why the β 1, 4 linkage is an excellent geometry for structural support • Can withstand compression and tension forces • The structure and function correlated

  15. Cellulose • A collection of about 80 cellulose molecules are cross linked by hydrogen bonding to create a tough fiber • These fibers further cross link forming a tough sheet • You can not digest cellulose • Cellulose water absorption • The termite gut and its protozoans

  16. Chitin • Similar in structure and orientation to cellulose • The monomer is N-acetylglucosamine • Modified sugar with a nitrogen group • Cell wall material of fungus • Exoskeleton of insects

  17. Chitin

  18. Peptidoglycan • Backbone of two alternating sugars NAM-NAG • Backbone is cross-linked by peptide bridge • Certain antibiotics kill bacteria by not allowing the cross links to occur

  19. Peptidoglycan

  20. Polysaccharides & Chemical Evolution • Virtually all organisms manufacture or metabolize glycogen or starch • Despite their importance polysaccharides probably played little or no role in the origin of life

  21. Polysaccharides & Chemical Evolution • Problems • No plausible mechanism for polymerization without enzymes • Self-replicating life probably began as RNA • No ribozyme can catalyze formation of glycosidic linkages • Do not catalyze reactions themselves • Despite many hydroxyl groups lack the diversity of functional groups found on proteins • Simple secondary structure • No mechanism for self coping • No complementary base pairing • Obviously important for cellular life and probably evolved after cellular life

  22. Carbohydrates & Cell Identity • Structural polymers are repetitive • Glycoproteins • One or more distinct carbohydrate bound to a serine or theonine with various linkages (oligosaccharides) • Each cell in your body has a set of glycoproteins for recognition • Especially important for your immune system to recognize your cells from foreign cells • Each cell type has a different type of glycoproteins • Nerve, muscle, liver, etc

  23. Carbohydrates & Cell Identity

  24. Sugar Proteins • The biological advantages of adding oligosaccharides to proteins are not fully understood • Attachment influences hydration of joints & ECM • Large hydrophilic clusters of carbohydrate alter the polarity and solubility of proteins • Attachment may influence folding events • Numerous negatively charged oligosaccharides chains clustered in a region cause charge repulsion and the formation of a rod structure • Attachment may influence cell targeting

  25. Do Sperm Recognize Glycoproteins

  26. Do Sperm Recognize Glycoproteins

  27. Oligosaccharide function on Proteins • Luteinizing hormone is a glycosolated protein which is release from the pituitary gland • Causes follicle development in women • If a woman lacks the enzyme which glycosolates the protein the protein does not function and females fail to undergo sexual changes during puberty

  28. Proteoglycans • These interactions: • Anchor cells to ECM • Cause significant hydration • Convey information across plasma membrane • Cross-linked meshwork gives ECM strength and resilience

  29. ∆H in Molecules ∆H Decreases potential energy stored in these bonds goes down as you oxidize fats & sugars

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