Download Carbohydrates I

Carbohydrates I
Introduction
A carbohydrate is an organic compound that consists only of carbon, hydrogen, and oxygen,
usually with a hydrogen:oxygen atom ratio of 2:1 (as in water); in other words, with the
empirical formula Cm(H2O)n. (Some exceptions exist; for example, deoxyribose, a
component of DNA, has the empirical formula C5H10O4.) Carbohydrates are not technically
hydrates of carbon. Structurally it is more accurate to view them as polyhydroxy aldehydes
and ketones. The term is most common in biochemistry, where it is a synonym of saccharide.
The carbohydrates (saccharides) are divided into four chemical groupings: monosaccharides,
disaccharides, oligosaccharides, and polysaccharides. In general, the monosaccharides and
disaccharides, which are smaller (lower molecular weight) carbohydrates, are commonly
referred to as sugars. The word saccharide comes from the Greek word σάκχαρον
(sákkharon), meaning "sugar". While the scientific nomenclature of carbohydrates is
complex, the names of the monosaccharides and disaccharides very often end in the suffix ose. For example, blood sugar is the monosaccharide glucose, table sugar is the disaccharide
sucrose, and milk sugar is the disaccharide lactose.
Carbohydrates perform numerous roles in living organisms. Polysaccharides serve for the
storage of energy (e.g., starch and glycogen), and as structural components (e.g., cellulose in
plants and chitin in arthropods). The 5-carbon monosaccharide ribose is an important
component of coenzymes (e.g., ATP, FAD, and NAD) and the backbone of the genetic
molecule known as RNA. The related deoxyribose is a component of DNA. Saccharides and
their derivatives include many other important biomolecules that play key roles in the
immune system, fertilization, preventing pathogenesis, blood clotting, and development. In
food science and in many informal contexts, the term carbohydrate often means any food that
is particularly rich in the complex carbohydrate starch (such as cereals, bread, and pasta) or
simple carbohydrates, such as sugar (found in candy, jams, and desserts).
Structure
Formerly the name "carbohydrate" was used in chemistry for any compound with the formula
Cm (H2O) n. Following this definition, some chemists considered formaldehyde (CH2O) to be
the simplest carbohydrate, while others claimed that title for glycolaldehyde. Today the term
is generally understood in the biochemistry sense, which excludes compounds with only one
or two carbons.
Natural saccharides are generally built of simple carbohydrates called monosaccharides with
general formula (CH2O)n where n is three or more. A typical monosaccharide has the
structure H-(CHOH)x(C=O)-(CHOH)y-H, that is, an aldehyde or ketone with many hydroxyl
groups added, usually one on each carbonatom that is not part of the aldehyde or ketone
functional group. Examples of monosaccharides are glucose, fructose, and glyceraldehydes.
However, some biological substances commonly called "monosaccharides" do not conform
to this formula (e.g., uronic acids and deoxy-sugars such fucose), and there are many
chemicals that do conform to this formula but are not considered to be monosaccharides (e.g.,
formaldehyde CH2O and inositol (CH2O)6).
The open-chain form of a monosaccharide often coexists with a closed ring form where the
aldehyde/ketonecarbonyl group carbon (C=O) and hydroxyl group (-OH) react forming a
hemiacetal with a new C-O-C bridge.
Monosaccharides can be linked together into what are called polysaccharides (or
oligosaccharides) in a large variety of ways. Many carbohydrates contain one or more
modified monosaccharide units that have had one or more groups replaced or removed. For
example, deoxyribose, a component of DNA, is a modified version of ribose; chitin is
composed of repeating units of N-acetyl glucosamine, a nitrogen-containing form of glucose.
Monosaccharides
D-glucose is an aldohexose with the formula (C.H2O)6. The red atoms highlight the aldehyde
group, and the blue atoms highlight the asymmetric center furthest from the aldehyde;
because this -OH is on the right of the Fischer projection, this is a D sugar.
Monosaccharides are the simplest carbohydrates in that they cannot be hydrolyzed to smaller
carbohydrates. They are aldehydes or ketones with two or more hydroxyl groups. The general
chemical formula of an unmodified monosaccharide is (C.H2O)n, literally a "carbon hydrate."
Monosaccharides are important fuel molecules as well as building blocks for nucleic acids.
The smallest monosaccharides, for which n = 3, are dihydroxyacetone and D- and Lglyceraldehydes.
Monosaccharides are classified according to the total number of carbon atoms in their
structure. For example, an aldohexose is a monosaccharide that contains a total of six carbon
atoms including that of the aldehyde in its structure.
Similarly, a ketopentose has five carbons in its structure including the one in the keto group.
