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Proteins which are used as a catalyst in biochemical reaction are known as biocatalysts.

Specific characteristics

Enzymes have following two specific characters as :

(i) Specificity (ii) Efficiency

Specificity of enzymes

(a) Generally, one enzyme can catalyze only one biochemical reaction.

(b) It can increase the rate of reaction upto 1020 times.

(c) In some cases one enzyme can catalyze more than one reaction and one reaction can be catalyzed by more than one enzyme. eg. Enzymes present in Yeast (Zymase) can ferment both glucose and fructose into alcohol and also cane-sugar can be hydrolysed by invertase and sucrase enzymes.

Efficiency of enzymes:

(a) One molecule of enzyme can convert millions of substrate molecules into product per second.

eg. Carbonic anhydrase enzyme present in red blood cells has the highest turn over number.

(b) With having tertiary structure it can be collected as crystals.

Enzymes are denatured at higher temperatures.

(c) Enzymes can be stored at low temperature as they are inactivated.

Importance of enzymes

In the thousands of enzymes present in body if even a single enzyme would be absent or damaged then complex disease results in.

eg. Scarcity of Phenylalanine hydroxylase enzyme in human body results in Phenylketonuria disease.

Factors affecting Enzyme Action:

(i) Optimum temperature and pH. Enzyme catalysed reactions have maximum rate at physiological pH of around 7.4 and human body temperature of 37ºC (310 K) under one atmosphere pressure.

In fact, as the temperature or pH is increased, the rate rises to a maximum (at 37ºC or pH = 7.4) and then falls off.

(ii) Enzyme activators (coenzymes). The activity of certain enzymes is increased in the presence of certain substances, called co-enzymes. It has been observed that if a protein contains a small amount of vitamin as the non-protein part, its activity is enhanced considerably. The activators are generally metal ions such as Na , Mn2+ , Cu2+ , Co2+  etc. These metal ions are weakly bonded to the enzyme molecules and increase their catalytic activity. For example, the enzyme, amylase in presence of NaCl, which provides Na+  ion, shows a very high catalytic activity.

(iii) Enzyme inhibitors and poisons. Just as in the case of catalysts, the activity of enzyme is slowed down in the presence of certain substance. Such substances are called inhibitors or poisons. They act by combining with the active functional group thereby reducing or completely destroying the catalytic activity of the enzymes. The use of many drugs is on account of their action as enzyme inhibitors in our body.


Sodium, Potassium and Chlorine

(i) Na+  is the principal mineral cation in the extracellular fluid.
(ii) K + is the principal cation inside the cell.
(iii) Clis the principal mineral anion in the ECF.
(iv) Na+ and K+ are essential to the maintenance of water balance and acid-base balance.
(v) Na+ and K+ are important in nerve impulse transmission.

Calcium and Phosphorus

(i) Calcium and phosphorus are deposited in bones and teeth to give them strength and rigidity.

(ii) Ca2+  is also essential for blood coagulation, neuromuscular function, cardiac function and actions of many enzymes and hormones.

(iii) Phosphorus enters into many compounds such as nucleic acids and phospholipids, many co-enzymes and high energy compounds like ATP.

(iv) Calcium plays an essential role in sustaining intestinal peristalsis and growth of body tissues.


(i) Iron is required for haemoglobin synthesis.

(ii) Iron is essential both for transportation of oxygen to tissues and for operation of oxidative systems within the tissue cells.


(i) Magnesium is required as a catalyst for many intracellular enzymatic reactions, particularly those relating to carbohydrate metabolism.

(ii) Mg is the central metal atom in chlorophyll.


Iodine is used in the synthesis of thyroid hormones.


(i) Zinc is a constituent of carbonic anhydrase, present in RBCs helping in COtransport.

(ii) Zinc is a component to lactic dehydrogenase, important for the interconversion between pyruvic acid and lactic acid.

(iii) Zinc is a component part of some peptidases and therefore is important for digestion of proteins in the alimentary canal.


(i) Cobalt helps in erythropoiesis and in the activities of some enzymes.

(ii) It is present in vitamin B12 .


(i) Copper helps in the utilisation of iron.

(ii) Copper deficiency may produce anaemia because of failure in iron utilisation.


(i) Molybdenum is a constituent of oxidase enzymes (xanthine oxidase).

(ii) Molybdenum plays an important role in biological nitrogen fixation.


(i) Fluorine maintains normal dental enamel and prevents dental caries.

(ii) Excessive intake of fluorine cause fluorosis characterized by mottled teeth and enlarged bones.


It has been observed that certain organic compounds are required in small amounts in our diet but their deficiency causes specific diseases. These compounds are called vitamins.

Classification of Vitamins-

Vitamins are classified into two groups depending upon their solubility in water or fat.

(i) Fat soluble vitamins:

Vitamins which are soluble in fat and oils. But insoluble in water are kept in this group. These are vitamins A, D, E and K. They are stored in liver and adipose (fat storing) tissues.

