BIOMOLECULES
Life is made up of chemicals; living beings are constituted from
chemicals known as biomolecules such as carbohydrates, vitamins,
lipids, proteins and nucleic acids. These biomolecules interact with each
other and constitute the molecular logic of life processes. In addition,
some simple molecules like vitamins and mineral salts also play an
important role in the functions of organisms. In this chapter we will
study the structures and functions of some of these biomolecules.
1. Carbohydrates
2. Proteins 3. Enzymes
4. Vitamins
5. Nucleic Acids
After studying this chapter, students will be able to explain the characteristics of biomolecules like carbohydrates, proteins and nucleic
acids and hormones; classify carbohydrates, proteins, nucleic acids and vitamins on the basis of their structures; explain the difference between
DNA and RNA and describe the role of biomolecules in bio-system.
CARBOHYDRATES
Carbohydrates are probably the most abundant and widespread organic
substances in nature, and they are essential constituents of all living
things. Carbohydrates are formed by green plants from carbon
dioxide and water during the process of photosynthesis. Carbohydrates
serve as energy sources and as essential structural components in
organisms; in addition, part of the structure of nucleic acids, which
contain genetic information, consists of carbohydrate.
Examples: cane sugar, glucose, starch, etc. Most of them have a general
formula, Cx(H2O)y,
For example, the molecular formula of glucose (C6H12O6) fits into this
general formula, C6 (H2O)6.
But all the compounds which fit into this formula may not be classified
as carbohydrates.
For example acetic acid (CH3COOH) fits into this general formula,
C2 (H2O)2 but is not a carbohydrate.
A large number of their reactions have shown that they contain specific
functional groups. Chemically, the carbohydrates may be defined as
optically active polyhydroxy aldehydes or ketones or the compounds
which produce such units on hydrolysis. Some of the carbohydrates,
which are sweet in taste, are also called sugars. The most common sugar,
used in our homes is named as sucrose whereas the sugar present in milk is
known as lactose.
CLASSIFICATION
Monosaccharides: A carbohydrate that cannot be hydrolysed further to
give simpler unit of polyhydroxy aldehyde or ketone is called a
monosaccharide. About 20 monosaccharides are known to occur in nature.
Examples: glucose, fructose, ribose, etc.
Oligosaccharides: Carbohydrates that yield two to ten
monosaccharide units, on hydrolysis, are called oligosaccharides. They are
further classified as disaccharides, trisaccharides, tetrasaccharides, etc.,
Sucrose is common sugar. It is a disaccharide, a molecule composed of
two monosaccharides: glucose and fructose. It has the molecular formula
C12H22O11.
C12H22O11 + H2O → C6H12O6 + C6H12O6
Glucose fructose
Polysaccharides: Carbohydrates which yield a large number of
monosaccharide units on hydrolysis are called polysaccharides.
Examples: cellulose, glycogen, gums, etc. Polysaccharides are not sweet
in taste, hence they are also called non-sugars.
MONOSACCHARIDES
Monosaccharides are further classified on the basis of number of carbon
atoms and the functional group present in them. If a monosaccharide
contains an aldehyde group, it is known as an aldose and if it contains a
keto group, it is known as a ketose
ALDOSE KETOSE
Monosaccharides can be classified by the number x of carbon atoms
they contain: triose (3), tetrose (4), pentose (5), hexose (6), heptose (7),
and so on. Glucose, used as an energy source and for the synthesis of
starch, glycogen and cellulose.
PREPARATION OF GLUCOSE
From sucrose (Cane sugar): If sucrose is boiled with dilute HCl or
H2SO4 in alcoholic solution, glucose and fructose are obtained in equal
amounts.
C12H22O11 + H2SO4 or HCl → C6H12O6 + C6H12O6
Sucrose Glucose Fructose
From starch: Commercially glucose is obtained by hydrolysis of starch
by boiling it with dilute H2SO4 at 393 K under pressure.
(C6H10O5) + nH2O → nC6H12O6
Starch Glucose
STRUCTURE OF GLUCOSE
Glucose is an aldohexose and is also known as dextrose. It is the
monomer of many of the larger carbohydrates, namely starch, cellulose.
Its molecular formula was found to be C6H12O6.