Oligosaccharides are carbohydrates that yield from two to about nine monosaccharide
molecules when one molecule of the oligosaccharide is hydrolysed. Small oligosaccharides
are often classified according to the number of monosaccharide residues contained in their
structures. For example, disaccharides and trisaccharides contain two and three
monosaccharide residues respectively whilst polysaccharides yield larger numbers of
monosaccharide molecules per polysaccharide molecule on hydrolysis. All types of
carbohydrate occur widely in the human body. They exhibit a wide variety of biological
functions but in particular act as major energy sources for the body.
Fig: Examples of the cyclic and straight chain structures of monosaccharides. The carbon of
the carbonyl group has the lowest locant.
Fig: The cyclization of the straight chain form of glucose to form the b-hemiacetal cyclic
form of the molecule
For Eg. Six membered rings usually occur as chair conformations whilst five membered rings
exist as envelope conformations.
This internal nucleophilic addition introduces a new chiral centre into the molecule. The
carbon of the new centre is known as the anomeric carbon and the two new stereoisomers
formed are referred to as anomers. The isomer where the new hydroxy group and the CH2OH
are on opposite sides of the plane of the ring is known as the alpha anomer. Conversely, the
isomer with the new hydroxy group and terminal CH2OH on the same side of the plane of the
ring is known as the beta anomer.
The alpha and beta anomers of monosaccharides are drawn using the Haworth convention.
In many cases pure α- and β-anomers may be obtained by using appropriate isolation
techniques. For example, crystallization of D-glucose from ethanol yields a-D-glucose
[α]D+112.2° whilst crystallization from aqueous ethanol produces β-D-glucose [α]D+18.7°. In
the solid state these forms are stable and do not interconvert. However, in aqueous solution
these cyclic structures can form equilibrium mixtures with the corresponding straight chain
form equilibrium mixtures. The change in optical rotation due to the conversion of either the
pure α- or pure β-anomer of a monosaccharide into an equilibrium mixture of both forms in
aqueous solution is known as mutarotation.
Fig :The mutarotation of glucose anomers. The specific rotation of the aqueous +52°
All monosaccharides have a number of stereogenic centres. The configurations of these
centres may be indicated by the use of the R/S nomenclature system.
However, the historic system where the configurations of all the chiral centres are indicated
by the stem name of the monosaccharide is generally preferred. In addition, monosaccharides
are also classified as D or L according to the configuration of their pentultimate CHOH
group. In the D form this hydroxy group projects on the right of the carbon chain towards the
observer whilst in the L form it projects on the left of the carbon chain towards the observer
when the molecule is viewed with the unsaturated group at the top.
These configurations are usually represented, on paper, by modified Fischer projections with
the unsaturated group drawn at the top of the chain. The D and L forms of a monosaccharide
have mirror image structures, that is, are enantiomers.
Fig: Examples of the stem names used to indicate the configurations of the chiral centres in
monosaccharides. The system is based on the relative positions of adjacent hydroxy groups
with the carbonyl group being used as a reference point for the hydroxy groups. L
configurations are mirror images of the corresponding D configurations.
Some monosaccharides may also be classified as being epimers. Epimers are compounds that
have identical configurations except for one carbon atom. For example, a-D-glucose and a-Dfructose are epimers. Epimers sometimes react with the same reagent to form the same
product. For example, both a-D-glucose and a-D-fructose react with phenylhydrazine to form
the same osazone.
The chemical properties of monosaccharides are further complicated by the fact that they can
exhibit tautomerism in aqueous basic solutions.
This means that after a short time a basic aqueous solution of a monosaccharide will also
contain a mixture of monosaccharides that will exhibit their characteristic chemical
properties. For example, a solution of fructose will produce a silver mirror when treated with
an ammoniacal solution of silver nitrate (Tollen's reagent). This is because under basic
conditions fructose undergoes tautomerism to glucose, whose structure contains an aldehyde
group, which reduces Tollen's reagent to metallic silver.
Fig: The tautomerism of glucose in a basic aqueous solution. The approximate concentrations
of the isomers present at equilibrium are given in the brackets
Nomenclature of monosaccharides
Monosaccharides are normally known by their traditional trivial names. However, systematic
names are in use. The systematic names of 'straight chain' monosaccharides are based on a
stem name indicating the number of carbon atoms, a prefix indicating the configuration of the
hydroxy group and either the suffix -ose (aldoses) or -ulose (ketoses). In addition the name is
also prefixed by the D or L as appropriate. Five membered ring monosaccharides have the
stem name furanose whilst six membered ring compounds have the stem name pyranose
together with the appropriate configurational prefixes indicating the stereochemistry of the
anomers. Monosaccharides in which one of the hydroxy groups has been replaced by a
hydrogen atomhave the prefix deoxy- with the appropriate locant, except if it is at position 2,
when no locant is given.