(ii) Water soluble vitamins:

B group vitamins and vitamin C are soluble in water so they are grouped together. Water soluble vitamins must be supplied regularly in diet because they are readily excreted in urine and can not be stored (except vitamin B12) in our body.

Some important vitamins, their sources and diseases caused by their deficiency are listed in table.



Name of Vitamins


Deficiency Diseases


Vitamin A (Retinol)

Fish liver oil, carrots, butter and milk

Xerophthalmia (hardening of cornea of eye) Night blindness


Vitamin B1(Thiamine)

Yeast, milk, Green Vegetables and cereals and grams

Beri beri (loss of appetite, retarded growth)


Vitamin B(Riboflavin)

Milk, egg white, liver, Kidney

Cheilosis (fissuring at corners of mouth and lips), digestive disorders and burning sensation of the skin


Vitamin B(Pyridoxine)

Yeast, milk, egg yolk, cereals and grams



Vitamin B1


Meat, fish, egg and curd

Pernicious anaemia (RBC deficient in haemoglobin)


Vitamin C (Ascorbic acid)

Citrus fruits, amla and green leafy vegetables

scurvy (bleeding gums)


Vitamin D (Calciferol)

Exposure to sunlight, fish and egg yolk

Rickets (bone deformities in children) and osteo­malacia (soft bones and joint pain in adults)


Vitamin E or Tocopherol (α, β and γ) or Anti Sterility factor

Eggs, Milk, Fish, Wheat germ oil cotton seed oil etc.

Sterility (loss of sexual power and reproduction)

Nucleic Acids: Chemical Composition & Structure

Nucleic Acids

(a) These are special type of acids which are present in nucleus & cytoplasm.

(b) Control the metabolic activities of cell.

(c) They are also found in Mitochondria, centriole and chloroplast.

Types → These are of 2 types:

DNA-Deoxyribonucleic acid

RNA- Ribonucleic acid
(d) Fischer discovered Nitrogen bases in 1888.

Enzymes & Nucleic Acids | Chemistry Class 12 - NEET

(e) Levan found sugar.

Enzymes & Nucleic Acids | Chemistry Class 12 - NEET

Deoxyribonucleic Acid (D.N.A.):

(a) It is found in Nucleus.

(b) They  are on pneumococcus bacteria.

(c) DNA made up of 3 units-

Enzymes & Nucleic Acids | Chemistry Class 12 - NEET

(i) Thymine (i) Adenine

(ii) Cytosine (ii) Guanine

(d) Nucleoside

When nitrogen base is combined with deoxyribose sugar, it constitutes a nucleoside.

Adenine Deoxyribose → Deoxyadenosine
Guanine Deoxyribose → Deoxyguanosine
Cytosine Deoxyribose → Deoxycytidine
Thymine Deoxyribose → Deoxythymidine

(i) Nucleotide

(a) Nitrogen base Sugar Phosphate → Nucleotide.

(b) Nucleotide is a unit of DNA.

(c) All nucleotides combined and form a chain called polynucleotides by which RNA and DNA are formed.

Structure of DNA-

(a) Double Helical model of DNA was proposed by biochemist J.D.Watson, British chemist FHC Crick in 1953.

Enzymes & Nucleic Acids | Chemistry Class 12 - NEET

(b) DNA in double stranded structure is made up of two chains of polynucleotides.

(c) DNA is a polymer of Nucleotide.

(d) Nucleotide are joined by 3' → 5' phosphodiester bonds.
(e) Sugar and phosphorous are alternately arranged.

(f) In both chains, in between A and T, 2 Hydrogen bonds are present while in C and G, 3H bonds are present. (A = T) (C º G)

(g) A always attaches with T while C always attaches with G.

(h) Purine and pyrimidine are found in ratio 1 : 1 cells.

(i) DNA is attached with histone protein.

(j) In prokaryotic cell and mitochondria, circular DNA is present.

Functions of DNA:

(i) Self - Replication or self -Duplication

DNA has the property of self - replication. It is therefore a reproducing molecule. This unique property of DNA is at the root of all reproduction. Through its replication, DNA acts as the key to heredity. In the replication of DNA, the two strands of a double helix unwind and separate as a template for the formation of a new complementary strand.

(ii) Protein Synthesis

The specific sequence of base pair in DNA represents coded information for the manufacture of specific proteins. These coded instructions first are transcribed into the matching nitrogen-base sequences within mRNA and the instructions in such RNA subsequently are translated into particular sequence of amino acid units within the polypeptide chains and proteins.

The major steps in the utilization of the genetic information can be represented as :

D N A Enzymes & Nucleic Acids | Chemistry Class 12 - NEET D N A Enzymes & Nucleic Acids | Chemistry Class 12 - NEETR N A Enzymes & Nucleic Acids | Chemistry Class 12 - NEETProtein

Ribonucleic Acid (RNA):

⇒ Found in cytoplasm as well as in nucleus.

