Biomolecules

Use app

Notes

5 min read

Biomolecules

- A quick revision of all the important concepts

Carbohydrates Carbohydrates are polyhydroxy aldehydes or ketones or compounds which give rise to polyhydroxy aldehydes or ketones upon hydrolysis. They are optically active as they have chiral "C". The other name of carbohydrates is saccharides meaning "sweet". The word saccharide is derived from Greek word "sakkharon"

Classification of carbohydrates

Monosaccharides: Monosaccharides can further be classified on the basis of the number of carbon atoms. If it contains aldehyde group then it is known as aldose and if it has ketone group then it is called ketose.

Glucose: It occurs both in free as well as combined state Preparation of Glucose: It is obtained from sucrose and starch. From Sucrose: It is obtained by boiling sucrose with dilute

H C l

H_{2} S O_{4}

in alcoholic solution. Both glucose and fructose are obtained in equal amounts. $C_{12} H_{22} O_{11} + H_{2} O H^{+} C_{6} H_{12} O_{6} + C_{6} H_{12} O_{6}$
From Starch: It is obtained by hydrolysis of starch by boiling it with dilute

H_{2} S O_{4}

at 393 K temperature and 2-3atm pressure.
$(C_{6} H_{10} O_{5})_{n} + n H_{2} O H^{+}, 393 K n C_{6} H_{12} O_{6}$
Structure of Glucose: It is an aldohexose and is a monomer of many larger carbohydrates. It is also known as dextrose.The structure of glucose was established based on the following properties:

The molecular formula was found to be $C_{6} H_{12} O_{6}$ .
It gives n-hexane on prolonged heating with HI.

$C H O - (C H O H)_{4} - C H_{2} O H H I, δ C H_{3} (C H_{2})_{4} - C H_{3}$ From the above reactions observation it was inferred that six carbon in a glucose molecule forms a straight chain.

Glucose on treatment with hydroxylamine form an oxime

On reaction with hydrogen cyanide it gives cyanohydrin.

These reactions confirmed the presence of a carbonyl group in glucose.

Glucose on treatment with mild oxidising agents such as bromine water gets oxidised to gluconic acids indicates that carbonyl group is present as an aldehydic group.

Acetylation of glucose with acetic anhydride confirms the presence of 5 OH group as it gives glucose pentaacetate.

Glucose contains one primary alcoholic group. It was confirmed when both glucose and gluconic acid on treatment with dilute nitric acid gave the same dicarboxylic acid(saccharic acid).

Configuration of Glucose:

Cyclic Structure of Glucose: The Fischer structure of glucose explained most of the properties but still there were some properties which this structure failed to explain.

Glucose has an aldehydic group but it failed to give Schiff's test and also it doesn't form hydrogensulphite addition product with $N a H S O_{3}$ .
The pentaacetate of glucose does not react with hydroxylamine which indicates the absence of free -CHO group
Glucose exists in two different crystalline forms which are named as $α$ -D glucose and $β$ -D glucose. The $α$ -form of glucose is obtained by crystallisation from concentrated solution of glucose at 303 K while the $β$ -form is obtained by crystallisation from hot and saturated aqueous solution.

The ring structure of glucose is formed by reaction between -CHO group and alcoholic group at C-5 position. Thus, the ring structure is hemiacetal. The two hemiacetals differ only in the configuration of C-1. The two ring structures are called

α

and

β

anomers of glucose and C-1 is the anomeric carbon.

Fructose and its Structure: Fructose has molecular formula

C_{6} H_{12} O_{6}

and is an important ketohexose. It is obtained by hydrolysis of sucrose. On the basis of chemical reactions it was found that it contains a ketonic functional group at C-2. Fructose exists as a mixture of fructopyranose(major) and fructofuranose in free state. Fructose in a combined state exists as fructofuranose.

Disaccharides: Disaccharides on hydrolysis gives two units of the same or different monosaccharides. The general formula of disaccharides is

C_{12} H_{22} O_{11}

. The three common and important disaccharides are sucrose, maltose and lactose. Glycosidic Linkage: The two monosaccharides combine by an oxide linkage with the loss of water molecule. This linkage between two monosaccharides is known as glycosidic linkage. Sucrose: Sucrose is dextrorotatory and on hydrolysis gives an equimolar mixture of D-(+) glucose and D-(-) fructose. $C_{12} H_{22} O_{11} + H_{2} O H^{+} C_{6} H_{12} O_{6} + C_{6} H_{12} O_{6}$ The laevorotation of fructose (

- 92 . 4^{0}

) is more than dextrorotation of glucose (

+ 52 . 5^{0}

), so the mixture is laevorotatory. Thus, hydrolysis of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo (-) and the product is named as invert sugar.

Maltose: The maltose is composed of two

α

-D-glucose units in which C1 of one glucose (I) is linked to C4 of another glucose unit (II).

