Transcript The structure of cellulose

1
References
• Lubert Stryer - Biochemistry
• Lehninger- Principles of Biochemistry
• Harper's -Illustrated Biochemistry
• Text Book of Biochemistry with clinical
correlation, T M. Devlin
‫ انتشارات‬،‫ دانشگاه علوم پزشکی تهران‬، ‫ هیئت مولفان‬،‫•بیوشیمی پزشکی‬
‫ دو جلد‬،‫آییژ‬
Carbohydrates
General characteristics


Compounds composed of C, H, and O
(CH2O)n when n = 5 then C5H10O5

Not all carbohydrates have this empirical formula:
Deoxysugars, Aminosugars

Carbohydrates are the most abundant compounds
found in nature (cellulose: 100 billion tons annually)
General characteristics

Most carbohydrates are found naturally in bound
form rather than as simple sugars (Glycoconjugates)
 Polysaccharides (starch, cellulose, inulin, gums),
Mucopolysaccharides (hyaluronic acid)
 Glycoproteins and proteoglycans (hormones, blood
group substances, antibodies)
 Glycolipids (cerebrosides, gangliosides)
 Nucleic acids (Ribose and desoxyribose)
Functions

Sources of energy (Glucose, Fructose,..)

Intermediates in the biosynthesis of other basic
biochemical entities (Fats and proteins)

Form structural tissues in plants and in
microorganisms (cellulose, lignin, murein)

Participate in cell-cell recognition, Cell-cell
adhesion
Classification of carbohydrates

Monosaccharides (monoses or glycoses)

Oligosaccharides (condensation products of two to ten
monosaccharides)
 Di, tri, tetra, penta, up to 9 or 10
 Most important are the disaccharides

Polysaccharides or glycans (more than 10)
 Homopolysaccharides
 Heteropolysaccharides
Classification of Monosaccharides
A. Number of carbons in chain
 Trioses (3C)
 tetroses (4C)
 pentoses (5C)
 hexoses (6C)
 heptoses (7C)
B. Aldose or Ketose


Aldoses (e.g., glucose) have an aldehyde group at one end
Ketoses (e.g., fructose) have a keto group, usually at C2
Aldose sugars
H
(H
C
O
C
OH)n
CH2OH
Aldose
H
H
H
H
C
O
C
OH
CH2OH
Aldotriose
n=1
C
O
H
C
OH
H
C
OH
CH2OH
Aldotetrose
n=2
H
C
O
H
C
OH
H
C
H
C
C
O
H
C
OH
OH
H
C
OH
OH
H
C
OH
H
C
OH
CH2OH
Aldopentose
n=3
CH2OH
Aldohexose
n=4
Ketose sugars
CH 2OH
(H
C
O
C
OH)n
CH 2OH
C
O
CH 2OH
CH 2OH
Ketose
H
C
O
C
OH
CH 2OH
Ketotriose
n=0
CH 2OH
CH 2OH
Ketotetrose
n=1
C
O
CH 2OH
H
C
OH
C
O
H
C
OH
C
OH
H
CH 2OH
H
Ketopentose
n=2
H
OH
C
OH
CH 2OH
Ketohexose
n=3
Sugars Exhibit Various Forms
of Isomerism
Aldose and Ketose isomers
L and D Isomers
D Form:
Most of the monosaccharides occurring in mammals are D
sugars, and the enzymes responsible for their metabolism are
specific for this configuration.
L form:
1. L-arabinose
2. L-idoronic acid
Enantiomers
Epimers
Two sugars that differ only in the configuration
around one carbon atom are called epimers;
Optical isomerism

Asymmetric compounds rotate plane polarized light
 In general, a molecule with n chiral centers can have
2n stereoisomers
 Of the 16 possible aldohexoses, eight are D forms and
eight are L
 Most of the hexoses of living organisms are D isomers.
POLARIMETRY
 Measurement of optical activity in chiral or asymmetric
molecules using plane polarized light
 Measurement uses an instrument called a polarimeter
(Lippich type)
 Rotation is either (+) dextrorotatory or (-) levorotatory
polarimetry
Magnitude of rotation depends upon:
1. The nature of the compound
2. The length of the tube (cell or sample container) usually
expressed in decimeters (dm)
3. The wavelength of the light source employed; usually
either sodium line at 589.3 nm or mercury vapor lamp at
546.1 nm
4. Temperature of sample
5. Concentration of analyte in grams per 100 ml
Polarimetry
[] T
D
 observed x 100
=
lxc
D = Na+ line
T = temperature oC
 obs : observed rotation in degree (specify solvent)
l = length of tube in decimeter
c = concentration in grams/100ml
[] = specific rotation
Specific rotation of various
carbohydrates at 20oC
D-glucose +52.7
 D-fructose -92.4
 D-galactose +80.2
 L-arabinose +104.5
 D-mannose +14.2
 D-arabinose -105.0
 D-xylose
+18.8
 Lactose
+55.4
 Sucrose
+66.5
 Maltose+
+130.4
 Invert sugar -19.8
 Dextrin
+195