On heating with HI, it forms n-hexane, suggesting that all the six carbon
atoms are linked in a straight chain
Glucose gets oxidised to six carbon carboxylic acid (gluconic acid) on
reaction with a mild oxidising agent like bromine water. This indicates
that the carbonyl group is present as an aldehydic group.
Acetylation of glucose with acetic anhydride gives glucose
pentaacetate which confirms the presence of five –OH groups. Since
it exists as a stable compound, five –OH groups should be attached to
different carbon atoms.
D– and L– notations of Glucose
Glucose is correctly named as D(+)-glucose or L (-) glucose. D and L
before the name of glucose represents the configuration whereas ‘(+)’
and (-) represents the rotation of monochromatic light in the glucose
solution. Ie. (+) sign indicates dextrorotatory or clockwise and (-) sign
indicates laevorotatory or anticlockwise.
In the linear form (also called Fischer Projections) of glucose do the
following steps to determine D- from L-sugars
Find the aldehyde functional group of glucose. This carbon is counted as
one.
Number the remaining carbons in chronological order.ie 2,3,4,5 etc
Find the fifth carbon. This is the chiral carbon. This carbon is bonded to
four different groups.
If the hydroxyl group on the 5th carbon is to the right of the molecule is a
D-sugar. If the hydroxyl group on the 5th carbon is to the left of the
molecule is L-sugar.
PROTEINS
Proteins are large biomolecules, or macromolecules, consisting of one
or more long chains of amino acid residues. Proteins perform a vast
array of functions within organisms, including catalyzing metabolic
reactions, DNA replication, responding to stimuli, providing structure to
cells, and organisms, and transporting molecules from one location to
another. Proteins differ from one another primarily in their sequence of
amino acids. They occur in every part of the body and form the
fundamental basis of structure and functions of life.All proteins are
polymers of α-amino acids.
Chief sources of proteins are milk, cheese, pulses, peanuts, fish, meat,
etc.
AMINO ACID
Amino acid, are organic compounds that contain a basic amino group
(―NH2), an acidic carboxyl group (―COOH), and an organic R group
(or side chain, it may be H, alkyl group or Aryl group) that is unique to
each amino acid. Each molecule contains a central carbon (C) atom,
called the α-carbon, to which both an amino and a carboxyl group are
attached. The remaining two bonds of the α-carbon atom are generally
satisfied by a hydrogen (H) atom and the R group. The general formula
amino acid is:
CLASSIFICATION OF AMINO ACID
Amino acids are classified as acidic, basic or neutral depending upon the
relative number of amino and carboxyl groups in their molecule. Equal
number of amino and carboxyl groups makes it neutral; more number of
amino than carboxyl groups makes it basic and more carboxyl groups as
compared to amino groups makes it acidic.
ACIDIC AMINO ACID
Essential and non - essential amino acids
The amino acids, which can be synthesized in the body, are known as
non- essential amino acids. On the other hand, those which cannot be
synthesized in the body and must be obtained through diet, are known as
essential amino acids
Amino acids are usually colourless, crystalline solids. These are water-
soluble, high melting solids and behave like salts rather than simple
amines or carboxylic acids; this behavior is due to the presence of both
acidic and basic groups in the same molecule. In aqueous solution, the
carboxyl group can lose a proton and amino group can accept a proton,
giving rise to a dipolar ion known as zwitter ion.
STRUCTURE OF PROTEINS
Proteins are the polymers of α-amino acids and they are connected to
each other by peptide bond or peptide linkage. Chemically, peptide
linkage is an amide formed between –COOH group and –NH2 group.
Tripeptide
Tetrapeptide
Polypeptide
CLASSIFICATION OF PROTEIN
Proteins can be classified into two types on the basis of their molecular
shape.
(1) Fibrous proteins
Fibrous proteins are made up of polypeptide chains that are elongated
and fibrous in nature or have a sheet like structure. These fibers and
sheets are mechanically strong and are water insoluble. They are often
structural proteins that provide strength and protection to cells and
tissue.
Eg: keratin (present in hair, wool, silk) and myosin (present in muscles),
etc
GLOBULAR PROTEINS
This structure results when the chains of polypeptides coil around to give
a spherical shape. These are usually soluble in water.
Eg: Insulin and albumins.
Structure and shape of proteins can be studied at four different levels,
i.e., primary, secondary, tertiary and quaternary.