Lactose: It is known as milk sugar. Lactose is formed from two monosaccharide units know as D-galactose and D-glucose. The glycosidic linkage is formed between C-1 of

β

-D galactose and C-4 of glucose. Therefore the linkage is called

β

-1,4-glycosidic linkage. It is called reducing sugar because the free aldehyde group may be produced at C-1 of glucose unit.

Polysaccharides: They are formed by joining a large number of monosaccharide units by glycosidic linkage. The general formula of polysaccharides is

(C_{6} H_{10} O_{5})_{n}

. The most common natural polysaccharides are starch, cellulose, and glycogen. Starch: Starch is a polymer of

α

-D glucose. It consists of two components amylose and amylopectin. Amylose constitutes 15-20% of starch and is a water soluble component. It has a long unbranched chain of 200-1000

α

-D-glucose units connected together by C1-C4 glycosidic linkage.

Amylopectin constitutes about 80-85% of starch and it is a branched chain polymer of

α

-D glucose. It has C1-C4 and C1-C6 glycosidic linkage

Cellulose: Cellulose is a straight chain polysaccharide of

β

-D glucose units which are joined together by glycosidic linkage between C1 of one glucose and C4 of another glucose unit.

Glycogen: It has a similar structure as amylopectin but is more branched. It is also known as animal starch.

Proteins Proteins are very important for living beings. They are the main constituents of living beings and essential for growth and development. The word protein is derived from Greek word Proteios meaning prime importance. They are highly complex nitrogenous compounds with high molecular mass. They are formed from

α

- amino acids. Amino Acids: The compounds made up of carboxylic(

- C O O H

) group and amino(

- N H_{2}

) group are known as amino acids. The amino acids present in proteins are

α

amino acids.The general formula of

α

-amino acids is shown

α

-amino acids have trivial names which mostly reflect the property of that compound or that source. Generally, amino acids are represented by three letter symbols and sometimes one letter symbol is also used.

Classification of amino acid

On the basis of relative number of amino and carboxyl group

On the basis of their synthesis in the body

Properties of Amino acids

Amino acids are colourless, crystalline solids with high melting points
They are soluble in water

Structure of Proteins

α

-amino acids are the building blocks of proteins and they are linked with peptide bonds. Peptide bond is an amide formed by condensation of two amino acids with the elimination of water molecules.

Peptides are of different types depending upon the number of amino acids combined
Dipeptide: It is formed by the condensation of two same or different amino acids
Tripeptide: It is formed by condensation of 3 amino acids
Polypeptides: The peptides formed from large number of

α

-amino acid molecules are called polypeptides
A polypeptide with more than hundred amino acid residues and molecular mass higher than 10,000u is called a protein.

Classification of Proteins:

They are classified on the basis of their shape into two types
Fibrous Proteins:

The proteins consisting of linear thread-like polypeptide chains arranged to form fibres are called fibrous proteins.
They(polypeptide chains) are held together by hydrogen and disulphide bonds. Fibrous proteins are generally insoluble in water.
Some fibrous proteins are keratin, myosin, etc.

Globular Proteins:

Polypeptide chains folded to give spherical shape to the protein molecules are called globular proteins.
They are water soluble.
Common examples of globular proteins are albumin and insulin.

Structure of Proteins are studied at four levels i.e. primary, secondary, tertiary and quaternary structure.

Primary Structure: It refers to the sequence of amino acids held together by peptide bond. If there is any change in the primary structure it will lead to a different protein.

Secondary Structure: The secondary structure of protein refers to the shape in which long polypeptide chains can exist. They have two different structures i.e.

α

-helix and

β

-pleated structure.

α

-helix: It forms when a polypeptide chain twists into a right handed or clockwise spiral with the -NH group attached to the -C=O of an adjacent turn of the helix by the hydrogen bonding.

β

-pleated:
The

β

-pleated structure of protein consists of extended strands of polypeptide chains held together by hydrogen bonding.

Tertiary Structure:

It refers to the three dimensional shape of the protein molecule which results from twisting, folding and bending the helix.

It involves the folding of the entire molecule.
On the basis of folding, two types of molecular shapes are possible i.e. fibrous and molecular.

The main forces which stabilise the $2^{0}$ and $3^{0}$ structures of proteins are hydrogen bonds, disulphide linkages, van der Waals and electrostatic forces of attraction.

Quaternary Structure: When two or more polypeptide chains with folded tertiary structures come together into one protein complex, the resulting shape is called a quaternary structure.

Denaturation of Proteins:
The process of losing the molecular structure of proteins without breaking the peptide/amide bonds is called denaturation. It also results in the loss of biological activity of the proteins.
Secondary and tertiary structures are destroyed during denaturation of proteins while primary structure remains intact.
Examples of denaturation of protein are curdling of milk and coagulation of egg while cooking.

