Download Chapter 7 - Chemistry

Biochemistry
Chapter 7
Carbohydrates
Biochemical Substances
biochemistry
- study of the chemical substances found in living organisms and
the chemical interactions of these substances with each other
biochemical substance
- chemical substance found within a living organism
- divided into two groups:
i) bioinorganic substances
- includes water and inorganic salts
ii) bioorganic substances
- includes carbohydrates, lipids, proteins and nucleic
acids
Occurrence and Functions of
Carbohydrates
carbohydrates
- most abundant class of bioinorganic molecules on planet
- abundance in human body relatively low, yet carbohydrates
constitute about 75% by mass of dry plant materials
- green (chlorophyll-containing) plants produce carbohydrates
via photosynthesis
Chlorophyll
CO2 + H2O + solar energy
carbohydrates + O2
Plant enzymes
Fig. 7.1 (p.221)
Mass consumption data for the human body in terms
of major types of biochemical substances.
Plants have two main uses for carbohydrates they produce:
i) in form of cellulose
- carbohydrates serve as structural elements
ii) in form of starch
- provide energy reserves for plants
- dietary intake of plant materials is major carbohydrate source
for humans and animals
- Carbohydrates have following functions in humans:
1) carbohydrate oxidation provides energy
2) carbohydrate storage, in form of glycogen, provides shortterm energy reserve
3) carbohydrates supply carbon atoms for synthesis of other
biochemical substances (proteins, lipids and nucleic acids)
4) carbohydrates form part of structural framework of DNA
and RNA molecules
5) carbohydrates linked to lipids and structural components of cell
membranes
6) carbohydrates linked to proteins function in variety of cell-cell
and cell-molecule recognition processes
Classification of Carbohydrates
- simple carbohydrates have empirical formula that fits general
formula CnH2nOn [may see this written as Cn(H2O)n]
Carbohydrate
- a polyhydroxy aldehyde, a polyhydroxy ketone, or a cmpd that
yields polyhydroxy aldehydes or ketones upon hydrolysis
Note: carbohydrates have large number of functional groups
1
Classified on basis of molecular size:
i) Monosaccharide
- carbohydrate that contains single polyhydroxy
aldehyde or polyhydroxy ketone unit
- cannot be broken down into simpler units by hydrolysis
- naturally occurring monosaccharides have 3-7 C atoms
with 5-6 C species being most common
- pure monosaccharides are water-soluble, white,
crystalline solids
ii) Oligosaccharide
- carbohydrate containing 2-10 monosaccharide units
covalently bonded to each other
- most common are disaccharides
- types of carbohydrates are related to each other through
hydrolysis:
polysaccharides
hydrolysis
disaccharide
- carbohydrate that contains two monosaccharide units
covalently bonded to each other
- crystalline, water-soluble substances
Note: oligosaccharides often found associated with proteins and
lipids in complexes that have both structural and regulatory
functions
- free oligosaccharides, other than disaccharides, seldom
encountered in biochemical systems
- complete hydrolysis of oligosaccharides produces
monosaccharides
iii) Polysaccharide
- polymeric carbohydrate that contains many
monosaccharide units covalently bonded to each other
Chirality: Handed in Molecules
- most monosaccharides have important general property called
handedness
- exist in two forms: left-hand form and right-hand form
which are related to one another as mirror images
oligosaccharides
Fig. 7.3 (p.223)
The mirror image of
the right hand is the
left hand.
Conversely, the
mirror image of the
left hand is the right
hand.
hydrolysis
monosaccharides
mirror image - the reflection of an object in a mirror
Objects divided into two classes on basis of mirror image:
i) objects with superimposable mirror images
- images that coincide at all points when the images are
laid upon each other
ii) objects with nonsuperimposable mirror images
- images where not all points coincides when the images
are laid upon each other
Fig. 7.4 (p. 223)
A person s left and
right hands are not
superimposable upon
each other.
Chirality
- What is the molecular structural feature that generates
handedness ?
- any organic molecule that contains a C atom with four
different group attached to it in a tetrahedral orientation
possesses handedness or has a chiral center
chiral center
- an atom in a molecule that has four different group
tetrahedrally bonded to it
chiral molecule
- molecule whose mirror images are not superimposable
- contains a chiral center and has handedness
achiral molecule
- molecule whose mirror images are superimposable
- do not possess handedness
2
Chirality
bromochloroiodomethane
glyceraldehyde
Why is handedness important?