Racemic mixture
 In chemistry, a racemic mixture, or racemate, is
one that has equal amounts of left- and righthanded enantiomers of a chiral molecule.
 A racemic mixture is denoted by the prefix (±)- or dl(for sugars the prefix DL- may be used), indicating an
equal (1:1) mixture of dextro and levo isomers
 A racemate is optically inactive
Tartaric acid
•
•
•
It occurs naturally in many plants, particularly
grapes, bananas
Add to foods to give a sour taste
Used as an antioxidant
levotartaric dextrotartaric
acid
acid
(D-(−)(L-(+)tartaric acid) tartaric acid)
mesotartaric acid
Structural representation of sugars

Fisher projection: straight chain representation

Haworth projection: simple ring in perspective

Conformational representation: chair and boat
configurations
Rules for drawing Haworth projections

draw either a six or 5-membered ring including
oxygen as one atom
O
Pyran
O
Furan

most aldohexoses are six-membered

Aldotetroses, Aldopentoses, ketohexoses are fivemembered
Rules for drawing Haworth projections

Next number the ring clockwise starting next to
the oxygen
5
O
1
4
3

O
2
1
4
3
2
If the substituent is to the right in the Fisher
projection, it will be drawn down in the Haworth
projection (Down-Right Rule)
Rules for drawing Haworth projections
 For
D-sugars the highest numbered carbon
(furthest from the carbonyl) is drawn up. For
L-sugars, it is drawn down
 For
D-sugars, the OH group at the anomeric
position is drawn down for α and up for β. For
L-sugars α is up and β is down
Formation of the two cyclic forms of D-glucose
Anomers:
Isomeric forms of monosaccharides that
differ only in the configuration about the
hemiacetal or hemiketal carbon atom are
called
Mutarotation:
interconvert of The  and b anomes of Dglucose in aqueous solution
<1%
Conformations:
interconvertible without the breakage of
covalent bonds,
configurations :
interconvertible only by breaking a covalent
bond—for example, in the case of α and β
configurations,
one-third
two-thirds
Condensation reactions: acetal and ketal formation
D-ribose and other five-carbon
saccharides can form either
furanose or pyranose structures
Chair and boat conformations of a pyranose sugar
2 possible chair conformations
of b-D-glucose
Envelope Conformations of β - D-ribose
Envelope Conformations
Oxidation reactions

Aldoses may be oxidized to 3 types of acids
◦ Aldonic acids: aldehyde group is converted to a carboxyl
group ( glucose – gluconic acid)
◦ Uronic acids: aldehyde is left intact and primary alcohol
at the other end is oxidized to COOH
 Glucose --- glucuronic acid
 Galactose --- galacturonic acid
◦ Saccharic acids: (glycaric acids) – oxidation at both
ends of monosaccharide)
 Glucose ---- saccharic acid
 Galactose --- mucic acid
 Mannose --- mannaric acid
Solutions of cupric ion (known as Fehling's solution)
provide a simple test for sugars such as glucose. Sugars
that react are called reducing sugars; those that do not
are called nonreducing sugars
Reduction






either done catalytically (hydrogen and a catalyst) or
enzymatically
the resultant product is a polyol or sugar alcohol
(alditol)
glucose form sorbitol (glucitol)
mannose forms mannitol
fructose forms a mixture of sorbitol
glyceraldehyde gives glycerol
Sructures of some sugar alcohols
used as a sugar substitute in
foods, especially for diabetics.
In cosmetics it is commonly
used in aftershave lotions, mild
soaps and baby shampoos.
Sorbitol is used as a humectant
and skin conditioning agent
Special monosaccharides: deoxy sugars

