Define the following as related to proteins (i) Peptide linkage (ii) Primary structure (iii) Denaturation

(i)Peptide linkage

When two molecule of –amino acids similar or different combine in this way that the amino group of one molecule with carboxy group of the other molecule, this result in the elimination of water molecule and the formation of new bond-CO NH- . This type of bond is called peptide bond or peptide linkage. The product of reaction is called dipeptide. For example, COOH group of valine combine with NH2 group of alanine, we get dipeptide Valylalanine (Val-Ala) as final product, as shown below:-

(ii)Primary structure:

Proteins may have one or more polypeptide chains. Each polypeptide in a protein has amino acids linked with each other in a specific sequence and due to this sequence of amino acids the structure of proteins is said to be primary structure. Any change in this sequence of amino acids creates a different proteins. In other words, the sequence in which various amino acids are arranged in a protein is called primary structure. The amino acid sequence of a protein determines its function and its critical of its biological activity.

(iii)Denaturation

Each protein in the biological system has a unique 3-D structure and has specific biological activity. This is called native form of a protein. When a protein in its native form is subjected to physical changes such as change in temperature, pH, etc. hydrogen bonds are broken. Due to cleavage of hydrogen bonds, unfolding of protein molecule occurs and the protein loses its biological activity. This loss of biological activity is called denaturation. During denaturation, 2-degree and 3-degree structures of proteins are destroyed but 1-degree structure remains intact. As a result of denaturation, globular proteins are converted into fibrous proteins. In other words, denaturation leads to coagulation. That is why coagulated proteins are also called denaturated proteins.

 

 

 

 

 

 

 

 

 

13.What are the common types of secondary structure of proteins?

 

Ans:-There are two common types of secondary structure of proteins:

 

(i)α–helix structure

 

(ii)β–pleated sheet structure

 

 

 

α–Helix structure

 

If the size of R-groups is quite large then the intramolecular bonds are formed between the C=O of one amino acid and the N-H group of the forth amino acid residue in the chain. This causes the polypeptide chain to coil up into a spiral structure called righ handed α-helix structure.

 

 

 

β -pleated sheet structure

 

In this conformation, the polypeptide chains lie side by side in a zig0zag manner with alternate R groups on the same side situated at fixed distances apart. The two such neighbouring polypeptide chains are held together by interbounded to form a sheet. These sheets are then stacked one above the other like the pages of the book to form a 3-D structure. This structure resembles pleated folds of drapery and hence is called β–pleated sheet structure. The polypeptide chains can link together in parallel and anti-parallel sequence. Such sheet like structure can easily slip on each other. Proteins of this structure are soft.

 

 

 

 

 

 

 

 

 

 

 

14.What type of bonding helps in stabilising the α-helix structure of proteins?

 

Ans:-The stability of α-helix structure is due to intramolecular hydrogen bonding between –NH and –CO groups of polypeptide chain. The α-helix is known as 3.613 because each turn of α-helix contains approximately 3.6 amino acids and a 13-membered  ring is formed by hydrogen bonding.

 

 

 

 

 

15.Differentiate between globular and fibrous proteins.

 

Ans:-

 

 

 

 

 

How do you explain the amphoteric behaviour of amino acids?

 

Ans:-Amino acids contain an acidic (carboxyl group) and a basic (amino) group within the same molecule. In aqueous solution, they neutralize each other. The carboxyl group loses a proton while the amino group accepts it. S a result, A dipolar or zwitter-ion is formed.

 

 

 

 

 

In zwitter ionic form, the amino acid show amphoteric behaviour as they react with both acids and bases. In the acidic medium, COO- ion of the zwitter-ion accepts a proton to form the cation first, while in the basic medium,+NH3 ion loses a proton to form the anion, as shown below:-

 

 

 

 

 

Thus, amino acids show amphoteric behaviour.

 

 

 

 

 

 

 

16.What are enzymes?

 

Ans:-Enzymes are naturally occurring simple conjugate proteins acting as specific catalysts in all processes. In contrast to ordinary chemical catalyst, it loses activity by pH or temperature change.

 

 For example:- the enzyme used to catalyze the hydrolysis of maltose into glucose is named as maltase.

 

 

 

Enzymes are highly specific, i.e., a particular enzyme catalyses a specific reaction. For example, urase attacks on urea. This specific action is due to active sites present in the enzyme molecule (E) that fits into substrate (S) and forms E-S complex which changes into product P and E. 

 

Enzymes increase the speed of reactions. They can catalyze several million of reactions per second.

 

 

 

17.What is the effect of denaturation on the structure of proteins?

 

Ans: - During denaturation, 2-degree and 3-degree structures of proteins are destroyed but 1-degree structure remains intact. As a result of denaturation, The globular proteins (soluble in water) are converted into Fibrous proteins (insoluble in water) and their biological activity is lost. For example, boiled egg which contains coagulated proteins cannot be hatched to produce chickens. 

