Proteins: Study Notes

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Protein:

Amino acids

• Amino acids are the building blocks of proteins.
• 20 amino acids are known as standard amino acids that commonly occur in proteins.
• A protein may also possess non-coded amino acids. Such amino acids are called as rare amino acids. E.g. Hydroxyproline from proline, hydroxylysine from lysine
• A typical amino acid is formed of an amino group (basic) (-NH,), an acid group (-COOH), -H and a variable group (R) that gives each amino acid its uniqueness. The simplest amino acid is glycine having R group is hydrogen. If R group is CH3 then it is alanine.
• The chemical and physical properties of amino acids depend upon the amino group, carboxyl group and R group.
• More carboxyl group- acidic amino acid
• More amino group-basic amino acid
• Equal amino and carboxylic group-neutral amino acid.

Amino acids are amphoteric compounds and contain both a basic group and an acidic group. Such ions are described as dipolar and are called zwitter ion.

Based on nutritional requirements, amino acids are grouped into two classes:

(1) Essential amino acids which cannot be synthesized by the body E.g. Arginine, valine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, and tryptophan. Arginine and histidine are called semi-essential amino acids as they can be partly synthesized in our body.

(ii) Non-essential amino acids can be synthesized in our body to meet the biological needs E.g. Glycine, alanine, serine, cysteine, aspartic acid, glutamic acid, aspargine, glutamine, tyrosine, and proline.

TYPES OF BONDS IN PROTEIN

• Peptide Bond Formation
• Ionic bond
• DISULFIDE bond: Between two molecules of cysteine due to sulfhydryl groups
• Hydrogen bond
• Hydrophobic interactions

Proteins

• Proteins are variously folded linear heteropolymer of amino acids.
• Molecules containing more than 10 amino acids are called polypeptides.
• The linear chains of amino acids are linked by peptide bonds.
• A protein having two or more polypeptides is called multimeric protein.
• Proteins make up 50% of the total dry weight of the cell.

Amino acid (1) -> Peptide (<50 amino acids) -> Peptones (50-100 amino acids) -> Protein (>100 amino acids)

Structure of Protein:

Proteins are macromolecules formed by the polymerization of amino acids. On the basis of configuration proteins are divided into four levels.

(a) Primary structure: It is the linear sequence of amino acids in a polypeptide chain. Eg. Insulin.

(b) Secondary structure: The polypeptide chain is coiled to form a three-dimensional structure. It has only right handed helices. There are three types of secondary structures:

**(i) α-helix:**In α -helix, the polypeptide chain is coiled spirally, in right handed manner. The helix is stabilized by hydrogen bonds between two amino acids. E.g. keratin, myosin, epidermis, fibrin etc.

(ii) β –pleated: In β-pleated secondary structure, two or more polypeptides are interconnected by hydrogen bonds. They are arranged in a parallel direction or in the same direction or in opposite direction. A sheet is produced instead of a fiber or rod as in α-helix E.g. β-keratin, fibroin of silk

(iii) Collagen helix: In collagen helix, three polypeptide chains are wound around each other like the strands of a rope to form a triple helix. Each chain is itself in the form of a loose helix (not a α helix). The three strands or chains are held together by hydrogen bonds.

(c) Tertiary structure:

• The helical polypeptide chain is further coiled and folded to form a complex structure. It gives 3-D view of protein.
• They are covalent bond ionic bond, hydrogen bonds, disulphide bond and hydrophobic interaction.
• The process of breaking down of tertiary structure is known as denaturation.
• e.g. Myoglobin

(d) Quaternary structure:

• Many highly complex proteins consist of more than one polypeptide chain.
• The separate chains are held together by hydrophobic interactions and hydrogen and ionic bonds.
• Hemoglobin shows such a structure. It consists of four separate polypeptide chains of two types, namely two α chains and two ß chains.

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