Enzymes

Enzymes are essential for the life process as they help in catalysing the biochemical reactions occurring in plants and animals bodies. Without enzymes, living processes will be too slow to sustain life. They are also known as globular proteins.
Enzymes are very specific in their action on substrates.
For example; enzyme urease hydrolyses urea to $N H_{3}$ and $C O_{2}$ but it does not hydrolyse N-methylurea( $C H_{3} N H C O N H_{2}$ ).

N H_{2} C O N H_{2} + H_{2} O U r e a s e 2 N H_{3} + C O_{2}

C H_{3} N H C O N H_{2} + H_{2} O U r e a s e N o r e a c t i o n

Mechanism of Enzyme Action:
Fischer suggested a Lock and Key Mechanism to explain the action of an enzyme as a catalyst. Other mechanisms came but the lock and key mechanism holds prominence.
Lock and key mechanism involves following steps:
Step I: Binding of enzyme E to the substrate S to form an enzyme substrate complex

E + S \to [E S]

Step II: Product formation in the complex

[E S] \to E P

Step III: Release of the product from the enzyme

E P \to E + P

Vitamins
The term vitamine was coined from vital+amine. Later on letter 'e' was dropped as most of the vitamins do not contain the amino group.
Vitamins are organic substances present in small amounts in natural foods. Having too little of any particular vitamin may increase the risk of developing certain health issues.
Most of the vitamins are not synthesised in our body and produced by foods.
Vitamins are represented by alphabets A,B,C,D,E,K. Some of them are further categorised into sub groups like vitamin B:

B_{1}

B_{2}

B_{6}

, etc.

Sources and Deficiency of Vitamins

Nucleic Acids
Nucleic acids are an important class of biomolecules which are present in nuclei of all living cells. They are large molecules with high molecular mass. They play a very important role in reproduction and development of living beings.
Nucleic acids are biopolymers, where the monomer unit is nucleotide. They can also be called polynucleotides. They are of two types:

Deoxyribo nucleic acid (DNA)
Ribonucleic acid (RNA)

Chemical Composition of Nucleic Acid

DNA molecule contains

β

-D-2-deoxyribose sugar and RNA contains

β

-D-ribose
The nitrogen bases present in nucleotide is either derivative of purine or pyrimidine

The bases present in DNA are adenine(A), guanine(G) cytosine(C) and thymine(T) whereas RNA contains adenine(A), guanine(G). cytosine(C) and uracil(U).

The base sugar unit present in a nucleic acid chain is termed as nucleoside.
Nucleoside = Base + Sugar
In nucleosides, the sugar carbons are numbered as 1', 2', 3', etc. in order to distinguish these from the bases. When nucleoside is linked to phosphoric acid at the 5'-position of sugar moiety, we get a nucleotide.

As shown in the image below, nucleotides are joined together by phosphodiester linkage between 5' and 3' carbon atoms of pentose sugar:

The simplified form of nucleic acid is shown below:

The sequence in which four nitrogenous bases are attached to the sugar-phosphate backbone of a nucleotide chain in a nucleic acid is called the primary structure of that nucleic acid. They also have secondary structures.
A double strand helix structure for DNA was given by James Watson and Francis Crick. Two nucleic acid chains are coiled 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.
In the secondary structure of RNA, a single stranded helix is present which sometimes folds back on itself. RNA molecules are of three types and they perform different functions. They are named as messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA).

Hormones
Hormones act as the intercellular messenger in human beings and other living organisms.They are secreted by endocrine glands and are transported by blood circulation in different parts of the body to produce stimulatory or inhibitory effects.
Hormones can be classified into two types based on their chemical nature:

Steroid Hormones
Peptide Hormones

Steroid Hormones:

Steroid hormones are fat-soluble
Example-Testosterone, Estrogen and Progesterone are examples of steroid hormones.

Peptide Hormones:

Polypeptide hormones are composed of amino acids and are soluble in water
Example-Insulin, oxytocin, vasopressin

Function of peptide hormones:

Regulating Glucose level:

Insulin is released in response to the rapid rise in blood glucose level.

Insulin helps control blood glucose levels by signaling the liver and muscle and fat cells to take in glucose from the blood.

On the other hand, hormone glucagon tends to increase the glucose level in the blood.

The two hormones together regulate the glucose level in the blood.

Response to External stimuli:

Epinephrine and norepinephrine mediate responses to external stimuli.

Maintaining thyroxine level:

Thyroxine produced in the thyroid gland is an iodinated derivative of amino acid, tyrosine.

Abnormally low levels of thyroxine leads to hypothyroidism which is characterised by lethargy and obesity.

Increased levels of thyroxine causes hyperthyroidism.

Maintaining body function

Steroid hormones are produced by adrenal cortex and gonads

Hormones released by the adrenal cortex play a very important role in the functions of the body

Example-glucocorticoids control the carbohydrate metabolism, modulate inflammatory reactions and are involved in reactions to stress.

The mineralocorticoids control the level of excretion of water and salt by the kidney.

Hormones released by gonads are responsible for development of secondary sex characters

Example- Testosterone and Progesterone