Question

Describe te primary ,secondary,tertiary and quartenary structure of protein?

Answer 1

Primary Structure

There are 20 different standard L-α-amino acids used by cells for protein construction. Amino acids, as their name indicates, contain both a basic amino group and an acidic carboxyl group. This difunctionality allows the individual amino acids to join together in long chains by forming peptide bonds: amide bonds between the -NH2 of one amino acid and the -COOH of another. Sequences with fewer than 50 amino acids are generally referred to as peptides, while the terms protein or polypeptide are used for longer sequences. A protein can be made up of one or more polypeptide molecules. The end of the peptide or protein sequence with a free carboxyl group is called the carboxy-terminus or C-terminus. The terms amino-terminus or N-terminusdescribe the end of the sequence with a free α-amino group.

Secondary Structure

Stretches or strands of proteins or peptides have distinct characteristic local structural conformations orsecondary structure, dependent on hydrogen bonding. The two main types of secondary structure are the α-helix and the ß-sheet.

The α-helix is a right-handed coiled strand. The side-chain substituents of the amino acid groups in an α-helix extend to the outside. Hydrogen bonds form between the oxygen of the C=O of each peptide bond in the strand and the hydrogen of the N-H group of the peptide bond four amino acids below it in the helix. The hydrogen bonds make this structure especially stable. The side-chain substituents of the amino acids fit in beside the N-H groups.

Tertiary Structure

The overall three-dimensional shape of an entire protein molecule is the tertiary structure. The protein molecule will bend and twist in such a way as to achieve maximum stability or lowest energy state. Although the three-dimensional shape of a protein may seem irregular and random, it is fashioned by many stabilizing forces due to bonding interactions between the side-chain groups of the amino acids.

Quaternary Structure

Many proteins are made up of multiple polypeptide chains, often referred to as protein subunits.hese subunits may be the same (as in a homodimer) or different (as in a heterodimer). The quaternary structure refers to how these protein subunits interact with each other and arrange themselves to form a larger aggregate protein complex. 

Protein Stability

Due to the nature of the weak interactions controlling the three-dimensional structure, proteins are very sensitive molecules. The term native state is used to describe the protein in its most stable natural conformation in situ. This native state can be disrupted by a number of external stress factors including temperature, pH, removal of water, presence of hydrophobic surfaces, presence of metal ions and high shear. The loss of secondary, tertiary or quaternary structure due to exposure to a stress factor is called denaturation. Denaturation results in unfolding of the protein into a random or misfolded shape.

Answer 2

Answer:

1. Primary structure of proteins: Refers to sequence of amino acids linked together through peptide bonds in a polypeptide chain. It also refers to position of disulphide bonds (-S-S) that are formed between the -SH groups of cysteine residues that are adjacent in space but not in the linear sequence of amino acids.
2. Secondary structure of proteins: The polypeptide chain undergoes regular folding or coiling to give rise secondary structure of protein. The coiling of polypeptide chain is produced and stabilized by weak, low energy non-covalent hydrogen bonds and covalent disulpide bonds. Each hydrogen bond is weak, but collectively hydrogen bonds are strong enough to stabilize the secondary structure. The two most prominent types of secondary structure of proteins are (i) alpha-Helix & (ii) beta-Pleated sheet.
3. Tertiary structure of proteins: Refers to overall three-dimensional structure (native conformation) of a protein molecule. The tertiary structure is derived by further folding and super-folding of polypeptide chain(s) exhibiting secondary structure. The tertiary structure is stabilized by covalent disulfide bonds and various weak non-covalent bonds (hydrophobic interactions/van der Waal forces, ionic interactions and hydrogen bonding).
4. Quaternary structure of proteins: Quaternary structure refers to arrangement of multiple protein subunits (polypeptide chains) in a multi-subunit protein. Proteins having two or more polypeptide chains (protein subunits), are known as multi-subunit proteins or multimeric or oligomeric proteins (like CPK, LDH, glycogen phosphorylase, Hb, etc.) and exhibit quaternary structure. Different multi-subunit proteins have 2-200 subunits. In a multi-subunit protein, all the polypeptide chains (subunits) are arranged in a characteristic manner and held together by non-covalent bonds (hydrogen bonds, ionic bonds & hydrophobic interactions) & covalent disulfide bonds to give rise a quaternary structure.