- in human body chemistry, right-handed and left-handed form
of molecule often elicit different responses with in body
Possibilities:
i) sometimes both are biologically active, each form giving
different response
ii) sometimes both elicit same response but one form s responses
is many times greater than that of the other
iii) sometimes only one of two forms is biochemically active
Fig. 7.5 (p.224)
Examples of simple molecules that are chiral.
Stereoiosmerism: Enantiomers and Diastereomers
- left- and right-handed forms of chiral molecules are isomers
stereoisomers
- isomers that have same molecular and structural formulas but
differ in the orientation of atoms in space
- structural features that generate stereoisomerism:
1) presence of a chiral center in a molecule
2) presence of structural rigidity in a molecule
- caused by restricted rotation about chemical bonds
Two types of stereoisomers:
i) enantiomers
- stereoisomers whose molecules are nonsuperimposable
mirror images of each other
ii) diastereomers
- stereoisomers whose molecules are not mirror images of
each other
Stereoisomerism
Fig. 7.7 (p.226)
The thought process used in classifying molecules as
enantiomers or diastereomers.
Stereoisomerism
Fig. 7.6 (p.226)
Enantiomers are stereoisomers whose molecules are
nonsuperimposable mirror images of each other.
Designating Handedness Using Fischer
Projections
Fischer projection
- a two-dimensional structural notation for showing the spatial
arrangement of groups about chiral centers in molecules
- chiral center represented as intersection of vertical and
horizontal lines
- atom at chiral center, which is almost always C, is not explicitly
shown
- tetrahedral arrangement of four groups attached to atom a
chiral center is governed by the following conventions:
1) vertical line from chiral center represent bonds to groups
directed into the printed page
2) horizontal lines from chiral cetner represent bonds to groups
directed out of printed page
3
- for monosaccharides, carbon chain positioned vertically with
carbonyl group (aldehyde or ketone) at or near top
Consider glyceraldehyde:
- two enantiomers with handedness (left or right) specified
using D and L
i) enantiomer with chiral center OH group on right is
defined as the right-handed isomer (designated D from
Latin dextro which means right )
ii) enantiomer with chiral center OH group on left is
defined as the left-handed isomer (designated L from
Latin levo which means left )
Properties of Enantiomers
Constitutional isomers
- differ in most chemical & physical properties
Diastereomers
- differ is most chemical and physical properties
Enantiomers
- nearly all properties are the same
- exhibit different properties in only two areas:
i) interaction with plane-polarized light
ii) interaction with other chiral molecules
Plane-polarized light
Fig. 7.10 (p.232)
Instruments used to measure degree to
rotation of plane-polarized light by
enantiomeric cmpds are called
polarimeters. Schematic depiction of
how a polarimeter works.
Fig. 7.9 (p.232)
Vibrational characteristics of
ordinary and polarized light.
Carbohydrates
Constitutional
isomerism and
stereoisomerism
(p.231)
Enantiomers and plane-polarized light
- ordinary light waves (unpolarized light waves) vibrate in all
planes at right angles to their direction of travel
- plane-polarized light waves, vibrate in only one plane at right
angles to their direction of travel
- ordinary light converted to plane-polarized light by passing it
through polarizer
- when plane-polarized light passed through sol n containing
single enantiomer , plane of polarized light is rotated
counterclockwise (left) or clockwise (right) depending on
enantiomer
- extent of rotation depends on concentration of enatiomer and
its identity
- the two enantiomers of pair rotate plane-polarized light same
number of degrees, but in opposite directions
Dextrorotatory and Levorotatory
optically active cmpds
- cmpds that rotate the plane of polarized light (includes
enantiomers)
Note: achiral molecules optically inactive
chiral molecules optically active
dextrorotatory cmpd; (+) (Latin dextro means right )
- chiral cmpd that rotates the plane of polarized light in
clockwise direction
levorotatory cmpd; (-) (Latin levo means left )
- chiral cmpd that rotates the plane of polarized light in
counterclockwise direction
Note: - if one member of enantiomeric pair is dextrorotatory, the
other member must be levorotatory (optical isomers)
- handedness of enantiomers (D or L) and direction of
rotation of plane-polarized light by enantiomers (+ or -) are
not connected
4
Interactions between Chiral Compounds
Two members of an enantiomeric pair have same interactions with
achiral molecules and different interactions with chiral molecules
Observations:
1) Enantiomers have identical boiling points, freezing points, and
densities b/c these properties depend on strength of
intermolecular forces (which does not depend on chirality).
