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Carbohydrates II; Lipids I

Carbohydrates II; Lipids I. Andy Howard Introductory Biochemistry 18 September 2008. Carbohydrates (concluded): Structural polysaccharides Glycoconjugates Proteoglycans Peptidoglycans Glycoproteins. Lipids Characteristics Fatty acids Phospholipids. What we’ll discuss.

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Carbohydrates II; Lipids I

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  1. Carbohydrates II; Lipids I Andy HowardIntroductory Biochemistry18 September 2008 Biochemistry: Carbo2/Lipid1

  2. Carbohydrates (concluded): Structural polysaccharides Glycoconjugates Proteoglycans Peptidoglycans Glycoproteins Lipids Characteristics Fatty acids Phospholipids What we’ll discuss Biochemistry: Carbo2/Lipid1

  3. Structural polysaccharides I • Insoluble compounds designed to provide strength and rigidity • Cellulose: glucose b-14 linkages • Rigid, flat structure: each glucose is upside down relative to its nearest neighbors (fig.7.27) • 300-15000 glucose units • Found in plant cell walls • Resistant to most glucosidases • Cellulases found in termites,ruminant gut bacteria • Chitin: GlcNAc b-14 linkages:exoskeletons, cell walls (fig. 7.29) Biochemistry: Carbo2/Lipid1

  4. Structural polysaccharides II • Alginates: poly(-D-mannuronate),poly(-L-guluronate), linked 14 • Cellulose-like structure when free • Complexed to metal ions:3-fold helix (“egg-carton”) • Agarose: alternating D-gal, 3,6-anhydro-L-gal, with 6-methyl-D-gal side chains • Forms gels that hold huge amounts of H2O • Can be processed to use in the lab for gel exclusion chromatography • Glycosaminoglycans: see next section Biochemistry: Carbo2/Lipid1

  5. iClicker question 1 • Suppose you isolate a polysaccharide with 5000 glucose units, and 3% of the linkages are 1,6 crosslinks. This is: • (a) amylose • (b) amylopectin • (c) glycogen • (d) chitin • (e) none of the above. Biochemistry: Carbo2/Lipid1

  6. iClicker question 2 • Suppose you isolate an enzyme that breaks down -1,4-glycosidic linkages between GlcNAc units. This would act upon: • (a) glycogen • (b) cellulose • (c) chitin • (d) all of the above • (e) none of the above. Biochemistry: Carbo2/Lipid1

  7. Glycoconjugates • Poly or oligosaccharidescovalently linkedto proteins or peptides • Generally heteroglycans • Categories: • Proteoglycans (protein+glycosaminoglycans) • Peptidoglycans (peptide+polysaccharide) • Glycoproteins (protein+oligosaccharide) Image courtesy Benzon Symposia Biochemistry: Carbo2/Lipid1

  8. Proteoglycans: Glycosaminoglycans • Unbranched heteroglycans of repeating disaccharides • One component isGalN, GlcN, GalNAc, or GlcNAc • Other component: an alduronic acid • —OH or —NH2 often sulfated • Found in cartilage, joint fluid Biochemistry: Carbo2/Lipid1

  9. Proteoglycans in cartilage • Highly hydrated, voluminous • Mesh structure (fig.7.47 or this fig. from Mathews & Van Holde) • Aggrecan is major proteoglycan • Typical of proteoglycans in that it’s extracellular Biochemistry: Carbo2/Lipid1

  10. Peptidoglycans • Polysaccharides linked to small proteins • Featured in bacterial cell walls:alternating GlcNAc + MurNAclinked with -(14) linkages • Lysozyme hydrolyzes these polysaccharides • Peptide is species-specific: often contains D-amino acids Biochemistry: Carbo2/Lipid1

  11. Peptidoglycans in bacteria • Gram-negative: thin peptidoglycan layer separates two phospholipid bilayer membranes • Gram-positive: only one bilayer, with thicker peptidoglycan cell wall outside it • Gram stain binds to thick wall, not thin layer • Fig. 7.36 shows multidimensionality of these walls Biochemistry: Carbo2/Lipid1

  12. Peptide component (fig. 7.34) • Sugars are crosslinked with entities containing(L-ala)-(isoglutamate)-(L-Lys)-(D-ala) • Gram-neg: L-Lys crosslinks via D-ala • Gram-pos: L-lys crosslinks via pentaglycine followed by D-ala Biochemistry: Carbo2/Lipid1

  13. Gram-negative bacteria:the periplasmic space (fig. 7.37) • Periplasmic space: space inside cell membrane but inside just-described peptidoglycan layer (note error in fig. legend!) • Peptidoglycan is attached to outer membrane via 57-residue hydrophobic proteins • Outer membrane has a set of lipopolysaccharides attached to it; these sway outward from the membrane Biochemistry: Carbo2/Lipid1

  14. Gram-negative membranes and periplasmic space Figure courtesy Kenyon College microbiology Wiki Biochemistry: Carbo2/Lipid1

  15. Glycoproteins • 1-30 carbohydrate moieties per protein • Proteins can be enzymes, hormones, structural proteins, transport proteins • Microheterogeneity:same protein, different sugar combinations • Eight sugars common in eukaryotes • PTM glycosylation much more common in eukaryotes than prokaryotes Biochemistry: Carbo2/Lipid1

  16. Diversity in glycoproteins • Variety of sugar monomers •  or  glycosidic linkages • Linkages always at C-1 on one sugar but can be C-2,3,4,6 on the other one • Up to 4 branches • But:not all the specific glycosyltransferases you would need to get all this diversity exist in any one organism Biochemistry: Carbo2/Lipid1

