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09/16/08Biochemistry: Carbohydrates 1
Carbohydrates
Andy HowardIntroductory Biochemistry, Fall 2008
16 September 2008
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Now we’ll study sugars!
Sugars are vital as energy sources, and they also serve as building blocks for lipid-carbohydrate and protein-carbohydrate complexes
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What we’ll discuss
Notes about upcoming midterm
Sugar Concepts Monosaccharides Oligosaccharides Glycosides
Polysaccharides Starch & glycogen Cellulose and chitin
Glycoconjugates Proteoglycans Peptidoglycans Glycoproteins
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Midterm is Tuesday 23 Sep
Internet students can take it between 9am Tuesday and 5pm Wednesday
Find a proctor or arrange to take it in class
Details about how the midterm works are in the Course Introduction document
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What the midterm will cover Everything up through today’s lecture Thursday’s lecture will be on the second
midterm Exam syllabus will be posted by the
weekend to help you study Exam help-sheet too (don’t memorize
what’s on the help sheet!) Yes, I curve these exams; but the grade
cutoffs are determined at the end of the course, not now
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Carbohydrates These are polyhydroxylated aldehydes and
ketones, many of which can exist in cyclic forms General monomeric formula (CH2O)m, 3 < m < 9 With one exception (dihydroxyacetone) they
contain chiral centers Highly soluble Can be oligomerized and polymerized Oligomers may or may not be soluble Most abundant organic molecules on the planet
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How do we measure solubility for very soluble compounds?
(Note: this is not a serious chemical topic: it’s an example of how statistics can be abused…)
The assertion is that, with highly soluble compounds like sugars, it’s difficult to use conventional approaches to compare their solubilities
The suggestion is that we might use the amount of time it takes to dissolve (for example) 50g of solute in 100mL of cold water: if it’s fast, the solute is more soluble than if it’s slow.
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Solubility measured by dissolution time
Assertion: more polar groups means shorter dissolution time for a given class of compounds
# of
pol
ar g
roup
s
Time required for dissolution
1
2
3
4
5
6
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What if we extrapolate to n=6?
We get a negative dissolution time!
That is, the solid goes into solution 6 seconds before we put it in the water!
This causes serious psychological problems (what if I change my mind?) and philosophical problems (is this pre-ordained?)
# of
pol
ar g
roup
s
Time required for dissolution
1
2
3
4
5
6
Observed points
Extrapolated point
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Whose idea is this? Isaac Asimov, that’s who! “The endochronic properties of resublimated
thiotimolene”:Astounding Science Fiction, March1948
My point: extrapolations and other misuses of statistics are dangerous
Benjamin Disraeli (popularized by Mark Twain):There are three kinds of untruth:lies, damn lies, and statistics.
Okay: let’s get back to the science.
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Aldoses & ketoses If the carbonyl moiety is at
the end carbon (conventionally counted as 1), it’s an aldose
If carbonyl is one carbon away (counted as 2), it’s a ketose
If it’s two or more carbons from the end of the chain, it’s not a sugar
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Simplest monosaccharides Glyceraldehyde and
dihydroxyacetone Only glyceraldehyde is chiral:
D-enantiomer is more plentiful in biosphere
All longer sugars can be regarded as being built up by adding-(CHOH)m-1 to either glyceraldehyde or dihydroxyacetone, just below C2
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How many aldoses are there? Every -(CHOH) in the interior offers one
chiral center An m-carbon aldose has (m-2) internal
-(CHOH) groups Therefore: 2m-2 aldoses of length m For m=3, that’s 21=2; for m=6, it’s 24=16.
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How many ketoses are there? Every -(CHOH) in the interior offers
one chiral center An m-carbon ketose has (m-3) internal
-(CHOH) groups Therefore: 2m-3 ketoses of length m For m=3, that’s 20 = 1; for m=6, that’s
23=8.
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Review: stereochemical nomenclature Stereoisomers: compounds with identical
covalent bonding apart from chiral connectivity Enantiomers: compounds for which the opposite
chirality applies at all chiral centers Epimers: compounds that differ in chirality at
exactly one chiral center One chiral center: enantiomers are epimers. > 1 chiral center: enantiomers are not epimers.
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Example: 2 chiral centers Chiral centers u,v; compounds A,B,C,D
Compound Stereo @ u
Stereo @ v
Enantio-morph of
Epimer of
A + + D B,C
B + - C A,D
C - + B A,D
D - - A B,C
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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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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 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 Hemiacetals and hemiketals are compounds that
have an –OH and an –OR group on the same carbon Cyclic monosaccharides are hemiacetals &
hemiketals 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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09/16/08 Biochemistry: Carbohydrates p. 46 of 66
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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09/16/08 Biochemistry: Carbohydrates p. 47 of 66
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 from
G-6-P G-1-P UDP-Glucose units
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Glycogen structure