 

 

 

 

 

 

 

19. How are vitamins classified? Name the vitamin responsible for the coagulation of blood.

 

Ans:-Vitamins can be defined as the essential dietary factors required by an organism in minute quantities and whose absence causes specific deficiency diseases. Vitamins are broadly classified into two types:-

 

(i)Fat-soluble vitamins:

 

These are oily substances not readily soluble in water. These include vitamins A, D , E and K.

 

(ii)Water-soluble vitamins:-

 

Vitamins that are soluble in water belong to this group. For example :B group vitamins (B1 ,B2 ,B6, B12 ,etc.) and vitamin C. 

 

 

 

Vitamin K is responsible for the coagulation of blood.

 

Vitamin H (Biotin) as an exception, it is neither soluble in water nor in fat

 

 

 

 

 

 

 

20. Why are vitamin A and vitamin C essential to us? Give their important sources.

 

Ans:-The deficiency of Vitamin A leads to Xerophthalmia which is hardening of the cornea of the eye and night blindness as well. Vitamin-C is essential to us because its deficiency causes scurvy which is a phenomenon of bleeding gums; and pyorrhoea which is phenomenon of loosening-bleeding of teeth.

 

The sources of vitamin A are fish, cod liver oil, carrots, butter and milk. 

 

The sources of vitamin C are citrus fruits, lemon, amla and green leafy vegetables.

 

 

 

 

 

 

 

21. What are nucleic acids? Mention their two important functions.

 

Ans:-Nucleic acids are Biomolecules which are found in the nuclei of all living cells, inform of nucleoproteins or chromosomes (proteins containing nucleic acids as the prosthetic group). Nucleic acids are of two types: – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are also known as Polynucleotide as they are long- chain polymers of nucleotides.

 

The two important functions of nucleic acids are listed below:-

 

 

 

(i)DNA  which is responsible for the transference of hereditary effects from one generation to another, which is due to their property of replication during cell division as a result of which two identical DNA strands are transferred to the daughter cells.

 

(ii)Nucleic acids (both DNA and RNA) are responsible for synthesis of all proteins needed for the growth and maintenance of our body. Actually, the proteins are synthesised by various RNA molecules in the cell but the message for the synthesis of a particular protein is given by DNA molecules.

 

 

 

 

 

 

 

 

 

22. What is the difference between a nucleoside and a nucleotide?

 

Ans:-A nucleoside is formed when l-position of a pyrimidine (cytosine, thymine or uracil) or 9-position of a purine (guanine or adenine) base is attached to C-l of sugar (ribose or deoxyribose) by a linkage. Thus in general, nucleosides may be represented as: Sugar-Base.

 

 Nucleoside= sugar + base

 

On the other hand , all the three basic components of nucleic acids (i.e., pentose sugar, phosphoric acid and base) are present in a nucleotide.These are obtained by esterification of C5’ –OH group of the pentose sugar by phosphoric acid. Thus, in general, a nucleotide is represented as:-

 

Nucleotide= sugar + base + phosphoric acid

 

 

 

 

 

 

 

 

 

23. The two strands in DNA are not identical but are complementary. Explain.

 

Ans:-In the helical structure of DNA, the two strands are held together by hydrogen bonds between specific pairs of bases. Cytosine from hydrogen bond with guanine, while adenine forms hydrogen bond with thymine. As a result , the two strands are complementary to each other.

 

 

 

DNA consists of two strands of nucleic acid chains coiled around each other in the form of a double helix. The base of one strand of DNA is paired with bases on other strand by means of hydrogen bonding. This hydrogen bonding is very specific as the bases can only base pair in a complementary manner. Adenine pairs with only thymine via 2 hydrogen bonds and guanine pairs with cytosine through 3 hydrogen bonds. Thus, the two strands of DNA are complementary to each other in the sense that the sequence of bases in one strand automatically determines that of the other. So the DNA stands cannot be identical, but they are complementary to each other.

 

 

 

 

 

 

 

25. What are the different types of RNA found in the cell?

 

Ans:-The different types of RNA found in the cell are listed below:-

 

(i)Messenger RNA (m-RNA)

It carries the genetic message code from the DNA to ribosomes. It is produced by the DNA; m-RNA is also single stranded and constitutes about 15% of total RNA. 

(ii)Ribosomal RNA (r-RNA)

It is found in the ribosomes and it is usually associated with protein to form the ribosomes. It is synthesised in the nucleus by DNA. It is single stranded, comprising  about 80% of total RNA. It is metabolically stable.

(iii)Transfer RNA (t-RNA)

It is synthesised in nucleus by DNA. It is also called soluble RNA. It is single stranded. There are 20 different kinds of t-RNA and each type has specificity for a particular amino acid. It constitutes about 5% of total RNA. It has very short life.