Enzymes are divided into six classes each with 4-13 sub-classes and named accordingly by a four-digit number.
(i) Oxidoreductases/dehydrogenases These enzymes take part in oxidation and reduction or transfer of $e^{-}$.
$$\mathrm{S} \text { (reduced) }+\mathrm{S}^{\prime} \text { (oxidised) } \longrightarrow \mathrm{S} \text { (oxidised) }+\mathrm{S}^{\prime} \text { (reduced) }$$
(ii) Transferaes These enzymes transfer a functional group from one molecule to another (other than hydrogen). The chemical group transfer does not occur in free state.
$$S-G+S^{\prime} \xrightarrow{\text { Transferase }} S+S^{\prime}-G$$
(iii) Hydrolases These enzymes catalyse the hydrolysis of bonds like ester, ether, peptide, glycosidic C-C, C-halide, P-N etc.
$$\mathrm{C}_{12} \underset{\text { Maltose }}{\mathrm{H}_{22}} \mathrm{O}_{11}+\mathrm{H}_2 \mathrm{O} \xrightarrow{\text { Maltase }} \underset{\text { Glucose }}{2 \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}$$
(iv) Lyases These enzymes causes cleavage, removal of groups without hydrolysis and addition of groups to double bonds or removal of groups producing double bond.
(v) Isomerases These enzymes cause rearrangement of molecular structure to effect isomeric changes. They are of three types, i.e., isomerases, epimerases and mutases.
$$\begin{gathered} \text { Glucose - } 6 \text { - phosphate } \xrightarrow[\text { (Aldose to ketose group or vice-versa) }]{\text { Isomerase }} \text { Fructose } 6 \text { - phosphate } \\ \text { } \end{gathered}$$
Glucose - 6 - phosphate $\xrightarrow{\text { Mutase }}$ Glucose-1-phosphate (Shifting the position of side group)
Xylulose 5-phosphate $\xrightarrow{\text { Epimerase }}$ Ribulose-5-phosphate (Change in position of one constituent or carbon group)
(vi) Ligases are enzymes catalysing bonding of two chemicals with the help of energy obtained from ATP resulting formation of bonds such as $\mathrm{C}-\mathrm{O}, \mathrm{C}-\mathrm{S}, \mathrm{C}-\mathrm{N}$ and P-O e.g., pyruvate carboxyl use
$$\mathrm{Ab}+\mathrm{C} \longrightarrow \mathrm{~A}-\mathrm{C}+\mathrm{b}$$
$$\text { Pyruvric acid }+\mathrm{CO}_2+\mathrm{ATP}+\mathrm{H}_2 \mathrm{O} \stackrel{\text { pyruvate carboxylase }}{\rightleftharpoons} \text { Oxaloacetic }+\mathrm{ADP}+\mathrm{Pi}-$$
Nucleic acids are long chain macromolecules which are formed by end to end polymerisation of large number of repeated units called nucleotides. Nucleic acids show a wide range of secondary structures. A secondary structure is the set of interactions between bases and sugar phosphate backbone and is responsible for the shape that nucleic acid.
James Watson and Francis Crick proposed a secondary structure of DNA molecules based on the crystallographic studies.
(i) DNA or deoxyribonucleic acid is a helically twisted double-chain polydeoxyribonucleotide macromolecule.
(ii) The two strands of DNA run anti-parallely to each other called as DNA duplex.
(iii) The spiral twisting of DNA has two types of alternate grooves, i.e., major and minor.
(iv) One turn of $360^{\circ}$ of the spiral has about 10 nucleotides on each strand of DNA, occupying a distance of about 3.4 nm .
(v) The nucleotides within each strand are held together by the phosphodiester bonds between the $5^{\prime}$ carbon of one nucleotide and the $3^{\prime}$ carbon of the adjacent nucleotide. These strong covalent bonds holds the sugar/phosphate backbone together.
(vi) The two strands of DNA are held together by weak hydrogen bonds between the nitrogenous bases. These hydrogen bonds are base specific. That is adenine forms 2 hydrogen bonds with thymine $\mathrm{CA}=\mathrm{T}$ and cytosine forms 3 hydrogen bonds with guanine ( $\mathrm{C} \equiv \mathrm{G}$ ).
(vii) As specific and different nitrogen bases occur on two DNA chains, they are said to be complementary, i.e., purine lies opposite to pyrimidine. This purine-pyrimidine pairing also contributes to the thickness of strand, i.e., 2 nm , and makes the two chains complementary.
Difference between a nucleotide and nucleoside is as follows
| Nucleoside | Nucleotide |
|---|---|
| Nucleoside is a compound formed by the union of the nitrogenous base with a pentose sugar | Nucleotide is compound formed by union of nitrogen base, a pentose sugar and phosphae. |
| It is slightly basic in nature | A nucleotide is acidic in nature |
| It is a component of nucleotide and forms with both ribose and deoxyribose sugars. | Nucleotide is formed through phosphorylation of nucleoside |
e.g., cytidine, uridine, adenosine, guanosine, thymidine and inosine. |
e.g., AMP, GMP, CMP, UMP, dTMP (deoxythymidine monophosphate)  |
Lipids are the esters of higher fatty acid with alcohol, such as glycerol, etc.
These can be classified as
1. Simple Lipids are esters of fatty acids with alcohol. These may be
(i) Fats These are esters of higher fatty acids with glycerol (triglycerides).
(ii) Waxes These are esters of higher fatty acids with alcohol other than glycerol.
2. Compound or conjugated lipids, are those compounds which contain simple lipids and prosthetic (other additional) group. They include
(i) Glycerophospholipids, also known as phospholipids in which one of the fatty acid is replaced by phosphoric acid which is linked to nitrogenous groups like choline, ethanolamine, serine etc, e.g., Lecithin and cephalin, etc.
(ii) Sphingo lipids, are lipides having phosphoric acid with amine alcohol 4-sphinganine or sphingosine instead of glycerol in addition to fatty acid and choline.
(iii) Glycolipids, i.e., those which contain spinganine with a fatty acid and a monosaccharide sugar, e.g., cerebrosides and gangliosides.
3. Steroids are compounds with different chemical nature but similar physical properties. Their structure is based upon a 4 ring cyclopentenoperhydro phenanthrene, e.g., cholesterol.
4. Prostoglandins are derivatives of arachidonic acid and contain 20 C -atoms. These are biologically active lipids.
