09/22/2009 Biochem: Carbohydrates II Carbohydrates II Andy Howard Introductory Biochemistry, Fall 2009 22 September 2009.

09/22/2009Biochem: Carbohydrates II

Carbohydrates II

Andy HowardIntroductory Biochemistry,

Fall 2009 22 September 2009

09/22/2009

Biochem: Carbohydrates II p. 2 of 58

Sugars and polysaccharides

Sugars are vital as energy sources, and they also serve as building blocks for lipid-carbohydrate and protein-carbohydrate complexes

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Plans for Today Sugar Concepts

Monosaccharides Cyclization Reducing and nonreducing sugars

Sugar Derivatives

Oligosaccharides Glycosides

Polysaccharides Starch & glycogen

Cellulose and chitin

Glycoconjugates Proteoglycans Peptidoglycans Glycoproteins

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Sugar nomenclature All sugars with m ≤ 7 have specific names apart from their enantiomeric(L or D) designation,e.g. D-glucose, L-ribose.

The only 7-carbon sugar that routinely gets involved in metabolism is sedoheptulose, so we won’t try to articulate the names of the others

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Fischer projections

Convention for drawing open-chain monosaccharides

If the hydroxyl comes off counterclockwise relative to the previous carbon, we draw it to the left;

Clockwise to the right.

Emil Fischer

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Cyclic sugars Sugars with at least four carbons can readily interconvert between the open-chain forms we have drawn and five-membered(furanose) or six-membered (pyranose) ring forms in which the carbonyl oxygen becomes part of the ring

There are no C=O bonds in the ring forms

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Furanoses Formally derived from structure of furan

Hydroxyls hang off of the ring; stereochemistry preserved there

Extra carbons come off at 2 and 5 positions

3

2

1

4

5

furan

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Pyranoses Formally derived from structure of pyran

Hydroxyls hang off of the ring; stereochemistry preserved there

Extra carbons come off at 2 and 6 positions

3

2

4

5

1

6

pyran

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How do we cyclize a sugar?

Formation of an internal hemiacetal or hemiketal (see a few slides from here) by conversion of the carbonyl oxygen to a ring oxygen

Not a net oxidation or reduction;in fact it’s a true isomerization.

The molecular formula for the cyclized form is the same as the open chain form

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Family tree of aldoses Simplest: D-, L- glyceraldehyde (C3) Add —CHOH: D,L-threose, erythrose (C4) Add —CHOH:D,L- lyxose, xylose, arabinose, ribose (C5)

Add —CHOH:D,L-talose, galactose, idose, gulose,mannose, glucose, altrose, allose (C6)

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Family tree of ketoses Simplest: dihydroxyacetone (C3) Add —CHOH: D,L-erythrulose (C4) Add —CHOH:D,L- ribulose, xylulose (C5)

Add —CHOH:D,L-sorbose, tagatose, fructose, psicose (C6)

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Haworth projections

…provide a way of keeping track the chiral centers in a cyclic sugar, as the Fischer projections enable for straight-chain sugars

Sir Walter Haworth

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The anomeric carbon In any cyclic sugar (monosaccharide, or single unit of an oligosaccharide, or polysaccharide) there is one carbon that has covalent bonds to two different oxygen atoms

We describe this carbon as the anomeric carbon

C

O

O

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iClicker quiz, question 1

Which of these is a furanose sugar?

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iClicker quiz, question 2 Which carbon is the anomeric carbon in this sugar?

(a) 1 (b) 2 (c) 5 (d) 6 (e) none of these.

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iClicker, question 3 How many 7-carbon D-ketoses are there?

(a) none. (b) 4 (c) 8 (d) 16 (e) 32

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-D-glucopyranose

One of 2 possible pyranose forms of D-glucose

There are two because the anomeric carbon itself becomes chiral when we cyclize

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-D-glucopyranose

Differs from -D-gluco-pyranose only at anomeric carbon

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Count carefully!

