Carbohydrates & Conjugates

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Carbohydrates & Conjugates
2010-10-17 22:46:12

Biochem block3 Lecture 24 Chapter 7
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  1. What are disaccharides and how can the linkage occur?
    • Two monosaccharides linked by a glycosidic bond
    • Linkages can occur between the anomeric carbon of one sugar and a hydroxyl on another sugar
  2. What are the name of linkages in disaccharides based on and in what form are sugars abosorbed in the body?
    • Linkages are named based on the number of the carbon atoms involved
    • In the body, sugars are absorbed as monosaccharides and digestion requires specific enzymes (e.g., lactase)
  3. What are the key disaccharides?
    • Lactose
    • Maltose
    • Cellabiose
    • Sucrose
  4. What is lactose, how is it formed, and what is its linkage?
    • A disaccharide found in milk.
    • A combination of galactose and glucose.
    • Beta(1,4) linkage.
    • Requires lactase to digest (expression decreases following childhood).
  5. What is maltose, how is it formed, and what is its linkage?
    • A disaccharide that is the intermediate product of starch hydrolysis
    • Alpha (1,4) linkage between two D-glucose molecules
    • Does not appear freely in nature
    • In solution the free anomeric carbon undergoes mutarotation
  6. What is cellobiose, how is it formed, and what is its linkage?
    • A disaccharide that is the degradation product of cellulose
    • Contains two D-glucose molecules with a beta (1,4) linkage
    • Does not occur freely in nature.
  7. What is sucrose, how is it formed, and what is its linkage?
    • A disaccharide: table sugar (cane or beet sugar)
    • Formed from alpha-glucose and beta-fructose
    • alpha, beta (1,2) linkage
    • Produced in the leaves and stems of plants
    • Non-reducing sugar
  8. What are polysaccharides?
    • Also called glycans
    • Composed of a large number of monosaccharides connected by glycosidic linkages
  9. What are oligosaccharides?
    • Smaller glycans with up to 10-15 monomers
    • Most often attached to polypeptides in glycoproteins and some glycolipids.
  10. What are the two classes of polysaccharides?
    • Homoglycans
    • Heteroglycans
  11. What are homoglycans?
    Polysaccharides composed of a single type of monosaccharide
  12. What are heteroglycans?
    Polysaccharides composed of two or more different types of monosaccharides.
  13. What are the most abundant homoglycans found in nature and what are they made of?
    • Starch, glycogen, and cellulose are all made from D-glucose
    • Chitin is made of N-acetylglucosamine
  14. What is the molecular weight polysaccharides?
    • Polysaccharides like starch and glycogen have no fixed molecular weight, unlike proteins and nucleic acids. This means there is not necessarily a limit as to how many monomers can be combined
    • In a “well fed” state, the liver synthesizes glycogen until the blood sugar levels reach proper levels
  15. What is starch?
    • Energy reservoir in plant cells and a significant energy (carbohydrate) source in the human diet (e.g., potatoes, rice, corn, wheat)
    • A homoglycan
  16. What are the two different polysaccharides that form starch?
    Amylose and Amylopectin
  17. What is amylopectin, how is it formed, and digested?
    • One of the polysaccharides that form starch
    • Made of branched chains of D-glucose with alpha(1,4) and alpha(1,6) linkages
    • Digestion involves amylase in the mouth and small intestine, followed by other enzymes designed to release the glucose monomers
  18. What is amylose, how is it formed, and what is its useful function?
    • One of the polysaccharides that form starch
    • Made of unbranched chains of D-glucose with alpha (1,4) linkages
    • Ideal for storage
  19. What are glycogens and where are they most abundant?
    • A homoglycan
    • Carbohydrate storage form used by vertebrates
    • Most abundant in liver and muscle cells
  20. Describe the sturcture of glycogen
    • Involves a compact branched chains of D-glucose similar to the structure of amylopectin
    • The numerous non-reducing ends allows the cell to rapidly break glycogen down and release glucose
  21. Describe the similarities of linkages in gylcogen and amylopectin
    These polysaccharides have both alpha(1,4) [for the chain] and alpha(1,6) [for thebranches] linkages
  22. What is cellulose, where is it most abundant?
    • A homoglycan and structural polysaccharide in plants
    • Probably the most abundant organic substance on earth
    • Considered a good dietary fiber for animals
  23. Describe the formation, structure, and linkage of cellulose
    • Made of D-glucopyranose
    • Beta (1,4) linkages
    • Unbranched chains pair together via H-bonds (microfibril)
    • 40 pairs combine to form a bundle, which is the structural form found in plant cells
  24. What are 5 types of heteroglycans?
    • N-glycans
    • O-glycans
    • Glycosaminoglycans (GAGs)
    • Glycan components of glycolipids
    • GPI anchors (glycosylphosphatidylinositol)
  25. What are N-glycans, how are they formed, and what is its linkage?
    • A heteroglycan
    • Sugar groups linked to the nitrogen of the side chain amide of an asparagine residue
    • Involves a beta-glycosidic bond between nitrogen and the anomeric carbon of N-acetylglucosamine
    • Involves several different monosaccharides
  26. What are O-glycans, how are they formed, and what is its linkage?
    • Core of galactosyl-b-(1,3)-N-acetylgalactosamine linked to the protein with an a-glycosidic bond to
    • the oxygen of the side chain of serine or threonine residues
    • Collagens have a similar linkage with hydroxylysine using a different core
    • Several other kinds sugars are attached to the core
  27. What are glycosaminoglycans (GAGs) and what does it generally contain?
    • A heteroglycan
    • Linear polymers of repeating dissacharides
    • Generally contain a 6-carbon uronic acid (CH2OH to carboxylic acid)
  28. What does the repeating groups of Glycosaminoglycans (GAGs) contain and what are its features?
    • Repeating groups contain both carboxyl and sulfate groups (negative charges!)
