Protein Structure INTRODUCTION OF PROTIEN

Organic compounds containing C,H,O,N,P,S Comprise 50% of dry weight of cell. Made up of Amino acids. Protein are found in; Nails, feather, skin, hair, animal fur, Food sources; Meat, egg, pulses, milk.

STRUCTURE OF AMIINO ACID:

CLASSIFICATION ACCORDING TO STRUCTURE Primary structure Secondary structure Tertiary structure Quaternary structure PRIMARY STRUCTURE: The specific sequence of amino acids in the polypeptide is called primary structure.

By convention, the beginning of any polypeptide chain at the amino group and the end is at the carbonyl group. The repeating section of the polypeptide is also known as backbone. The variable portion of the polypeptide are the side chains. The polypeptide chain has the ability to form the hydrogen bond.

H-bond donor: N-H group H-bond acceptor: C-O group. Peptide bonds are resonance stabilized.

Double bond nature of the peptide bond ; Makes the peptide bond planer. Prevent any rotation about the peptide bond. In the majority of cases trans polypeptide bonds are energetically more favourable than cis because there is no bumping of atoms. Trans is thermodynamically more stable and lower in energy than the cis configuration.

Φ; angle of rotation about the single bond between the N and αCarbon. Ψ; angle of rotation about the single bond between the αcarbon and carbonyl carbon. SECONDARY STRUCTURE: Once the primary structure of the polypeptide is formed it begins to twist into regular pattern that made up the secondary structure. These include the α helix, the β pleated sheet, the β turns and Ω loop.

These twists are formed as a result of the regular pattern of H-bond between N-H and C- O group as polypeptide chain. α -helix; The α-helix is a rod like structure that contain the backbone on the inner portion and the side chain on the outer portion. The screw sense of the α-helix describes the direction in which the helix rotate w.r.t its axis. A right handed helix rotate clockwise while the left handed helix rotate counter clockwise.

The right handed helix predominates because there is less stearic hindrance between the side chains.

In the anti parallel beta sheet two linear polymer of amino acid runs in opposite directions. In this arrangement the N-H and C-O group of an amino acid are one strand form H-bond with C-O and N-H groups of the opposing amino acid on the other strand. Β -TURNS: An amino acid on one strand connect to two amino acids on the opposing end via H-bond.

The compact nature of the protein is part due to polypeptide ability to make sudden turns in the chain. These turns are called βturns or reverse turns and are stabilized by H-bonding. Tertiary structure; The tertiary structure refers to the spatial arrangement of amino acids that are found far way from one another along the polypeptide chain.

There are several important interactions that are involved. HYDROPHOBIC INTERACTIONS: Those amino acids with hydrophobic side chain will tend to be found inside the protein. Those amino acids that contain hydrophilic side chain will tend to be found outside the protein. A protein in an aqueous environment will have a hydrophobic core and a hydrophilic surface.

Vander Waals interaction Hydrogen bond Ionic interaction Di sulphide bridge Quaternary structure: some proteins are made up of multiple polypeptide chains, also known as subunits. When these subunits come together, they give the protein its quaternary structure. Weak interactions such as hydrogen bonding and London dispersion forces also hold the subunits together to give quaternary structure.

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