Transcript Enzyme Mechanisms - Illinois Institute of Technology

Carbohydrates
Andy Howard
Introductory Biochemistry, Fall 2008
16 September 2008
Biochemistry: Carbohydrates
1
09/16/08
Now we’ll study sugars!
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Sugars are vital as energy sources,
and they also serve as building
blocks for lipid-carbohydrate and
protein-carbohydrate complexes
09/16/08 Biochemistry: Carbohydrates
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What we’ll discuss
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Notes about
upcoming midterm
Sugar Concepts
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Polysaccharides
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Starch & glycogen
Cellulose and chitin
Monosaccharides  Glycoconjugates
 Proteoglycans
Oligosaccharides
 Peptidoglycans
Glycosides
 Glycoproteins
09/16/08 Biochemistry: Carbohydrates
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Midterm is Tuesday 23 Sep
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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
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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
09/16/08 Biochemistry: Carbohydrates
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Carbohydrates
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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
09/16/08 Biochemistry: Carbohydrates
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How do we measure solubility
for very soluble compounds?
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(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.
09/16/08 Biochemistry: Carbohydrates
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Solubility measured by
dissolution time
6
5
4
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Assertion: more polar
groups means shorter
dissolution time for a given
class of compounds
3
2
1
Time required for dissolution
09/16/08 Biochemistry: Carbohydrates
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What if we
extrapolate
to n=6?
Extrapolated
6
point
5
4
3
2
1
Observed
points
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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?)
Time required for dissolution
09/16/08 Biochemistry: Carbohydrates
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Whose idea is this?
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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.
09/16/08 Biochemistry: Carbohydrates
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Aldoses & ketoses
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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
09/16/08 Biochemistry: Carbohydrates
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Simplest monosaccharides
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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
09/16/08 Biochemistry: Carbohydrates
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How many aldoses are there?
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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.
09/16/08 Biochemistry: Carbohydrates
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How many ketoses are there?
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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.
09/16/08 Biochemistry: Carbohydrates
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Review: stereochemical
nomenclature
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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.
09/16/08 Biochemistry: Carbohydrates
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Example: 2 chiral centers
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Chiral centers u,v; compounds A,B,C,D
Compound
Stereo
@u
Stereo
@v
Enantio- Epimer
morph of of
A
+
+
D
B,C
B
+
-
C
A,D
C
-
+
B
A,D
D
-
-
A
B,C
09/16/08 Biochemistry: Carbohydrates
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Properties
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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
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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
09/16/08 Biochemistry: Carbohydrates
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Fischer projections
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Convention for drawing openchain monosaccharides
If the hydroxyl comes off
counterclockwise relative to
the previous carbon, we draw
it to the left;
Clockwise to the right.
09/16/08 Biochemistry: Carbohydrates
Emil
Fischer
p. 19 of 66
Cyclic sugars
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Sugars with at least four carbons can
readily interconvert between the openchain forms we have drawn and fivemembered(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
09/16/08 Biochemistry: Carbohydrates
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Furanoses
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Formally derived from
structure of furan
Hydroxyls hang off of the
ring; stereochemistry
preserved there
Extra carbons come off at 2
and 5 positions
09/16/08 Biochemistry: Carbohydrates
1
5
2
4
3
furan
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1
Pyranoses
6
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Formally derived from
structure of pyran
Hydroxyls hang off of the
ring; stereochemistry
preserved there
Extra carbons come off at 2
and 6 positions
09/16/08 Biochemistry: Carbohydrates
2
3
5
4
pyran
p. 22 of 66
How do we cyclize a sugar?
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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
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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)
09/16/08 Biochemistry: Carbohydrates
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Family tree of ketoses
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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)
09/16/08 Biochemistry: Carbohydrates
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Haworth projections
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…provide a way of
keeping track the chiral
centers in a cyclic sugar,
as the Fischer
projections enable for
straight-chain sugars
09/16/08 Biochemistry: Carbohydrates
Sir Walter
Haworth
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O
The anomeric carbon
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C
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
09/16/08 Biochemistry: Carbohydrates
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O
iClicker quiz, question 1
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Which of these is a furanose sugar?
09/16/08 Biochemistry: Carbohydrates
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iClicker quiz, question 2
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Which carbon is the anomeric
carbon in this sugar?