Intermolecular forces are same for both forms of chiral
molecule b/c both forms have identical sets of functional groups
2) Pair of enantiomers have same solubility in achiral solvent but
differing solubilities in chiral solvent
3) Rate and extent of rxn of enantiomers with another reactant
are same if reactant is achiral but differ if reactant is chiral
4) Receptor sites for molecules within body have chirality
associated with them. Enantiomers always generate different
responses with human body as they interact at such sites.
Interactions between Chiral Compounds
Fig. 7.12 (p. 233)
Epinephrine binds to the receptor at three points.
- monosaccharides often called sugars
- hexoses are 6-carbon sugars, pentoses are 5-carbon sugars, etc.
Note: sugar general designation for either monosaccharide or
disaccharide
- associated with sweetness with most
monosaccharides (but not all) and many
disaccharides (but not all) having sweet taste
- trioses are smallest monosaccharides that can exist:
i) one is an aldose (glyceraldehyde)
reference cmpds for all
ii) one is a ketose (dihydroxyacetone)
ketoses and aldoses
Note: major difference btw these two substances is that
dihydroxyacetone does not posses a chiral carbon; this
means that D and L forms are not possible
- ie. ketotetroses, ketopentoses and ketohexoses have half
the number of possible stereoisomers
Chiral-chiral interactions involving enantiomers in human body:
1) Taste perception
- distinctly different natural flavors of spearment (Lcarvone) and caraway (D-carvone) are generated by
molecules that are enantiomers interacting with chiral
taste receptors
2) body s response to hormone epinephrine (adrenaline)
- response to D isomer is 20X greater than response to
L isomer
- binding of D- epinephrine to cellular receptor site yields a
perfect 3-point contact yet L-epinephrine yields only a
2-point contact
- poorer fitting of L isomer lends a poorer physiological
response
Classification of Monosaccharides
- no limit to number of C atoms in monosaccharide, but only
monosaccharides with 3 to 7 C atoms are commonly found in
nature
triose 3-carbon monosaccharide
tetrose 4-carbon monosaccharide
pentose 5-carbon monosaccharide
hexose 6-carbon monosaccharide
Classification of monosaccharides
- classified by both their number of C atoms and their
functional group
i) aldose
- monosaccharide that contains a aldehyde
functional group
ii) ketose
- monosaccharide that contains a ketone functional
group
D-Aldoses
Fig. 7.13 (p. 236)
Fischer projections and common names for Daldoses three, four, five, and six carbon atoms.
5
Fig 7.13
- number of possible aldoses doubles each time additional
carbon atom is added since new carbon atom is chiral center
- chiral center farthest from aldehyde group determines D or L
designation for aldose
- configurations about other chiral centers present are
accounted for by assigning a different common name to each
set of D and L enantiomers
Note: only D-isomers shown in Fig 7.13; L isomers are mirror
images
Fig. 7.14
- projection formulas and common names of D forms of ketoses
Biochemically Important Monosaccharides
- all are white, water-soluble,crystalline solids
1 & 2) D- Glyceraldehyde and Dihydroxyacetone
- simplest monosaccharides
- important intermediates in process of glycolysis
glycolysis series of rxns whereby glucose is converted into
two molecules of pyruvate
3) D-Glucose
- most abundant monosaccharide in nature and most imp.
from human nutritional standpoint
- good source ripe fruit
- other names:
dextrose emphasizes optical activity of d-glucose
(rotation to right)
blood sugar - emphasizes that blood contains
dissolved glucose which is used as cells
primary energy source
5) D-Fructose
- most important ketohexose
- rotate plane-polarized light to left (levolose)
- sweetest-tasting of all sugars fruits and honey
- may be used as dietary sugar (less needed for same
amount of sweetness
6) D-Ribose
- pentose
- component of variety of complex molecules, including
ribonucleic acids (RNAs) and energy-rich cmpds
adenosine triphosphate (ATP)
- cmpd 2-Deoxy-D-ribose also important in nucleic acid
chemistry (component of DNA molecules)
Note: deoxy means minus an oxygen
- so, structure of ribose and 2-deoxy-ribose differ in
that 2-deoxyribose lacks O atom at C-2
D-Ketoses
Fig. 7.14 (p.237)
Fischer
projections and
common names
for ketoses
containing three,
four, five, and six
carbon atoms.
