Carbohydrates pdf - D. Adrees

Biochemistry Lectures: 1st year Lecturer: Hiba A. Alasadi

Lecture No.1

Biochemistry

Science

Science asks how the remarkable properties of living organisms arise

from the thousands of different lifeless biomolecules. When these molecules

are isolated and examined individually, they conform to all the physical and

chemical laws that describe the behavior of inanimate matter—as do all the

processes occurring in living organisms. The study of biochemistry shows how

the collections of inanimate molecules that constitute living organisms

interact to maintain and perpetuate life animated solely by the physical and

chemical laws that govern the nonliving universe.

The chemistry of living organisms is organized around carbon, which

accounts for more than half the dry weight of cells.

Carbon can form single

bonds with hydrogen atoms, and both single and double bonds with oxygen

and nitrogen atoms. Of greatest significance in biology is the ability of carbon

atoms to form very stable carbon–carbon single bonds. Each carbon atom can

form single bonds with up to four other carbon atoms. Two carbon atoms also

can share two (or three) electron pairs, thus forming double (or triple) bonds.

Covalently linked carbon atoms in biomolecules can form linear chains,

branched chains, and cyclic structures. To these carbon skeletons are added

groups of other atoms, called functional groups, which confer specific

chemical properties on the molecule.

Macromolecules

Many biological molecules are macromolecules, polymers of high

molecular weight assembled from relatively simple precursors. Proteins, are

long polymers of amino acids, Nucleic acids, DNA and RNA, are polymers of

nucleotides , Carbohydrates, are polymers of simple sugars such as glucose

and the Lipids, are greasy or oily hydrocarbon derivatives.

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Biochemistry Lectures: 1st year Lecturer: Hiba A. Alasadi

Lecture No.1

CARBOHYDRATES

Occurrence

Most abundant molecules on earth, most are produced by photo-

synthesis. They are present in humans, animal tissues, plants and micro-

organisms. Carbohydrates are also present in tissue fluids, blood, milk

secretions and excretions of animals.

Medical And Biological Importance

1. Carbohydrates are the major source of energy for man. For example,

glucose is used in the human body for energy production.

2. Some carbohydrates serves as reserve food material in humans (glycogen)

and in plant (starch).

3. Some of carbohydrates are components of cell membrane and nervous

tissue.

4. Carbohydrates are components of nucleic acids and blood group

substances.

5. Carbohydrates are involved in cell-cell interaction.

6. Derivative of carbohydrates are drugs. For example, streptomycin an

antibiotic is a glycoside.

7. Aminosugar, derivatives of carbohydrates are components of antibiotics

like erythromycin and carbomycin.

8. Ascorbic acid, a derivative is water-soluble vitamin.

9. Bacterial invasion involves hydrolysis of mucopolysaccharides.

Biochemistry Lectures: 1st year Lecturer: Hiba A. Alasadi

Lecture No.1

Chemical Nature Of Carbohydrates

Carbohydrates are polyhydroxy alcohols with a functional aldehyde or

keto group. They are represented with general formula C

. usually the

ratio of carbon and water is one in most of the carbohydrates hence the name

carbohydrate (carbonhydrate).

Classification of carbohydrates

Carbohydrates are classified into three major classes based on number of

carbon chains present. They are:

1. Monosacchrides

2. Oligosacchrides

3. Polysacchrides

All the three classes contain a saccharose group and hence the name

saccharides.

Saccharose group

MONOSACCHARIDES

Monosaccharides are those carbohydrates which cannot be hydrolyzed to

small compounds. Their general formula is C

. They are also called as

simple sugars. Monosaccharides containing three to nine carbon atoms occur

in nature.

Nomenclature

Monosaccharides have common (trivial) names and systematic names.

Systematic name indicates both the number of carbon atoms present and

aldehyde or ketone group. For example, glyceraldehyde is a simple sugar

Biochemistry Lectures: 1st year Lecturer: Hiba A. Alasadi

Lecture No.1

containing three carbon atoms and an aldehyde group. Simple sugars

containing three carbon atoms are referred as triose. In addition, sugar

containing aldehyde group or keto group are called as aldoses or ketoses,

respectively. Thus, the systematic name of glyceraldehydes is aldotriose.

Similarly, a simple sugar with three carbon atoms and keto group is called

ketotriose. Some monosaccharides along with their common and systematic

names are shown below

Fig.1.1 Some Important Monosaccharides (Systematic Names Are Given Too)

Biochemistry Lectures: 1st year Lecturer: Hiba A. Alasadi

Lecture No.1

PROPERTIES OF MONOSACCHARIDES

1. Optical Isomers

All the monosaccharides except dihydroxyacetone contain one or more

asymmetric (chiral) carbon atoms and thus occur in optically active isomeric

forms. The simplest aldose, glyceraldehyde, contains one chiral center (the

middle carbon atom) and therefore has two different optical isomers, or

enantiomers (Fig. 1.2). By convention, one of these two forms is designated

the D isomer, the other the L isomer. To represent three-dimensional sugar

structures on paper, we often use Fischer projection formulas. In general, a

molecule with n chiral centers can have 2

stereoisomers. Glyceraldehyde has

= 2; the aldohexoses, with four chiral

center has 2

= 2; the aldohexoses,

with four centers, have 2

= 16 stereoisomers.

FIGURE  1–2  Three  ways  to  represent  the  two
stereoisomers  of  glyceraldehyde.  The  stereo-
isomers  are  mirror  images  of  each  other.  Ball-
and-

stick

models

show

the

actual

configuration  of  molecules.  By  convention,  in
Fischer  projection  formulas,  horizontal  bonds
project  out  of  the  plane  of  the  paper,  toward
the  reader;  vertical  bonds  project  behind  the
plane of the paper, away from the reader.

Biochemistry Lectures: 1st year Lecturer: Hiba A. Alasadi

Lecture No.1

The stereoisomers of monosaccharides of each carbon-chain length can be

divided into two groups that differ in the configuration about the chiral center

most distant from the carbonyl carbon. Of the 16 possible aldohexoses, eight

are D forms and eight are L. Most of the hexoses of living organisms are D

isomers.

2. Optical Activity

Monosaccharides except dihydroxy acetone exhibit optical activity because of

the presence of asymmetric carbon atom. If a sugar rotates plane polarized

light to the right then it called as dextrorotatory and if a sugar rotates plane

polarized light to left then it called the levorotatory. Usually '+' sign or 'd'

indicates dextrorotation and ' - ' or ' l ' indicates levorotatory. For example, D-

glucose which is dextrorotatory is designated as D(+) glucose and D-fructose,

which is levorotatory is designated as D(-)fructose.

3. Epimers

Are those monosaccharides that differs in the configuration of -OH group on

2nd, 3rd and 4th carbon atoms. Epimers are also called diasterioisomers.

Glucose, galactose and mannose are examples for epimers. Galactose is an

epimer of glucose because, configuration of hydroxyl group on 4th carbon

atom of galactose is different from glucose. Similarly, mannose is an epimers

of glucose because configuration of hydroxyl group on 2nd carbon atom of

mannose is different from glucose (Fig.1.3). Ribulose and xylulose (both are

ketoses) are also epimers. They are differ in configuration of -OH group on

third carbon atom (Fig. 1.4).

Biochemistry Lectures: 1st year Lecturer: Hiba A. Alasadi

Lecture No.1

FIGURE 1–3 Epimers. D-Glucose and two of its epimers are shown as

projection formulas. Each epimer differs from D-glucose in the configuration

at one chiral center

Figure 1-4 Epimers of some ketoses