Glycosides
Many endogenous compounds occur as glycosides. These are compoundswhich consist of a
carbohydrate residue, known generally as a glycone, bondedto a non-sugar residue, known
generally as an aglycone, by a so calledglycosidic link to the anomeric carbon of the glycone.
Since the glycosidiclink is formed to the anomeric carbon both a- and b-isomers of a
glycosideare known. The structures of glycosidic links vary, the most common beingan ether
group (oxygen glycosidic links) but amino (nitrogen glycosidic links), sulphide (sulphur
glycosidic links) and carbon to carbon links(carbon glycosidic links) are known. Each type of
glycosidiclink will exhibit the characteristics of the structure forming the link.
For example, oxygen glycosidic links are effectively acetals and so undergohydrolysis in
aqueous solution. Both trivial and systematic nomenclatureis used for glycosides. In
systematic nomenclature the radicalname of the aglyconepreceeds the name of the glycone,
which has thesuffix -oside.
(a)
(b)
Fig: (a) oxygen glycosides, (b) nitrogen glycosides and (c) carbon glycosides.
Glycoproteins are glycosides that have a protein aglycone. The protein isusually linked to a
polysaccharide by an O or N glycosidiclink. Glycoproteins are found in all forms of life.
They exhibit a wide range ofbiological activities. For example, they may act as receptors,
hormones andenzymes.
Use in living organisms
Monosaccharides are the major source of fuel for metabolism, being used both as an energy
source (glucose being the most important in nature) and in biosynthesis. When
monosaccharides are not immediately needed by many cells they are often converted to more
space-efficient forms, often polysaccharides. In many animals, including humans, this storage
form is glycogen, especially in liver and muscle cells. In plants, starch is used for the same
purpose.
Disaccharides
Two joined monosaccharides are called a disaccharide and these are the simplest
polysaccharides. Examples include sucrose and lactose. They are composed of two
monosaccharide units bound together by a covalent bond known as a glycosidic linkage
formed via a dehydration reaction, resulting in the loss of a hydrogen atom from one
monosaccharide and a hydroxyl group from the other. The formula of unmodified
disaccharides is C12H22O11. Although there are numerous kinds of disaccharides, a handful of
disaccharides are particularly notable.
Sucrose, pictured to the right, is the most abundant disaccharide, and the main form in which
carbohydrates are transported in plants. It is composed of one D-glucose molecule and one
D-fructose molecule. The systematic name for sucrose, O-α-D-glucopyranosyl-(1→2)-Dfructofuranoside, indicates four things:
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

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Its monosaccharides: glucose and fructose.
Their ring types: glucose is a pyranose, and fructose is a furanose.
How they are linked together: the oxygen on carbon number 1 (C1) of α-D-glucose is
linked to the C2 of D-fructose.
The -oside suffix indicates that the anomeric carbon of both monosaccharides
participates in the glycosidic bond.
Lactose, a disaccharide composed of one D-galactose molecule and one D-glucose molecule,
occurs naturally in mammalian milk. The systematic name for lactose is O-β-Dgalactopyranosyl-(1→4)-D-glucopyranose. Other notable disaccharides include maltose (two
D-glucoses linked α-1,4) and cellulobiose(two D-glucoses linked β-1,4). Disaccharides can
be classified into two types.They are reducing and non-reducing disaccahrides if the
functional group is present in bonding with another sugar unit it is called a reducing
disaccharide or biose.
Polysaccharides
Polysaccharides
(glycans) are carbohydrates whose structures
monosaccharideresidues joined together by oxygen glycosidic linkages.
consist
of
The links between monosaccharide residues of a polysaccharidemolecule are usually referred
to in terms of the type of the numbers of thecarbon atoms forming the link and the
stereochemistry of the anomeric position.
For example, the glycosidic link formed in maltose is referred to as an a-1,4-link because the
anomeric carbon of an a-D-glucose residueis linked to carbon number 4 of the other (second)
glucose residue in thestructure. The anomeric carbon atom of the second glucose residue can
undergomutarotation and so maltose will exist as two isomers in aqueous solution. The prefix
(a, b) is used for residues that can undergo mutarotation.
Fig: Examples of simple disaccharides. The structures are normally drawn so that the oxygen
atom forming the glycosidic link isabove or below the plane of the ring system. This
sometimes requires the structure of a residue to beturned around and/or over in order to
obtain the correct alignment of the oxygen atom. Hexagonalcardboard cutouts can be useful
in determining how a particular glycosidic link was formed.
