Biomolecules

Biomolecules are chemical compounds found in living organisms. They include organic and inorganic compounds.

BIOMICROMOLECULES

Include amino acids, sugars, nitrogen bases, lipids etc.

1. Amino acids

A typical amino acid is formed of an amino group (-NH₂), an acid group (-COOH), H & a variable group (R). –NH₂ and  –COOH are attached to the same carbon atom (α-carbon).

20 amino acids are used for protein synthesis. They include

·   Acidic amino acids (e.g. glutamic acid)

·   Basic amino acids (e.g. Lysine)

·   Neutral amino acids (e.g. Valine)

Some amino acids are aromatic (e.g. tyrosine, phenyl alanine, and tryptophan).

Amino acids have ionizable nature of –NH₂ & –COOH groups.

2. Lipids

-    Water insoluble. Contains fatty acids and glycerol.

-    A fatty acid has –COOH group attached to an R- group.

-    Palmitic acid has 16 carbons (CH₃-(CH₂)₁₄-COOH) and Arachidonic acid has 20 Carbons.

-    Fatty acids are 2 types:

o Saturated fatty acids: No double or triple bonds between 2 carbon atoms.

o Unsaturated Fatty acids: One or more C=C bonds.

-    Structure of glycerol (trihydroxy propane):

CH₂-OH

|

CH-OH

|

CH₂-OH

-    Many lipids have both glycerol and fatty acids to form fats. (Fatty acids are esterified with glycerol).

-    It includes monoglycerides, diglycerides & triglycerides.

-    Based on melting point lipids are 2 types: fats and oils.

3. Sugars (Carbohydrates)

Sugars are sweet and water soluble carbohydrates. They are formed of C, H and O in the ratio of 1:2:1.

4. Nitrogen Bases

Includes Purines (Adenine & Guanine) and Pyrimidines (Cytosine, Thymine & Uracil) 

•   Nucleoside: Nitrogen base + Sugar.

Adenine + sugar           =         Adenosine

Guanine + sugar           =         Guanosine

Cytosine + sugar         =         Cytidine

Thymine + sugar         =         Thymidine

Uracil + sugar             =         Uridine 

•   Nucleotide: Nitrogen base + Sugar + phosphate.

Adenine + sugar + phosphate    =     Adenylic acid

Guanine + sugar + phosphate    =     Guanylic acid

Cytosine + sugar + phosphate   =     Cytidylic acid

Thymine + sugar + phosphate    =     Thymidylic acid

   Uracil + sugar + phosphate        =      Uridylic acid 

Nucleotides are heterocyclic compounds.

Nucleic acids (DNA & RNA) are made up of nucleotides.

BIOMACROMOLECULES

-    Molecules having molecular weight greater than 1000 Dalton (Da) is called biomacromolecules.

-    Molecules having molecular weight less than 1000 Da are called micromolecules.

-    Molecular weight of compounds found in the acid soluble pool ranges from 18 to 800 Da. The acid soluble pool represents the cytoplasmic composition.

-    Biomacromolecules are found in acid insoluble fraction.

-   

Molecular weight is 10,000 Da and above except lipids.

Acid insoluble fraction includes

o Proteins

o Nucleic acids            

o Polysaccharides

o Lipids

-    Molecular weight of lipids does not exceed 800 Da. But it comes under acid insoluble fraction because lipids are arranged into structures like cell membranes. When a tissue is grinded, these membranes are broken and form vesicles which are water insoluble, i.e. lipids are not strictly macromolecules.

-    Acid insoluble fraction: Includes macromolecules from cytoplasm and organelles.

Average composition of cells

Water                : 70-90 %

Protein              : 10-15%

Carbohydrates    : 3%

Lipids               : 2%

Nucleic acids     : 5-7%

                        Ions                  : 1%

2. Polysaccharides

These are polymers of sugars (monosaccharides). E.g.

§ Starch (polymer of glucose)

§ Cellulose (polymer of glucose)

§ Inulin (polymer of fructose)

§ Glycogen

§ Glucosamine

§ N-acetyl galactosamine

  § Chitin (Exoskeleton of arthropods) 

§ Glycosidic bond in polysaccharides: It is the bond formed when individual monosaccharides are linked between 2 carbon atoms by dehydration.

§ Starch forms helical secondary structure. Starch holds I₂ molecules in the helical portion giving blue colour.

§ Cellulose has no complex helices and so cannot hold I_2.

§ Diagrammatic representation of a portion of glycogen is given below:

3. Nucleic acids (DNA and RNA)

Secondary structure of DNA (Watson - Crick Model)

-    DNA exists as a double helix. The 2 polynucleotide strands are arranged antiparallely.

-    One full turn of helical strand have 10 steps (10 base pairs).

-    Length of one full turn = 34 Å (i.e. 3.4 Å for each step).

-    At each step the strand turns 36⁰ (360⁰ for a full turn).

-    A polynucleotide chain is heteropolymer of nucleotides.

-    A nucleotide consists of nitrogen bases, deoxyribose sugar and phosphate group. 

-    The backbone of DNA is formed by the sugar-phosphate-sugar chain. Nitrogen base pairs form the steps of DNA.

-    Nitrogen bases include Adenine (A), Guanine (G), Thymine (T) and Cytosine (C).

-    A pairs with T (A=T) by 2 hydrogen bonds.

-    G pairs with C (G≡C) by 3 hydrogen bonds.

-    A phosphate molecule links the 3’-carbon atom of one sugar of one nucleotide to the 5’-carbon of the sugar of the succeeding nucleotide.

-    There is an ester bond between PO₄^3- and –OH group of sugar. As there is one such ester bond on either side, it is called phosphodiester bond.