Proteins are the large-sized, heteropolymeric macromolecules having one or more polypeptides (chains of amino acid). Primary Structure The primary structure of a protein is the linear sequence of amino acid structural units and partially comprises its overall biomolecular structures. The amino acids are linked together in a sequence by peptide bonds.

In the primary structure of protein initiate from an amino-terminal $(\mathrm{N})$ to the carboxyl terminal (C) end,

Secondary Structure It is a three dimensional form of local segments of bipolymers such as proteins. Secondary structure of proteins is defined by hydrogen bonds between backbone amino and carboxyl groups. Mainly secondary structure in proteins possess two forms, i.e., $\alpha$-helix and $\beta$-pleated sheet.

$\alpha$-helix is a polypeptide chain spirally coiled to form a right handed helix. This helix may be coiled regularly at places and at some places randomly coiled. The helix is stabilised by many hydrogen bonds which are formed between - CO of one amino acid and - NH group of next fourth amino acid.

$\beta$-pleated sheets two or more polypeptide chains are joined together by intermolecular hydrogen bonds to produce a sheet like structure instead of fibre as in $\alpha$-helix. The polypeptide strands in a sheet may run parallel in same direction, e.g., keratin or in opposite direction called antiparallel $\beta$-sheet, e.g., fibroin.

Tertiary structure involves interactions that are caused by the bending and folding of $\alpha$-helix or $\beta$-sheets leading to the formation of rods, spheres of fibres. Such interactions are typically conferred by H-bonds, ionic bonds, covalent bonds, van der Waat's interactions and hydrophobic interactions or disulphide linkages. It gives the protein a three dimensional conformation.

Nucleic acids are polymeric macromolecules or large biological molecules, essential for all known forms of life. The secondary structrure of a nucleic acid molecule refers to the base pairing interactions within a single molecule or set of interacting molecules. DNA and RNA represent two main nucleic acids, however their secondary structures differ e.g., the secondary structure of DNA comprises of two complementary strands of polydeoxyribonucleotide, spirally coiled on a common axis forming a helical structure. This double helical structure of DNA is stabilised by phosphodiester bonds (between 5' of sugar of one nucleotide and 3 of sugar of another nucleotide), hydrogen bonds (between bases, i.e., hydrogen of one base and nitrogen of oxygen of other base) and ionic interactioins.

Living organism are not in equilibrium because system at equilibrium cannot perform work. The living organisms exist in a steady state characterised by concentration of each of the biomolecules. These biomolecules are in a metabolic flux. Any chemical or physical process moves simultaneously to equilibrium. As living organisms work continuously, they cannot afford to reach equilibrium. Hence, the living state is in a non-equilibrium steady-state to be able to perform work. This is achieved by energy input provided lay metobolism.

Each enzyme molecule has an active site for specific binding of substrate molecules. The enzyme work by altering the activation energy of the reaction.

The catalytic site of an enzyme can be described as follows

(i) The substrate process to the active site of the enzyme, fitting into it.

(ii) Binding of the substrate induces the enzymes to alter its shape leading to formation of the Enzyme Substrate (ES) complex.

(iii) The active site of the enzyme, now is in close proximity with the substrate and break its chemical bonds and a new enzyme product complex is formed.

(iv) The enzyme releases the products of the reaction and the free enzyme is ready to bind to another molecule of substrate and run through the catalytic cycle once again.

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

$$\mathrm{Ab}+\mathrm{C} \longrightarrow \mathrm{~A}-\mathrm{C}+\mathrm{b}$$

$$\text { Pyruvric acid }+\mathrm{CO}_2+\mathrm{ATP}+\mathrm{H}_2 \mathrm{O} \stackrel{\text { pyruvate carboxylase }}{\rightleftharpoons} \text { Oxaloacetic }+\mathrm{ADP}+\mathrm{Pi}-$$