It’s tempting to think that hexoses are pyranoses and pentoses are furanoses;

But that’s not always true The ring always contains an oxygen, so even a pentose can form a pyranose

In solution: pyranose, furanose, open-chain forms are all present

Percentages depend on the sugar

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Substituted monosaccharides

Substitutions on the various positions retain some sugar-like character

Some substituted monosaccharides are building blocks of polysaccharides

Amination, acetylamination, carboxylation common

O

OH

HO

HO

HNCOCH3

OH

OHO

HO

HOOH

O O-

GlcNAcD-glucuronic acid(GlcUA)

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Sugar acids (fig. 7.10) Gluconic acid: glucose carboxylated @ 1 position In equilibrium with lactone form

Glucuronic acid:glucose carboxylated @ 6 position

Glucaric acid:glucose carboxylated @ 1 and 6 positions

Iduronic acid: idose carboxylated @ 6

D--gluconolactone

1

2

5

3

4

6

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Sugar alcohols (fig.7.11)

Mild reduction of sugars convert aldehyde moiety to alcohol

Generates an additional asymmetric center in ketoses except dihyroxyacetone

These remain in open-chain forms Smallest: glycerol Sorbitol, myo-inositol, ribitol are important

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Sugar esters (fig. 7.13) Phosphate esters of sugars are significant metabolic intermediates

5’ position on ribose is phosphorylated in nucleotides

Glucose 6-phosphate

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Amino sugars

Hydroxyl at 2- position of hexoses is replaced with an amine group

Amine is often acetylated (CH3C=O)

These aminated sugars are found in many polysaccharides and glycoproteins

O

OH

HO

HO

HNCOCH3

OHGlcNAc

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Hemiacetals and hemiketals

Hemiacetals and hemiketals are compounds that have an –OH and an –OR group on the same carbon

Cyclic monosaccharides are hemiacetals & hemiketals

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Acetals and ketals Acetals and ketals have two —OR groups on a single carbon

Acetals and ketals are found in glycosidic bonds

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Oligosaccharides and other glycosides A glycoside is any compound in which the hydroxyl group of the anomeric carbon is replaced via condensation with an alcohol, an amine, or a thiol

All oligosaccharides are glycosides, but so are a lot of monomeric sugar derivatives, like nucleosides

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Sucrose: a glycoside

A disaccharide Linkage is between anomeric carbons of contributing monosaccharides, which are glucose and fructose

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Other disaccharides

Maltose glc-glc with -glycosidic bond from left-hand glc

Produced in brewing, malted milk, etc. Cellobiose

-glc-glc Breakdown product from cellulose

Lactose: -gal-glc Milk sugar Lactose intolerance caused by absence of enzyme capable of hydrolyzing this glycoside

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Reducing sugars

Sugars that can undergo ring-opening to form the open-chain aldehyde compounds that can be oxidized to carboxylic acids

We describe those as reducing sugars because they can reduce metal ions or amino acids in the presence of base

Benedict’s test:2Cu2+ + RCH=O + 5OH- Cu2O + RCOO- + 3H2O

Cuprous oxide is red and insoluble

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Ketoses are reducing sugars In presence of base a ketose can spontaneously rearrange to an aldose via an enediol intermediate, and then the aldose can be oxidized.

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Sucrose: not a reducing sugar Both anomeric carbons

are involved in the glycosidic bond, so they can’t rearrange or open up, so it can’t be oxidized

Bottom line: only sugars in which the anomeric carbon is free are reducing sugars

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Reducing & nonreducing ends Typically, oligo and polysaccharides have a reducing end and a nonreducing end

Non-reducing end is the sugar moiety whose anomeric carbon is involved in the glycosidic bond

Reducing end is sugar whose anomeric carbon is free to open up and oxidize

Enzymatic lengthening and degradation of polysaccharides occurs at nonreducing end or ends

