PROTIENS STRUCTURE, CLASSIFICATION AND FUNCTION

                   BY: Sreelakshmi S Nair

Proteins:

Proteins are macromolecules obtained from the one or more amino acids chain linked by peptide bonds. They are the natural polymers of amino acids. It contains nitrogen, carbon, hydrogen and oxygen. They act as a biological catalyst form structural parts of different organisms, participate in different cell reactions.

CLASSIFICATIONS OF PROTIENS

Proteins are classified on the basis of

• Structure of Protein
• Composition of Protein
• Functions of Protein

Based on the structure of Protein

• Fibrous Protein

They are linear in shape. Usually they do not have tertiary structure. They are physically strong and are insoluble in water. They perform the structural functions in the cell. Examples are: Keratin, Collagen, and Myosin.

• Globular Protein

They are spherical or globular in shape.Teritary structure is the most functional structure. Physically they are soft while comparing to fibrous proteins. They are readily soluble in water. Most of the proteins present in the cell belongs to globular protein. It forms enzymes, antibodies and some hormones. Examples are Insulin, haemoglobin, DNA polymerase and RNA polymerase.

Based on the composition of Protein

• Simple proteins

They are composed of only amino acids. They may be fibrous or globular. They are generally simple in structure. Examples are Collagen, Myosin, Insulin

• Conjugated Proteins

They are complex proteins which contains one or more amino acid components. The non-protein parts are called prosthetic group. Prosthetic group may contain metals, ions, carbohydrates, lipids, nucleic acid. Conjugated proteins are generally water soluble and globular in structure. Most of the enzymes are conjugated proteins.

Based on the Function

• Structural Proteins

Most of them are fibrous proteins. Components in connective tissue, bone, tendons, cartilage, skin, feathers, nail, hairs and horn are made of structural proteins.

• Enzymes

They are the biological catalyst. Mostly globular conjugated proteins. Examples are DNA polymerase, Nitrogenase, and Lipase.

• Hormones

They include proteinaceous hormone in the cells. Examples are Insulin, ACH

• Respiratory Pigments

They are coloured proteins. All of them are conjugated proteins. Examples are haemoglobin, Mycoglobin.

• Transport Proteins

They transport the materials in the cell. They form channels in the plasma membrane. They also form a component in blood and lymph in animals.

STRUCTURE OF PROTEIN

There are four levels

1. Primary Structure
2. Secondary structure
3. Tertiary Structure
4. Quaternary Structure

1. Primary Structure

They give details about the amino acid sequence of a protein. It tells about the number of amino acid residues in the protein and also about the sequence of amino acids. It is stabilized by peptide bonds. Each component of an amino acid is called residue or moiety. It starts from the amino terminal (N) end and ends in the carboxyl terminal(C) end.

• Secondary structure

It is formed by hydrogen bond between backbones atoms. Three most important secondary structure in protein are:

• α-helix
• β- plates
• β-Turns

α -Helix

It is the most common secondary structure which repeat every 5.4It is the simplest arrangement of a polypeptide chain which was proposed by Puling and Corey in 1954.It is so common because it makes optimal use of internal hydrogen bond. The interactions between the amino acid chains can stabilize or destabilize the α-helix.

                    β- Plates

It is an extended form of a polypeptide chain.Polypetide backbone forms a zigzag structure. Similar to α-Helix structure is stabilized by hydrogen bond. The R-groups of adjacent amino acids protrude from the zigzag structure to the opposite direction forming an alternative pattern. It can be arranged in either parallel or anti-parallel direction.

                    β-Turns

         It’s a very common in proteins where peptide make a reverse direction. It forms a 180 degree turn involving four amino acids. The carbonyl oxygen of the first residue forms a hydrogen bond with the amino group hydrogen in the fourth amino acid in the turn. Glycine and proline allows the β-Turns frequently.

3. Tertiary Structure

                   The tertiary structure will have a single polypeptide backbone consisting of one or more    secondary structures. It can be defined by atomic coordinates. It is stabilized with the help of both covalent and non-covalent bond.

4. Quaternary Structure

Proteins which have more than one polypeptide subunit and which do not have a permanent (covalent) interaction between the subunits (like disulphide bond) are classified under quaternary structure. Bonds stabilizing quaternary structure includes hydrogen bonds, hydrophilic interactions, hydrophobic interactions, van der Waals interactions. A protein with a single subunit cannot have a quaternary structure.

Functions of Protein

• Boosts Immune System
•  Provides Structure
• Maintains pH
• Transports and stores nutrients