The stereochemical nature of these oxygen glycosidic links is important in thecontrol of the
metabolism of polysaccharides. Enzymes that catalyse the aqueoushydrolysis of the
glycosidic links of polysaccharides will only usually catalysethe cleavage of a link formed by
a particular anomer or anomers. For example, an α-glucosidase catalyses the hydrolysis of
glycosidic links formed byan α-glucose residue acting as a glycone in the polysaccharide
chain.
Nomenclature of polysaccharides
Trivial names are normally used for all types of polysaccharide. Systematicnames may be
used for small polysaccharides. These names are based on thesystematic names of the
monosaccharides corresponding to the residues. However,the suffix -osyl is used for a
substituent residue joined through its anomericcarbon to the next residue in the chain and the
suffix -oside is used for the lastresidue in the chain. Appropriate locants may or may not be
usedin systematic names.
Naturally occurring polysaccharides
Naturally occurring polysaccharides can occur either as individual carbohydratemolecules or
in combination with other naturally occurring substances,such as proteins (glycoproteins) and
lipids (glycolipids). In all cases the polysaccharidesection may have linear or branched chain
structures, which oftencontain the derivatives of both monosaccharides and aminosugars.
Fig: Some derivatives of monosaccharides and amino sugars commonly found in
polysaccharides
Polysaccharides and molecules whose structures contain polysaccharide residueshave a wide
variety of biochemical roles. They occur as integral parts of thestructures of specific tissues:
the mureins, for example, are glycoproteins that form part of the cell walls of bacteria while
thechondroitins are glycosaminoglycans that occur in cartilage, skin and connectivetissue.
Other polysaccharides have specific biological activities. For example,heparin inhibits the
clotting of blood whilst starch and glycogen,are the main energy stores of mammals, plants
and microorganisms. Polysaccharideresidues also form parts of some enzyme and receptor
molecules.
Fig: A schematic representation of a fragment of the structure of the glycoprotein
Fig: primary structure of glycophorin A, a glycoprotein that spans the plasma membrane
("Lipid bilayer") of human red blood cells. Each RBC has some 500,000 copies of the
molecule embedded in its plasma membrane.
Fig: A representation of the structureof glycogen and starch. Both structures are based on
chains of a-glucose residues joined by α-(1,4)glycosidic links in a similar manner to that
found in amylose. In glycogen, these chains arebranched every eight to 10 glucose residues,
the branches being attached by α-(1,6) glycosidiclinks similar to those found in the
amylopectins. Starch consists of unbranched amylose chains(10–20%) and amylopectins with
branches occurring every 20–30 glucose residues.
Nutrition
Fig:Grain products: rich sources of carbohydrates
Foods high in carbohydrate include fruits, sweets, soft drinks, breads, pastas, beans, potatoes,
bran, rice, and cereals. Carbohydrates are a common source of energy in living organisms;
however, no carbohydrate is an essential nutrient in humans.
Carbohydrates are not necessary building blocks of other molecules, and the body can obtain
all its energy from protein and fats. The brain and neurons generally cannot burn fat for
energy, but use glucose or ketones. Humans can synthesize some glucose (in a set of
processes known as gluconeogenesis) from specific amino acids, from the glycerol backbone
in triglycerides and in some cases from fatty acids. Carbohydrate and protein contain 4
kilocalories per gram, while fats contain 9 kilocalories per gram. In the case of protein, this is
somewhat misleading as only some amino acids are usable for fuel.
Organisms typically cannot metabolize all types of carbohydrate to yield energy. Glucose is a
nearly universal and accessible source of calories. Many organisms also have the ability to
metabolize other monosaccharides and Disaccharides, though glucose is preferred. In
Escherichia coli, for example, the lac operon will express enzymes for the digestion of
lactose when it is present, but if both lactose and glucose are present the lac operon is
repressed, resulting in the glucose being used first .Polysaccharides are also common sources
of energy. Many organisms can easily break down starches into glucose, however, most
organisms cannot metabolize cellulose or other polysaccharides like chitin and
arabinoxylans. These carbohydrates types can be metabolized by some bacteria and protists.
Ruminants and termites, for example, use microorganisms to process cellulose. Even though
these complex carbohydrates are not very digestible, they may comprise important dietary
elements for humans. Called dietary fiber, these carbohydrates enhance digestion among
other benefits.
Based on the effects on risk of heart disease and obesity, the Institute of Medicine
recommends that American and Canadian adults get between 45–65% of dietary energy from
carbohydrates.The Food and Agriculture Organization and World Health Organization jointly
recommend that national dietary guidelines set a goal of 55–75% of total energy from
carbohydrates, but only 10% directly from sugars (their term for simple carbohydrates).