These are monosaccharides which lack one or more
hydroxyl groups on the molecule

one quite ubiquitous deoxy sugar is 2’-deoxy ribose
which is the sugar found in DNA

6-deoxy-L-mannose (L-rhamnose) is used as a
fermentative reagent in bacteriology
examples of deoxysugars
Several sugar esters important
in metabolism
Special monosaccharides: amino sugars
Constituents of mucopolysaccharides
The anomeric forms of
methyl-D-glucoside
Properties
Differences in structures of sugars are responsible for
variations in properties

Physical
 Crystalline form; solubility; rotatory power

Chemical
 Reactions (oxidations, reductions, condensations)

Physiological
 Nutritive value (human, bacterial); sweetness; absorption
Oligosaccharides

Most common are the disaccharides
 Sucrose, lactose, maltose, terehalose
 Maltose :
2 molecules of D-glucose
 Lactose : glucose + galactose
 Sucrose : glucose + fructose
 Terehalose : 2 molecules of D-glucose
Sucrose

α-D-glucopyranoside + β-D-fructofuranoside

commercially obtained from sugar cane or sugar beet

hydrolysis yield glucose and fructose (invert sugar)
(sucrose: +66.5o ; glucose +52.5o; fructose –92o)

used pharmaceutically to make syrups
Sources of sucrose
Sugar cane
Sugar beet
Structure of sucrose
Non reducing sugar
Lactose

β-D-galactose (β 1,4) α-D-glucose

used in infant formulations, medium for penicillin
production and as a diluent in pharmaceuticals
Reducing sugar
Maltose

2-glucose molecules joined via α(1,4) linkage

known as malt sugar

produced by the partial hydrolysis of starch (either
salivary amylase or pancreatic amylase)

used as a nutrient (malt extract; Hordeum vulgare); as
a sweetener and as a fermentative reagent
Structure of maltose
Glc(α1 4)Glc
reducing sugar
Trehalose
Non reducing sugar
Lactulose

Galactose-β-(1,4)-fructose

A semi-synthetic disaccharide (not naturally
occurring)

Not absorbed in the GI tract

Used either as a laxative

Metabolized in distal ileum and colon by bacteria to
lactic acid, formic acid and acetic acid (remove
ammonia)
Oligosaccharides

Trisaccharide: raffinose (glucose, galactose and
fructose)

Tetrasaccharide: stachyose (2 galactoses, glucose
and fructose)

Pentasaccharide: verbascose (3 galactoses, glucose
and fructose)
raffinose
Raffinose can be hydrolyzed to D-galactose and
sucrose by the enzyme α-galactosidase (α-GAL),
an enzyme not found in the human digestive
tract.
α-GAL also hydrolyzes other α-galactosides such
as stachyose, verbascose, and galactinol, if
present. The enzyme does not cleave β-linked
galactose, as in lactose.
 stachyose (2 galactoses, glucose and
fructose)
verbascose (3 galactoses, glucose and fructose)
Polysaccharides or glycans

homoglycans (starch, cellulose, glycogen, inulin)

heteroglycans (mucopolysaccharides)
 characteristics:
 polymers (MW from 200,000)
 White and amorphous products (glassy)
 not sweet
 not reducing; do not give the typical aldose or ketose
reactions)
 form colloidal solutions or suspensions
Homoglycans
Starch

most common storage polysaccharide in plants
composed of 10 – 30% a-amylose and 70-90%
amylopectin depending on the source


the chains are of varying length, having molecular
weights from several thousands to half a million
Amylose and amylopectin are the 2 forms of starch. Amylopectin
is a highly branched structure, with branches occurring every 12
to 30 residues
Amylose and amylopectin, the polysaccharides of starch
amylopectin
Strands of amylopectin form double helical
structures with each other or with amylose strands

Salivary amylase : α (1-4) endoglycosidase
G
 1-4 link
G
G
G
G
G
G
G
G
G
G
G
G
G
G
amylase
 1-6 link
G
G
G
G
G
G  Limit
G G
G
G G
G G
G
dextrins
maltotriose
G G
G
maltose
G
G
isomaltose
Dextrins

produced by the partial hydrolysis of starch along
with maltose and glucose

used as mucilages (glues)

also used in infant formulas (prevent the curdling of
milk in baby’s stomach)
Cellulose