4) D-Galactose
- seldom encountered as free monosaccharide
- is component in numerous important biological substances
- synthesized from glucose in mammary glands for use in
lactose (milk sugar); lactose disaccharide consisting of
glucose unit and galactose unit
- brain sugar is component of glycoproteins found in brain
and nerve tissue
- also present in chemical markers that distinguish various
types of blood A, B, AB and O
Fig. 18.15
A 5% glucose solution is often used in hospitals
as an intravenous source of nourishment for
patients.
Cyclic Forms of Monosaccharides
- experimentally, monosaccharides containing 5- or more C
atoms are in equilibrium with open-chain structures and two
cyclic structures with cyclic forms being dominant @
equilibrium
- cyclic form (cyclic hemiacetal) results from ability of carbonyl
group to react intramolecularly with hydroxyl group
6
Cyclization of D-Glucose
Fig. 7.16 (p.239)
The cyclic
hemiacetal forms
of D-glucose
result from the
intramolecular
reaction between
the carbonyl
group and the
hydroxyl group
on carbon 5.
Important points of figure 7.16
Stucture 2
- rearrangement of projection formula for D-glucose with C
atoms have locations similar to those found for carbon atoms
in 6 membered rings
- all hydroxyl groups to right in Fischer projection appear
below ring
- those to left in Fischer projection appear above ring
Structure 3
- rotate groups attached to C-5 in counterclockwise direction to
visualize intramolecular hemiacetal formation
- intramolecular rxn occurs btw OH on C-5 and carbonyl
group (C-1)
- OH adds across C=O bond, producing heterocyclic ring
containing 5 C atoms and one oxygen atom
- results in production of ring formation with chiral center at C-1
and the formation of two stereoisomers differing in orientation
of OH group on hemiacetal C atom (carbon 1)
in glucose:
i) -D-Glucose - OH group on opposite side of ring from
CH2OH group attached to C-5
ii) -D-Glucose - OH group and CH2OH group are on the
same side of the ring
- in aqueous sol n, dynamic equilibrium exists among cyclic and
open-chain structures with the following distribution:
-D-Glucose
open-chain D Glucose
-D-Glucose
(37%)
(less than 0.01%)
(63%)
- all aldoses with 5- or more C atoms establish similar equilibria
but with different distribution btw cyclic and open-chain forms
- fructose and other ketoses will cyclize as well
Haworth Projection Formulas
- where or configuration does not matter, -OH group on C-1
is placed in horizontal position, and wavy line is used as bond
that connects to ring
- the specific identity of monosaccharide is determined by
positioning of other OH group in Haworth projection
- any OH group at chiral center to right in Fischer projection
points down in Haworth projection
- any OH group at chiral center to left in Fischer projection
points up in Haworth projection
Reactions of Monosaccharides
Haworth projection
- two-dimensional structural notation that specifies the threedimensional structure of a cyclic form of a monosaccharide
- the hemiacetal ring system is viewed edge-on with oxygen
ring atom at upper right (6-membered ring) or at top (5membered ring)
- D and L form of monosaccharide determined by position of
terminal CH2OH group on highest-numbered ring carbon
atom
D form CH2OH group positioned above ring
L form CH2OH group positioned below ring
- not usually found in biochemical systems
- - or - configuration determined by position of OH group
on C-1 relative to CH2OH group that determines D or L
-configuration both groups in same direction
-configuration two groups in opposite directions
-
five important reactions will be presented with glucose as our
example, but with realization that other aldoses and ketoses
