Carbohydrates II; Lipids I Andy Howard Introductory Biochemistry 28 February 2008

Carbohydrates II; Lipids

IAndy Howard

Introductory Biochemistry28 February 2008

Biochemistry: Lipids p. 2 of 26

What we’ll discuss Carbohydrates (concluded) Glycoconjugates Proteoglycans Peptidoglycans Glycoproteins

Lipids Lipid characteristics

Fatty acids Phospholipids

Lipids Glycosphingolipids Isoprenoids Steroids Other lipids

Membranes Bilayers Fluid mosaic model Physical properties

Lipid Rafts


Glycoconjugates Poly or oligosaccharidescovalently linkedto proteins or peptides

Generally heteroglycans Categories:

Proteoglycans (protein+glycosaminoglycans)

Peptidoglycans (peptide+polysaccharide)

Glycoproteins (protein+oligosaccharide)

Image courtesy Benzon Symposia


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


Proteoglycans in cartilage Highly hydrated, voluminous

Mesh structure (fig.8.29 or this fig. from Mathews & Van Holde)

Aggrecan is major proteoglycan

Typical of proteoglycans in that it’s extracellular


Peptidoglycans

Polysaccharides linked to small proteins

Featured in bacterial cell walls:alternating GlcNAc + MurNAc linked with -(14) linkages

Lysozyme hydrolyzes these polysaccharides

Peptide is species-specific: often contains D-amino acids


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(note misprint on p.244)

Fig. 8.31 shows multidimensionality of this wall


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


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


O-linked and N-linked oligosaccharides Characteristic sugar moieties and attachment chemistries


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


N-linked oligosaccharides

Generally linked to Asn

Types: High-mannose Complex(Sialic acid, …)

Hybrid(Gal, GalNAc, Man)

Diagram courtesy Oregon State U.


Lipids

Hydrophobic biomolecules;most have at least one hydrophilic moiety as well

Attend to fig. 9.1: periodic table of lipids

Functions Membrane components Energy-storage molecules Structural roles Hormonal and signaling roles


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


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


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


Sources for fatty acids

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


Triglyceride composition by source

Courtesy Charles Ophardt, Elmhurst College

Beef

Linoleic

Other

Oleic

Stearic

Palmitic

Soybean

Palmitic

Stearic

Oleic

Linoleic

Other


Nomenclature for fatty acids

IUPAC names: hexadecanoic acid, etc.

Trivial names from sources (Table 9.1) 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)


Saturated Fatty Acids

Melting points for saturated FAs

40

45

50

55

60

65

70

75

80

85

90

8 12 16 20 24 28

# of Carbons

Melting point, Deg C

Contrast withmelting points ofUnsaturated C18 FAs:16ºC, -5ºC -11ºC;C20, 4 double bonds: -50ºC


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


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


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


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


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!


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 . . .

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Carbohydrates II; Lipids I Andy Howard Introductory Biochemistry 28 February 2008