Polymer of β-D-glucose attached β) linkages

Yields glucose upon complete hydrolysis

Partial hydrolysis yields cellobiose

Most abundant of all carbohydrates
 Cotton flax: 97-99% cellulose
 Wood: ~ 50% cellulose
The structure of cellulose
(linear, unbranched homopolysaccharide)
Structure of cellulose
Glycogen

also known as animal starch

stored in muscle and liver

present in cells as granules (high MW)

contains both a(1,4) links and a(1,6) branches at every
8 to 12 glucose unit

complete hydrolysis yields glucose

hydrolyzed by both a and b-amylases and by glycogen
phosphorylase
Inulin

β-(1,2) linked fructofuranoses

linear only; no branching
Jerusalem artichokes

lower molecular weight than starch

hydrolysis yields fructose

sources include onions, garlic, dandelions and jerusalem
artichokes

used as diagnostic agent for the evaluation of glomerular
filtration rate (renal function test)
Chitin



Chitin is the second most
abundant carbohydrate
polymer
Present in the cell wall of
fungi and in the
exoskeletons of crustaceans,
insects and spiders
Chitin is used commercially
in coatings (extends the
shelf life of fruits and
meats)
Heteropolysacharides
Glycosaminoglycans
(Mucopolysacharides)





they are the polysaccharide chains of proteoglycans
they are linked to the protein core via a serine or
threonine (O-linked)
the chains are linear (unbranched)
the glycosaminoglycan chains are long (over 100
monosaccharides)
they are composed of repeating disaccharides
Glycosaminoglycans
Involved in a variety of extracellular functions; chondroitin
is found in tendons, cartilage and other connective tissues
Glycosaminoglycans
A characteristic of glycosaminoglycans is the presence
of acidic functionalities (carboxylate and/or sulfates)
Glycosaminoglycans
Proteoglycans
are
glycosaminoglycan-containing
macromolecules of the cell surface and extracellular matrix
Proteoglycan structure
Some proteoglycans can form proteoglycan
aggregates
Aggrecan interacts strongly with
collagen in the extracellular
matrix of cartilage, contributing
to the development and tensile
strength of this connective tissue
Proteoglycan aggregate of the extracellular matrix
Proteoglycans are a family of
glycoproteins whose carbohydrate
moieties
are
predominantly
glycosaminoglycans
structures are quite diverse as are
sizes examples: versican, serglycin,
decorin, syndecan
Functions:
- modulate cell growth processes
- provide flexibility and resiliency
to cartilage
Hyaluronate:
material
used
to
cement the cells into a
tissue
Interactions between cells
and the extracellular matrix
Cross-linked meshwork
that gives the whole
extracellular matrix strength
and resilience
Bacterial cell wall

provide strength and rigidity for the organism

consists of a polypeptide-polysaccharide known as
petidoglycan or murein

determines the Gram staining characteristic of the
bacteria
Structure of peptidoglycan
Structure of peptidoglycan
Cell wall of Gram-positive bacteria
Gram-negative bacteria
Cross-section of the cell
wall of a gram-negative
organism such as E.coli
Lipopolysaccharide
Lipopolysaccharide (LPS) coats the
outer membrane of Gram-negative
bacteria.
the lipid portion of the LPS is embedded in
the outer membrane and is linked to a
complex polysaccharide
Glycoproteins (Mucoprotins)

Usually done as a post-translational process

Proteins can contain either O-linked oligosaccharides
or N-linked oligosaccharides
Serine or threonine O-linked saccharides
Aspargine N-linked glycoproteins
Roles of oligosaccharides in recognition and adhesion
at the cell surface
Complex structure (branching, stereoisomerism)
•Hexasaccharid: 1.05*1012, hexapeptide: 4096, hexanucleotide: 6.4*107
Clone, Expresion, PCR-amplification
Sequencing
synthesis
Lectins
Lectins are glycoproteins that recognize and
bind to specific oligosaccharides.
Recognition/binding
of
CHO
moieties
of
glycoproteins, glycolipids & proteoglycans by animal
lectins is a factor in:
 cell-cell recognition
 adhesion of cells to the extracellular matrix
 recognition of disease-causing microorganisms
 initiation and control of inflammation.
Glycoarray
Glycome:
• to describe the complete rang of
oligosaccharide sequences that can be
found in a cell type, organ or organism
Glycomics:
• study of glycome by glycoarray
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