undergo similar rxns
1) Oxidation to produce acidic sugars
- redox chemistry of monosaccharides closely linked to
that of alcohol and aldehyde functional groups
- monosaccharide oxidation can yield 3 different yptes of
acidic sugars with oxidizing agent used determining
product
a) Weak oxidizing agents (Tollen s and Benedict s soln s)
- oxidize aldehyde end of aldose to give aldonic acid
- b/c aldoses act as reducing agents in these rxns, called
reducing sugars
Reducing sugar
- carbohydrate that gives positive test with Tollen s and
Benedict s soln s
7
- Under basic conditions of Tollen s and Benedict s sol ns, ketoses
are also reducing sugars
- here, ketoses undergo structural rearrangement to produce an
aldose which then reacts
- therefore, all monosaccharides, both aldoses & ketoses are
reducing sugars
b) Strong oxidizing agents
- can oxidize both ends of monosaccharide at same time to
produce
Dicarboxylic acid
- these polyhydroxy dicarboxylic acids are known as
aldaric acids
c) Enzyme oxidation of monosaccharides
- although difficult in lab, biochemical system enzymes can
oxidize 1o alcohols of aldose with oxidation of aldehyde
group to yield alduronic acid
3)
Glycoside Formation
- b/c cyclic forms of monosaccharides are hemiacetals, they
react with alcohols to form acetals
Glycoside
- an acetal formed from a cyclic monosaccharide by
replacement of hemiacetal carbon OH group with OR
group
- glycosides (like hemiacetals from which they are formed
(may be either or forms)
- glycosides are named by listng alkyl or aryl group attached to
oxygen, followed by name of monosaccharide involved, with
suffix ide appended to it
5) Amino Sugar Formation
- if one of hydroxyl groups of monosaccharide is replaced
by amino group produce an amino sugar
- naturally occurring amino sugars, amino group replaces
C-2 hydroxy group
- amino sugars and N- acetyl derivatives are important
building blocks of polysaccharides found in cartilage
- some N-acetyl derivatives are biochemical markers on red
blood cells, which distinguish various blood types
2) Reduction to Produce Sugar Alcohols
- carbonyl group in monosaccharide (either aldose or ketose)
can be reduced to hydroxyl group using H2 as reducing
agent
- corresponding polyhydroxy alcohol referred to as sugar
alcohol
- hexahydroxy alcohols have properties similar to the
trihydroxy alcohol glycerol
- uses: moisturizing agents in foods and costemics due
to great affinity for water
- d-sorbitol used as sweetening agent in
chewing gum: benefit - bacteria that cause
tooth decay cannot use polyalcohols as food
sources
4) Phosphate Ester Formation
- hydroxyl groups of monosaccharide can react with
inorganic oxyacids to from inorganic esters
- phosphate esters, formed from phosphoric acid and
various monosaccharides, are common
eg. consider glucose
- specific enzymes catalyze esterification of:
i) carbonyl group (C-1)
ii) primary alcohol group (C-6)
- phosphate esters of glucose are stable in aqueous sol n and
play important role in metabolism of carbohydrates
Sugar Terminology
Chemistry at a Glance (p. 247)
8
Disaccharides
disaccharide
- carbohydrate in which two monosaccharides are bonded
together
- reaction is similar to cyclic form of monosaccharide
(hemiacetal form) reacting with alcohol to form glycoside
(acetal)
- here, one of monosaccharide reactants functions as hemiacetal
and other as alcohol
Glycosidic linkage
- bond in disaccharide resulting from rxn btw hemiacetal
carbon atom OH group of one monosaccharide and a OH
group on the other monosaccharide
- always a C-O-C bond in a disaccharide
Forms of Maltose
Fig. 7.19 (p.248)
The three forms of maltose present in aqueous solution.