    • Charge repulsion keeps chains separated and the hydrophilicity attracts and maintains large volumes of water – vastly increasing the volume of space occupied
  29. What are the five classes of glycosaminoglycans (GAGs) and how are they connected to proteins?
    • hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparin and heparan sulfate, and keratan sulfate
    • All are connected to protein via serine or threonine except hyaluronic acid which connects via asparagine
  30. What is chondroitan sulfate?
    • A heteroglycan (GAG)
    • Component of cartilage
  31. What is dermatan sulfate?
    • A heteroglycan (GAG)
    • Found primarily in skin
  32. What is Heparin?
    • A heteroglycan (GAG)
    • Anticoagulant, found in mast cells
    • Heparan very similar but has fewer sulfate and more acetyl groups
  33. What is keratan sulfate?
    • A heteroglycan (GAG)
    • Found in cornea, cartilage, and intervertebral disks
  34. What is hyaluronic acid?
    • A heteroglycan (GAG)
    • Found in the vitreous humor of the eye and synovial fluid of joints
  35. What are glycoconjugates?
    • Compounds that covalently bind carbohydrates with either lipids or proteins
    • Serve structural and functional roles generally on the surface of cells or in the extracellular matrix
  36. What are the classes of glycoconjugates?
    • Glycolipids
    • Proteoglycans
    • Glycoproteins
  37. What is the difference between proteoglycans and glycoproteins
    • Proteoglycans have a higher carbohydrate content
    • Glycoproteins do not usually have uronic acids, sulfate groups, and disulfide repeating groups (GAGs)
  38. Where are proteoglycans present?
    Present on the cell surface or secreted into the extracellular matrix
  39. How are proteoglycans formed and what is its structure?
    • Made of GAGs linked to proteins (called core proteins) using N- and O-glycosidic linkages
    • Proteoglycan aggregates are a series of proteoglycans linked to a common central GAG backbone (hyaluronic acid)
  40. What are 3 examples of proteoglycans?
    • Syndecans
    • Glypicans
    • Aggrecan
  41. What are syndecans?
    A class of heparan sulfate and chondrotin sulfate containing proteoglycans which include a transmembrane core protein
  42. What are Glypicans?
    Are proteoglycans that contain heparan sulfate and are linked to membranes via GPI anchors
  43. What are aggrecans?
    • Proteoglycans found in cartilage
    • Combination of chondroitin sulfate and keratan sulfate attached to a core
  44. What are the functions of proteoglycans?
    Signaling and Structural (compressive stiffness)
  45. What is the backbone of a proteoglycan aggregate?
    Hyaluronic acid (GAG) backbone
  46. What are glycoproteins and how are they linked?
    • Glycoconjugates
    • Proteins that are linked through N- or O-linkages to carbohydrates (i.e., N- or O-glycans)
  47. What is the range of carbohydrate content in glycoproteins?
    • Carbohydrate content ranges from ~1-85%
    • Carbohydrates involved include monosaccharides, disaccharides, and several types of oligosaccharides
  48. How are the N and O linkages formed in gylcoproteins?
    • N-linked carbohydrate chains are formed initially and then added to the protein during synthesis (carbohydrate added on protein)
    • O-linked chains are built directly on the protein in the Golgi (carbohydrate built on protein)
  49. What are 5 examples of glycoproteins?
    • Immunoglobulin (antibodies)
    • Hormones: chorionic gonadotropin (HCG), follicle-stimulating hormone (FSH)
    • Metal transport proteins: transferrin and ceruloplasmin
    • Membrane protein such as Na+-K+-ATPase (Na+-K+pump)
    • ABO blood group system- most common example
  50. What is the glycocalyx?
    • Layer outside of most eukaryotic cells containing a large number of carbohydrates
    • These serve structural, signaling, and recognition roles for cells and tissues
    • Glycoproteins are components of the glycocalyx
  51. What are lectins?
    • Considered to be the translators of the sugar code.
    • Carbohydrate-binding proteins that are not abs and have no enzymatic activity.
  52. How does the cell recognize the vast array of different carbohydrates?
    • Proteins known as lectins are able to bind to specific carbohydrate groups
    • These are used in numerous types of interactions for recognition – especially cell-cell (e.g., selectins)
  53. What is glycolysis?
    Break down of glucose to produce energy
  54. What is gluconeogenesis?
    Production of glucose from specific precursors
  55. What is pentose phosphate?
    Involved in the production of ribose for nucleotides
  56. What is considered glycogen metabolism?
    The building (glycogenesis) or breakdown (glycogenolysis) of glucose
  57. What daily value percentage is considered low and high of total carbohydrates?
    • 5% or less is Low
    • 20% or more is High
  58. What are the dietary sources of carbohydrates?
    • Sugar
    • Starch- complex carbohydrate
    • Dietary fiber- complex carbohydrate
  59. Where is sugar found?
    • In a range of food sources such as milk (lactose), fruit (fructose), refined sugar for baking (sucrose), honey (fructose, glucose, maltose, sucrose)
    • These are digested by the body
  60. Where is starch found?
    • In bread, pasta, rice, potatoes, sweet potatoes,etc.
    • Are digested by the body
  61. Where is dietary fiber found?
    • In many plant foods (fruits and vegetables) and in whole grain foods
    • Cannot be broken down by the body (includes cellulose), but are important components of our diet