  17. O-linked and N-linked oligosaccharides • Characteristic sugar moieties and attachment chemistries Biochemistry: Carbo2/Lipid1

  18. O-linked oligosaccharides(fig. 8.34) • GalNAc to Ser or Thr;often with gal or sialic acid on GalNAc • 5-hydroxylysines on collagen are joined to D-Gal • Some proteoglycans joined viaGal-Gal-Xyl-Ser • Single GlcNac on ser or thr Biochemistry: Carbo2/Lipid1

  19. N-linked oligosaccharides • Generally linked to Asn • Types: • High-mannose • Complex(Sialic acid, …) • Hybrid(Gal, GalNAc, Man) Diagram courtesy Oregon State U. Biochemistry: Carbo2/Lipid1

  20. Lipids • Hydrophobic biomolecules;most have at least one hydrophilic moiety as well • Attend to “periodic table of lipids”(next slide) • Functions • Membrane components • Energy-storage molecules • Structural roles • Hormonal and signaling roles Biochemistry: Carbo2/Lipid1

  21. Periodic table of lipids Biochemistry: Carbo2/Lipid1

  22. Fatty acids • Unbranched hydrocarbons with carboxylate moieties at one end • Usually (but not always) even # of C’s • Zero or more unsaturations: generally cis • Unsaturations rarely conjugated (why?) • Resting concentrations low because they could disrupt membranes saturated unsaturated Biochemistry: Carbo2/Lipid1

  23. Trans fatty acids • Not completely absent in biology • But enzymatic mechanisms for breakdown of cis fatty acids are much more fully developed • Trans fatty acids in foods derived from (cis-trans) isomerization that occurs during hydrogenation, which is performed to solidify plant-based triglycerides Biochemistry: Carbo2/Lipid1

  24. Fatty acids:melting points and structures • Longer chain  higher MPbecause longer ones align readily • More unsaturations  lower MP • Saturated fatty acids are entirely flexible;tend to be extended around other lipids • Unsaturations introduce inflexibilities and kinks Biochemistry: Carbo2/Lipid1

  25. Bacterial lipids Mostly C12-C18  1 unsaturation Plant lipids High concentration of unsaturated f.a.s Includes longer chains Animal lipds Somewhat higher concentrations of saturated f.a.’s Unsaturations four carbons from methyl group (omega f.a.) common in fish oils Sources for fatty acids Biochemistry: Carbo2/Lipid1

  26. Triglyceride composition by source • Courtesy Charles Ophardt, Elmhurst College Biochemistry: Carbo2/Lipid1

  27. Nomenclature for fatty acids • IUPAC names: hexadecanoic acid, etc. • Trivial names from sources (Table 8.1) • Laurate (dodecanoate) • Myristate (tetradecanoate) • Palmitate (hexadecanoate) • Palmitoleate (cis-9-hexadecenoate) • Oleate (cis-9-octadecenoate) • Linoleate (cis,cis-9,12-octadecadienoate) • Arachidonate(all cis-5,8,11,14-eicosatetraeneoate) Biochemistry: Carbo2/Lipid1

  28. Saturated Fatty Acids Contrast withmelting points of Unsaturated C18 FAs: 16ºC, -5ºC -11ºC;C20, 4 double bonds: -50ºC Biochemistry: Carbo2/Lipid1

  29. How fatty acids really appear • Almost always esterified or otherwise derivatized • Most common esterification is to glycerol • Note that glycerol is achiral but its derivatives are often chiral • Triacylglycerols; all three OHs on glycerol are esterified to fatty acids • Phospholipids: 3-OH esterified to phosphate or a phosphate derivative glycerol Biochemistry: Carbo2/Lipid1

  30. Triacylglycerols • Neutral lipids • R1,2,3 all aliphatic • Mixture of saturated & unsaturated; unsaturatedmore than half • Energy-storage molecules • Yield >2x energy/gram as proteins or carbohydrates, independent of the water-storage issue … • Lipids are stored anhydrously; carbohydrates & proteins aren’t Biochemistry: Carbo2/Lipid1

  31. Catabolism of triacylglycerol • Lipases break these molecules down by hydrolyzing the 3-O esters and 1-O esters • Occurs in presence of bile salts(amphipathic derivatives of cholesterol) • These are stored in fat droplets within cells, including specialized cells called adipocytes Biochemistry: Carbo2/Lipid1

  32. Glycerophospholipids • Also called phosphoglycerides • Primary lipid constituents of membranes in most organisms • Simplest: phosphatides (3’phosphoesters) • Of greater significance: compounds in which phosphate is esterified both to glycerol and to something else with an —OH group on it Biochemistry: Carbo2/Lipid1

  33. Categories of glycerophospholipids • Generally categorized first by the polar “head” group; secondarily by fatty acyl chains • Usually C-1 fatty acid is saturated • C-2 fatty acid is unsaturated • Think about structural consequences! Biochemistry: Carbo2/Lipid1

  34. Varieties of head groups • Variation on other phosphoester position • Ethanolamine (R1-4 = H) (—O—(CH2)2—NH3+) • Serine (R4 = COO-)(—O—CH2-CH-(COO-)—NH3+) • Methyl, dimethylethanolamine(—O—(CH2)2—NHm+(CH3)2-m) • Choline (R4=H, R1-3=CH3) (—O—(CH2)2—N(CH3)3+) • Glucose, glycerol . . . Biochemistry: Carbo2/Lipid1

  35. Chirality in common lipids • Fatty acyl chains themselves are generally achiral • Glycerol C2 is often chiral (unless C1 and C3 fatty acyl chains are identical) • Phospholipid polar groups are achiral except for phosphatidylserine and a few others Biochemistry: Carbo2/Lipid1

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