4) Sucrose
- most abundant of all disaccharides and occurs in plant
kingdom
- produced from juice of sugar cane and sugar beets
- D-sucrose is composed of two monosaccharide units of aD-glucose and -D-fructose with a , (1 2) glycosidic
linkage
- is a nonreducing sugar
- no hemiacetal is present in the molecule since the
glycosidic linkage invloves the reducing ends of both
monosaccharides
- exists in only one form
Sucrase - enzyme needed to break , (1 2) linkage is present in
human body and sucrose is easily digested
- hydrolysis (digestion) produces equimolar mixture of
glucose and fructose (mixture referred to as invert sugar)
Four important disaccharides:
**focus on configuration ( or ) at C-1 of reacting
monosaccharides that function as hemiacetal
1) Maltose (malt sugar)
- produced when polysaccharide starch breaks down:
I) in plants as seeds germinate
ii) in human during starch digestion
- made of two D-glucose units where one must be -D
glucose
- is reducing sugar
- 3 forms: -maltose, maltose, and open-chain form
- will undergo hydrolysis, in the presence of acidic
conditions or enzyme maltase, to yield two molecules of Dglucose
2) Cellobiose
- produced as intermediate in hydrolysis of polysaccharide
cellulose
- contains two gucose monosaccharide units with the
hemiacetal monosaccharide in the configuration, that is
a (1 4) linkage
- reducing sugar which 3 isomeric forms in aqueous sol n
- produces 2 D-glucose molecules upon hydrolysis
3.) Lactose (milk sugar)
- two different monosaccharides; -D-galactose and D-glucose
joined by (1 4) glycosidic linkage
- reducing sugar
- major sugar found in mile
- can be hydrolyzed by acid or enzyme lactase
- Note: that galactose produced in this manner is then
converted to glucose by other enzymes
General Characteristics of Polysaccharides
polysaccharide (often called glycans)
- polymer that contains many monosaccharide units bonded to
each other by glycosidic linkages
- important parameters that distinguish various polysaccharides
(glycans) from each other:
1) identity of monosaccharide repeating unit(s) in polymer
chain
homopolysaccharides
polysaccharide in which only one type of monosaccharide
monomer is present
- most abundant in nature (includes starch, glycogen,
cellulose and chitin)
heteropolysaccharides
- polysaccharide in which more than one (usually two) types
of monosaccharide monomers are present
- includes hyaluronic acid and heparin
9
2) length of polymer chain
- can vary from less than 100 monomers to up to a million
monomer units
3) type of glycosidic linkage btw monomer units
- several different types of glycosidic linkages are encountered
4) degree of branching of polymer chain
- ability to from branched-chain structures distinguishes
polysaccharides from each other major types of biochemical
polymers
Fig. 7.22 (p.251)
The polymer chain of a
polysaccharide may be
unbranched or branched.
polysaccharide characteristics
- not sweet
- not test positive in Tollen s and Benedict s sol ns
- limited water solubility (due to size)
- however, -OH groups can individually become hydrated by
water molecules yielding thick colloidal suspension of
polysaccharide in water
Focus of certain polysaccharides:
i) storage polysaccharides
- starch and glycogen
ii) structural polysaccharides
- cellulose and chitin
iii) acidic polysaccharides
- hyaluronic acid and heparin
Storage polysaccharides
- polysaccharides that is a storage form for monosaccharides
are used as energy source in cells
- this lowers osmotic pressure within cells since osmotic
pressure depends on number of individual molecules present
i)
Starch energy-storage polysaccharide in plant cells
- homopolysaccharide containing only glucose monomers
- hydrolysis of starch releases glucose
- two different polyglucose polysaccharides may be isolated
from starches:
a) amylose (15-20% of starch)
- straight-chain glucose polymer (non-branched)
- 1 4) glycosidic linkages
- no. of units depending on source of starch; usually
btw 300-500 units
amylopectin
- high degree of branching with branching every 25-30
glucose units where branch point involves 1 6)
- linkages- up to 100,000 glucose units
- contains 1 4) and 1 6) linkages
ii) glycogen (animal starch)
storage polysaccharide in humans and animals
- liver cells and muscle cells are storage sites in humans
- similar structure to amylopectin with glycosidic linkages
being 1 4) and 1 6) linkages
- however, 3 times as many branches and upto 1,000,000
glucose units
- two opposing processes, glycogenesis and glycogenolysis,
represent formation and decomposition of glycogen, resp.
