09/16/2010 biochem: carbohydrates ii carbohydrates ii andy howard introductory biochemistry, fall...

09/16/2010Biochem: Carbohydrates II

Carbohydrates II

Andy HowardIntroductory Biochemistry,

Fall 2010 16 September 2010

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Mono-, oligo- and polysaccharides

These are the most abundant organic molecules on the planet, and they act as metabolites, components of complexes, and structural entities

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What we’ll discuss

Details of monosaccharide nomenclature

Cyclic sugars Sugar derivatives

Glycosides

Polysaccharides Starch & glycogen

Cellulose & chitin

Glycoconjugates Proteoglycans Peptidoglycans Glycoproteins

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Monosaccharide structures Remember that there is just one 3-carbon ketose and two 3-carbon aldoses

Addition of each –CHOH group gives us one more chiral center

Unique names for each enantiomorphic monosaccharide

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Properties

Enantiomers have identical physical properties (MP,BP, solubility, surface tension…) except when they interact with other chiral molecules

(Note!: water isn’t chiral!) Stereoisomers that aren’t enantiomers can have different properties; therefore, they’re often given different names

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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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D-aldose family tree

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D-ketose family tree

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How many of these are important?

D-sugars are more prevalent than L-sugars

3-, 5-, and 6-carbon sugars are the most important, but 4’s and 7’s play roles

Some 5’s and 6’s are obscure Glucose, ribose, fructose, glyceraldehyde play more important roles than the others

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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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Hemiacetals & 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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How do we cyclize a sugar? Formation of an internal hemiacetal or hemiketal (see previous slide) 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

Very low energy barriers between open-chain form and various cyclic 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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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

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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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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Why does this matter?

Partly historical: this cuprate reaction was one of the first well-characterized tools for characterizing these otherwise very similar compounds

But it also gives us a convenient way of distinguishing among types of glycosidic arrangements, even if we never really use Cu2+ ions in experiments

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Glycosides

Glycosides are covalent conjugates of a sugar with another species

Generally involve replacement of a sugar –OH group with a moiety that begins with an oxygen or a nitrogen

We describe them as N-linked and O-linked glycosides

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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