Primary structure
SECONDARY STRUCTURE
The secondary structures of proteins are found to exist in two different
types of structures α-helix a n d β-pleated sheet structure.
Both structures are held in shape by hydrogen bonds, which form
between the carbonyl ‘O’ of one amino acid and the amino ‘H’ of
another.
α-HELIX
In an α helix, the carbonyl (C=O) of one amino acid is hydrogen bonded
to the amino H (N-H) of an amino acid that is four down the chain. (E.g.,
the carbonyl of amino acid 1 would form a hydrogen bond to the N-H of
amino acid 5.) This pattern of bonding pulls the polypeptide chain into a
helical structure that resembles a curled ribbon.
β-PLEATED STRUCTURE
This structure occurs when two segments of a polypeptide chain overlap
one another and form a row of hydrogen bonds with each other. This can
happen in a parallel arrangement or anti parallel arrangement.
The hydrogen bonds form between carbonyl and amino groups of
backbone, while the R groups extend above and below the plane of the
sheet.
(4) QUATERNARY STRUCTURE
Protein quaternary structure is the number and arrangement of
multiple folded protein subunits in a multi-subunit complex. It includes
organizations from simple dimers to large complexes of subunits.
STRUCTURE OF PROTEIN
DENATURATION OF PROTEIN
Denaturation involves the breaking of many of the weak linkages, or
bonds (e.g., hydrogen bonds), within a protein molecule that are
responsible for the highly ordered structure of the protein in its natural
(native) state. Denatured proteins have a looser, more random structure;
most are insoluble. Denaturation can be brought about in various ways—
e.g., by heating, by treatment with alkali, acid, urea, or detergents, and
by vigorous shaking.
NUCLEIC ACIDS
Nucleic acids are the biopolymers, or large biomolecules, essential to all
known forms of life. Nucleic acids are the most important of all biomolecules. These are found in abundance in all living things, where
they function to create and encode and then store information of every
living cell of every life-form organism on Earth.
In turn, they function to transmit and express that information inside and
outside the cell nucleus—to the interior operations of the cell and
ultimately to the next generation of each living organism. The encoded information is contained and conveyed via the nucleic acid sequence,
which provides the 'ladder-step' ordering of nucleotides within the
molecules of RNA and DNA.
The term nucleic acid is the overall name for DNA and RNA.
CHEMICAL COMPOSITION OF NUCLEIC ACIDS
Deoxyribonucleic Acid (DNA)
Chemically, DNA is composed of a pentose sugar, phosphoric acid and
some cyclic bases containing nitrogen. The sugar unit present in DNA
molecules is β-D-2-deoxyribose. The cyclic bases that have nitrogen in
them are adenine (A), guanine (G), cytosine(C) and thymine (T). These
bases and their arrangement in the molecules of DNA play an important
role in the storage of information from one generation to the next one.
DNA has a double-strand helical structure in which the strands are
complementary to each other.
RIBONUCLEIC ACID (RNA)
The RNA molecule is also composed of phosphoric acid, a pentose
sugar and some cyclic bases containing nitrogen. RNA has β-D-ribose in
it as the sugar moiety. The heterocyclic bases present in RNA are
adenine (A), guanine (G), cytosine(C) and uracil (U). In RNA the fourth
base is different from that of DNA. The RNA generally consists of a
single strand which sometimes folds back; that results in a double helix
structure. There are three types of RNA molecules, each having a
specific function:
STRUCTURE OF NUCLEIC ACIDS
A unit formed by the attachment of a base to 1 position of sugar is
known as nucleoside
-position of sugar
moiety, we get a nucleotide
Nucleic acids are polynucleotides—that is, long chain like molecules
composed of a series of nearly identical building blocks
called nucleotides. Each nucleotide consists of a nitrogen-containing
aromatic base attached to a pentose (five-carbon) sugar, which is in turn
attached to a phosphate group. Each nucleic acid contains four of five
possible nitrogen-containing bases.
DOUBLE STRAND HELIX STRUCTURE FOR DNA
Two nucleic acid chains are wound about each other and held
together by hydrogen bonds between pairs of bases. The two strands
are complementary to each other because the hydrogen bonds are
formed between specific pairs of bases. Adenine forms hydrogen
bonds with thymine whereas cytosine forms hydrogen bonds with
guanine.
BIOLOGICAL FUNTION OF NUCLEIC ACID