- formation of glycogen from glucose greatly reduces osmotic
cell pressure
Structure of Amylopectin
Structural Polysaccharides
Storage Polysaccharides
Fig. 7.23 (p.254)
Two perspectives on the structure of the polysaccharide
amylopectin.
b)
- serve as structural element in plant cell walls and animal
exoskeletons
- both are homopolysaccharides
i) cellulose
- structural component of cell walls
- most abundant naturally-occurring polysaccharide
fibrous, water-insoluble substance found in woody portions
of plants stems, stalks and trunks
- unbranched glucose polymer with 1 4) glycosidic linkages
- linear structure which when aligned side by side become
water-insoluble fibers due to interchain H-bonding involving
many hydroxy groups present
10
-
ii)
cellulose chains approximately 5000 glucose units making MW
of approximately 900,000amu
not source of nutrition for humans since lack enzyme for
catalyzing hydrolysis of 1 4) linkages
grazing animals contain bacteria in intestinal tract that
produces cellulase which is the enzyme that can hydrolyze
1 4) linkages and produce free glucose
Chitin
- polysaccharide similar to cellulose in both function and
structure
- gives rigidity to exoskeletons of crabs, lobsters, shrimp and
other anthropods; also occurs in cell walls of fungi
- linear polymer (no branching) with all 1 4) glycosidic
linkages
- differs from cellulose in that monosaccharide present is
N acetyl amino derivative of D-glucose
Acidic Polysaccharides
- polysaccharides with disaccharide repeating unit in which one of
the disaccharide components is an amino sugar and one or
both disaccharide components has negative charge due to
sulfate group or carboxyl group
- heteropolysaccharides two different monosaccharides present
in alternating pattern
- involved in variety of cellular functions and tissues
- unbranched-chain structures
i) hyaluronic acid
- contains alternating residues of N-acetyl- -D-glucosamine
(repeating unit in chitin) and D-glucuronic acid (derived
from glucose by oxidation of OH group at C-6 to an acid
group)
- alternating pattern of glycosidic linkages of 1 3) and
1 4) and 50,000 disaccharide units per chain
Common Glycosidic Linkages in Di- and
Polysaccharides
Chemistry at a Glance (p. 257)
Structural Polysaccharides
Fig. 7.28
The structures of cellulose (a) chitin (b).
b)
Heparin
- an anticoagulant; helps prevent blood clots
- binds strongly to protein involved in terminating process
of blood clotting, thus inhibiting blood clotting
- small polysaccharide; 15-90 saccharide residues/chain
- monosaccharides in disaccharide repeating unit is Dglucuronate-2-sulfate and N-sulfo-D-glucosamine-6sulfate
Note: both contain two negatively charged acidic groups
Glycolipids and Glycoproteins
- oligosaccharides attached through glycosidic linkages to lipid or
protein molecules have wide variety of cellular functions
including process of cell recognition
glycolipids and glycoproteins
- often govern how individual cells of differing function
within biochemical system recognize each other and how
cells interact with invading bacteria and viruses
lipid or protein part of glycolipid or glycoprotein is
incorporated into cell membrane structure and
oligosaccharide part functions as marker on outer cell
membrane surface
cell recognition generally involves interaction btw
carbohydrate marker of one cell and protein imbedded into
cell membrane of another cell
11
Dietary Considerations and Carbohydrates
- balanced diet should ideally be about 60% carbohydrate
divided into simple and complex categories:
i) simple carbohydrate
- dietary monosaccharide or disaccharide
- usually sweet to taste and commonly referred to as sugars
- provide 20% of energy in U.S. diet
i) 50% from natural sugars
- sugars naturally present in whole foods (milk and
fresh fruit)
- provides energy and other nutrients
ii) 50% from refined sugars
- sugar that has been separated from plant source
(sugar beets and sugar cane)
- said to provide empty calories b/c provide energy and
no other nutrients
ii) complex carbohydrate
- dietary polysaccharide
- starch and cellulose, generally not sweet to taste
- major source in U.S. is grains (source of both starch and
fiber as well as protein, vitamins, and minerals)
- vegetables also a good source
- developing concern about dietary intake of carbohydrates
involves how fast a given dietary carbohydrate is broken down
to generate glucose within body
glycemic effect
- refers to how quickly carbohydrates are digested (broken
down into glucose, how high glucose levels rise, and how
quickly blood glucose levels return to normal
glycemic index (GI)
- developed for rating foods in terms of their glycemic
effect (see Chemical Connections, p.259)
12
This document was created with Win2PDF available at http://www.daneprairie.com.
The unregistered version of Win2PDF is for evaluation or non-commercial use only.