Cytoplasm → In the ribosome (higher amount).

Chemical Nature:

⇒ Ribonucleic acid is a polymer of purine and pyrimidine ribonucleotides linked by 3' → 5' phosphodiester bridges. The number of nucleotides in RNA ranges from as few as 75 to many thousands. Although sharing many features with DNA, RNA possesses several specific differences.

⇒ As indicated by its name, sugar in RNA to which the phosphate and nitrogen- bases are attached is ribose rather than the deoxyribose of DNA.

⇒ Although RNA contains the ribonucleotides of adenine, guanine, and cytosine, it does not posses thymine. Instead of thymine, RNA contains the ribonucleotides of uracil. Thus, the pyrimidine components of RNA differs from those of DNA.

⇒ RNA exists basically as a single-stranded molecule rather than as a double -stranded helical molecule, as does DNA. However the single strand of RNA is capable of folding back on itself like a hairpin and thus acquiring double-stranded characteristics. In these regions, A pairs with U and G pairs with C.

Thus a given segment of a long RNA molecule might, for example, be represented as follows:

Enzymes & Nucleic Acids | Chemistry Class 12 - NEET

⇒ where R stands for ribose ; A, U, G, and C for Adenine, Uracil, Guanine and Cytosine respectively.

Types of RNA and their Functions:

There are 3 main types of RNA molecules:

(i) Messenger RNA (mRNA)
(ii) Transfer RNA (tRNA)
(iii) Ribosomal RNA (rRNA)

(i) Messenger RNA (mRNA)

⇒ This type of RNA consists of single strand of variable length and serves as a template for protein synthesis. Code is in the chromosomes.

⇒ mRNA forms complementary copy of DNA as it carries chemical messages in the form of nitrogen-base sequence from the nucleus to the ribosomes, i.e. from DNA to cytoplasm where proteins are synthesized. Therefore, it is called messenger RNA or mRNA.

⇒ mRNA is sythesised from DNA in the nucleus.

⇒ It is called transcription.

(ii) Ribosomal RNA

⇒ A ribosome is a cytoplasmic nucleoprotein structure which serves as the organellar machinery for protein synthesis from mRNA templates.

⇒ On the ribosome, the mRNA and tRNA molecules interact to translate into a specific protein molecule the information transcribed from the DNA.

⇒ rRNA constitutes the largest part of total RNA (Highest) - 80%

(iii) Transfer RNA (RNA)

⇒ These are also called Soluble RNA.

⇒ Single stranded.

⇒ 10-15% of the total RNA.

Size - Smallest: 75 - 80 nucleotides only.

Synthesis - Within nucleus from DNA.

Function- It transports amino acid from cytoplasm to the site of protein synthesis.

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FAQs on Enzymes & Nucleic Acids - Chemistry Class 12 - NEET

1. What is the chemical composition of enzymes?
Enzymes are composed of proteins, which are made up of chains of amino acids. These amino acids are linked together by peptide bonds to form the primary structure of the enzyme. The specific sequence of amino acids determines the enzyme's three-dimensional structure and function.
2. How are nucleic acids structured?
Nucleic acids are composed of nucleotides, which are made up of three components: a sugar molecule (either ribose or deoxyribose), a phosphate group, and a nitrogenous base. The nitrogenous bases can be of two types: purines (adenine and guanine) or pyrimidines (cytosine, thymine, and uracil). Nucleotides are connected through phosphodiester bonds, forming a chain that makes up the primary structure of nucleic acids.
3. What is the role of enzymes in nucleic acids?
Enzymes play a crucial role in nucleic acids metabolism. For example, DNA polymerase is an enzyme that catalyzes the synthesis of new DNA strands during DNA replication. This enzyme adds nucleotides to the growing DNA chain, following the complementary base pairing rule. Similarly, RNA polymerase is an enzyme involved in transcription, synthesizing RNA molecules using a DNA template.
4. How do enzymes interact with nucleic acids?
Enzymes can interact with nucleic acids through specific binding sites. These binding sites on enzymes recognize and bind to specific regions on nucleic acids, allowing the enzymes to perform their respective functions. For example, restriction enzymes recognize and bind to specific DNA sequences, allowing them to cut the DNA at specific sites. This binding specificity ensures that enzymes only act on their intended targets.
5. How do changes in nucleic acid structure affect enzyme activity?
Changes in nucleic acid structure can have significant effects on enzyme activity. Mutations in the DNA sequence can alter the binding sites for enzymes, preventing them from effectively recognizing and binding to the nucleic acid. This can lead to a loss of enzyme function or a decrease in enzyme activity. Additionally, changes in the RNA structure can affect the ability of RNA-binding enzymes to interact with and process RNA molecules.
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