(a) 1
(b) 2
(c) 5
(d) 6
(e) none of these.
09/16/08 Biochemistry: Carbohydrates
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iClicker, question 3
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How many 7-carbon D-ketoses are
there?
(a) none.
(b) 4
(c) 8
(d) 16
(e) 32
09/16/08 Biochemistry: Carbohydrates
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a-Dglucopyranose
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One of 2 possible
pyranose forms of Dglucose
There are two
because the anomeric
carbon itself becomes
chiral when we
cyclize
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b-Dglucopyranose
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Differs from aD-glucopyranose only
at anomeric
carbon
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Count carefully!
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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, openchain forms are all present
Percentages depend on the sugar
09/16/08 Biochemistry: Carbohydrates
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Substituted monosaccharides
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Substitutions on the various positions
retain some sugar-like character
Some substituted monosaccharides are
building blocks of polysaccharides
Amination, acetylamination,
carboxylation common
O
OOH
HO
HO
O
OH
GlcNAc HNCOCH
3
HO
HO
D-glucuronic acid
HO
(GlcUA)
09/16/08 Biochemistry: Carbohydrates
O
OH
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6
Sugar acids
(fig. 7.10)
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4
Gluconic acid:
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5
1
3
D--gluconolactone
2
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
09/16/08 Biochemistry: Carbohydrates
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Sugar alcohols (fig.7.11)
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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)
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Phosphate esters of
sugars are significant
metabolic intermediates
5’ position on ribose is
phosphorylated in
nucleotides
09/16/08 Biochemistry: Carbohydrates
Glucose 6phosphate
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OH
Amino sugars
HO
HO
GlcNAc
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O
OH
HNCOCH3
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
09/16/08 Biochemistry: Carbohydrates
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Acetals and ketals
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Hemiacetals and hemiketals are compounds that
have an –OH and an –OR group on the same carbon
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Cyclic monosaccharides are hemiacetals &
hemiketals
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Acetals and ketals have two —OR groups on a single
carbon
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Acetals and ketals are found in glycosidic bonds
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Oligosaccharides and other
glycosides
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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
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A disaccharide
Linkage is between
anomeric carbons of
contributing
monosaccharides,
which are glucose
and fructose
09/16/08 Biochemistry: Carbohydrates
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Other disaccharides
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Maltose
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Cellobiose
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glc-glc with a-glycosidic bond from left-hand glc
Produced in brewing, malted milk, etc.
b-glc-glc
Breakdown product from cellulose
Lactose: b-gal-glc
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Milk sugar
Lactose intolerance caused by absence of
enzyme capable of hydrolyzing this glycoside
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Reducing sugars
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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
09/16/08 Biochemistry: Carbohydrates
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Ketoses are reducing sugars
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In presence of base a ketose can
spontaneously rearrange to an aldose
via an enediol intermediate, and then
the aldose can be oxidized.
09/16/08 Biochemistry: Carbohydrates
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Sucrose: not a reducing sugar
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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
09/16/08 Biochemistry: Carbohydrates
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Reducing & nonreducing ends
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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
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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
09/16/08 Biochemistry: Carbohydrates
Diagram courtesy of
World of Molecules
p. 47 of 66
Polysaccharides
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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
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Storage homoglycans (all Glc)
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Structural homoglycans
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Starch: amylose (a(14)Glc) , amylopectin
Glycogen
Cellulose (b(14)Glc)
Chitin (b(14)GlcNac)
Heteroglycans
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Glycosaminoglycans (disacch.units)
Hyaluronic acid (GlcUA,GlcNAc)(b(1  3,4))
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Storage polysaccharides
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Available sources of glucose for energy
and carbon
Long-chain polymers of glucose
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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
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Unbranched, a-14 linkages
Typically 100-1000 residues
Not soluble but can form hydrated
micelles and may be helical
Amylases hydrolyze a-14 linkages
Diagram courtesy
Langara College
09/16/08 Biochemistry: Carbohydrates
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Amylopectin
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Mostly a-14 linkages; 4% a-16
Each sidechain has 15-25 glucose
moieties
a-16 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
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Principal storage form of glucose in
human liver; some in muscle
Branched (a-14 + a few a-16)
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
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