Q.Give the differences between primary and secondary structure of proteins.

Protein structure is organised in levels: the primary structure is the amino-acid sequence, and the secondary structure is the general term for the regular, hydrogen-bonded shape the polypeptide backbone folds into — the $\alpha$-helix is just one specific example of a secondary structure, not the definition of the term itself.

Proteins exhibit four principal levels of structural organisation:

• Primary structure — the linear sequence of $\alpha$-amino acids joined by peptide bonds (option a describes this, not the secondary structure).
• Secondary structure — refers to the regular, repeating, fixed configuration (shape) that the polypeptide backbone adopts, stabilised by hydrogen bonding between the $C=O$ and $N-H$ groups of the peptide backbone. This general shape can take the form of an $\alpha$-helix (right-handed coil) or a $\beta$-pleated sheet — so "$\alpha$-helical backbone" (option c) names only one particular type of secondary structure, not the general definition asked for.
• Tertiary structure — the overall three-dimensional folding of the polypeptide chain, largely driven by hydrophobic interactions, hydrogen bonds, and disulphide linkages between side chains (option d describes a contributor to tertiary structure, not secondary structure).
• Quaternary structure — the spatial arrangement of two or more polypeptide subunits relative to each other.

So the secondary structure is correctly and generally defined as the fixed configuration of the polypeptide backbone.

Proteins are macromolecules formed when amino acid monomers polymerize via peptide bonds.

Amino acids are the monomeric (building-block) units of proteins. Each amino acid has an amino group (-NH2) and a carboxyl group (-COOH) attached to a central carbon. When the carboxyl group of one amino acid reacts with the amino group of another, water is eliminated and a peptide bond (-CO-NH-) is formed, linking them into a dipeptide; repeated linking of many amino acids in this way (polymerization) produces long chains called polypeptides, which fold into functional proteins.

Collagen — the fibrous structural protein of connective tissue — is present in the largest quantity of any protein across the animal world. Answer: (d) Collagen.

Step 1 — Recall the relative abundance of listed proteins.

• Collagen is a triple-helical fibrous protein that forms the major structural component of skin, bones, tendons, ligaments and cartilage in animals. Because connective tissue is distributed throughout every vertebrate (and many invertebrate) body, collagen constitutes the single largest fraction of total body protein — commonly cited as being about a quarter to a third of all protein in mammals.
• Tubulin forms microtubules (cytoskeleton, spindle fibres) — present in every cell but in much smaller total mass than structural collagen.
• Keratin is abundant but restricted mainly to epidermal structures (hair, nails, horns, feathers), a smaller total mass than collagen.
• Globulin refers to a class of globular proteins (e.g., blood plasma globulins, antibodies) present in comparatively small amounts.

Step 2 — Conclusion.

Because connective tissue containing collagen pervades the whole body (skin, bone matrix, tendons, blood vessel walls, etc.), collagen outweighs the other three proteins in total abundance.

A polypeptide's primary structure — its linear sequence of amino acids joined by peptide bonds — has two chemically distinct ends: the N-terminus and the C-terminus.

When amino acids join through peptide bonds (formed by condensation between the -COOH of one amino acid and the -NH2 of the next), the resulting polypeptide chain retains one free amino group at one end (the N-terminal/amino end) and one free carboxyl group at the other end (the C-terminal/carboxy end), since only the amino acids in the interior of the chain have both their functional groups tied up in peptide bonds. By convention, the amino acid sequence of a protein is always written starting from its N-terminus.

Collagen, the main protein of connective tissue (tendons, cartilage, bone matrix, skin), is present in such large quantities across the animal kingdom that it is the most abundant protein of all.

When biomolecules of a cell/organism are analysed quantitatively, structural proteins dominate by mass, and collagen tops that list.

• Collagen is a fibrous, triple-helical protein secreted by fibroblasts and forms the major structural component of connective tissues — tendons, ligaments, cartilage, bone, and skin (dermis).

• Because connective tissue is so widespread and forms a large fraction of an animal's body mass, collagen alone accounts for roughly 25–35% of total body protein in vertebrates.

• This makes it the most abundant protein in the animal world (by contrast, RuBisCO is often called the most abundant protein on Earth overall, because it also includes the plant kingdom).

Collagen is the most abundant protein among animals; RuBisCO holds that title for the biosphere as a whole (mainly due to plants).

Among proteins, RuBisCO (ribulose-1,5-bisphosphate carboxylase-oxygenase) is often cited as the most abundant protein on Earth overall, because of its huge quantity in plant chloroplasts worldwide. However, restricted specifically to the animal world, collagen is the most abundant protein — it is the major structural protein of connective tissues such as skin, bone, cartilage and tendons in animals. Cellulose is a polysaccharide (not a protein) and is the most abundant organic compound in plants; insulin is a hormone protein but present in far smaller quantities.