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Nucleosides Anomeric carbon of ribose (or deoxyribose) is linked to nitrogen of RNA (or DNA) base (A,C,G,T,U)

Generally ribose is in furanose form

This is an example of an N-glycoside

Diagram courtesy of World of Molecules

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Polysaccharides

Homoglycans: all building blocks same

Heteroglycans: more than one kind of building block

No equivalent of genetic code for carbohydrates, so long ones will be heterogeneous in length and branching, and maybe even in monomer identity

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Categories of polysaccharides Storage homoglycans (all Glc)

Starch: amylose ((14)Glc) , amylopectin

Glycogen Structural homoglycans

Cellulose ((14)Glc) Chitin ((14)GlcNAc)

Heteroglycans Glycosaminoglycans (disacch.units) Hyaluronic acid (GlcUA,GlcNAc)((1 3,4))

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Storage polysaccharides

Available sources of glucose for energy and carbon

Long-chain polymers of glucose Starch (amylose and amylopectin):in plants, it’s stored in 3-100 µm granules

Glycogen Branches found in all but amylose

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Amylose Unbranched, -14 linkages Typically 100-1000 residues Not soluble but can form hydrated micelles and may be helical

Amylases hydrolyze -14 linkages

Diagram courtesyLangara College

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Amylopectin Mostly -14 linkages; 4% -16

Each sidechain has 15-25 glucose moieties

-16 linkages broken down by debranching enzymes

300-6000 total glucose units per amylopectin molecule

One reducing end, many nonreducing ends

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Glycogen Principal storage form of glucose in human liver; some in muscle

Branched (-14 + a few -16) More branches (~10%) Larger than starch: 50000 glucose One reducing end, many nonreducing ends

Broken down to G-1-P units Built up fromG-6-P G-1-P UDP-Glucose units

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Glycogen structure

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Structural polysaccharides I

Insoluble compounds designed to provide strength and rigidity

Cellulose: glucose -14 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 -14 linkages:exoskeletons, cell walls (fig. 7.26)

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Structural polysaccharides II

Alginates: poly(-D-mannuronate),poly(-L-guluronate), linked 14 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

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Glycoconjugates Poly or oligosaccharidescovalently linkedto proteins or peptides

Generally heteroglycans Categories:

Proteoglycans (protein+glycosaminoglycans)

Peptidoglycans (peptide+polysaccharide)

Glycoproteins (protein+oligosaccharide)

Image courtesy Benzon Symposia

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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

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Proteoglycans in cartilage Highly hydrated, voluminous

Mesh structure (fig.7.36 or this fig. from Mathews & Van Holde)

Aggrecan is major proteoglycan

Typical of proteoglycans in that it’s extracellular

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Peptidoglycans(G&G fig. 7.29) Polysaccharides linked to small

proteins Featured in bacterial cell walls:alternating GlcNAc + MurNAclinked with -(14) linkages

Lysozyme hydrolyzes these polysaccharides

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

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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. 7.30 shows multidimensionality of these walls

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Peptide component(G&G fig. 7.29)

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

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Gram-negative bacteria:the periplasmic space(G&G fig. 7.30b, 7.31)

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

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Gram-negative membranes and periplasmic space

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Figure courtesy Kenyon College microbiology Wiki

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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

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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

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

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O-linked oligosaccharides(fig. fig 7.32a, 7.33 in G&G) 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

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N-linked oligosaccharides (fig. 7.32b,c in G&G) Generally linked to Asn

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

Hybrid(Gal, GalNAc, Man)

Diagram courtesy Oregon State U.

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iClicker question 4

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.

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iClicker question 5 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.

09/22/2009 Biochem: Carbohydrates II Carbohydrates II Andy Howard Introductory Biochemistry, Fall 2009 22 September 2009.

Documents

furan slide

pyran slide

emil fischer slide

open chain form slide

cyclic sugars sugars

psicose c6 slide

allose c6 slide

d designation