1. Proteins Molecules

1.4. Classification of proteins base on their level of organization

1. Primary proteins

2. Secondary proteins

3. Tertiary proteins and 

4. Quaternary proteins

a) Primary proteins

The primary structure of a protein is defined by the linear sequences of amino acid residues. Protein contains between 50 and 2000 amino acid residues.

The molecular mass of most proteins is between 5500 and 220,000 Da.

The amino acid composition of a peptide chain has a profound effect on its physical and chemical properties of proteins.

Proteins rich in polar amino acids are more water soluble while proteins rich in aliphatic or aromatic amino groups are relatively insoluble in water and more soluble in cell membranes (can easily cross the cell membrane).

b) Secondary Structure

The secondary structure of a protein refers to the local structure of a polypeptide chain, which is determined by hydrogen bond. 

There are two types of secondary structure, the ∝ - helix and the β- pleated sheet.

The α - helix

The α - helix is a rod like structure with peptide chains tightly coiled and the side chains of amino acid residues extending outward from the axis of spiral. Each amide carbonyl group is hydrogen bonded to the amide hydrogen of a peptide bond that is 4 - residues away along the same chain.

The β- pleated sheet

The β – pleated sheet is an extended structure as opposed the coiled ∝ - helix. It is pleated because the (C-C) bonds are tetrahedral and cannot exist in a planar configuration. If the polypeptide chain runs in the same direction, if forms a parallel β – sheet. It is said to be parallel, and when in opposite direction, antiparallel. 

for example, most immunoglobulins have such β-pleated conformation and some enzymes like Hexokinase contain a mixed α-β conformation.

c) Tertiary Structure

The three dimensional, folded and biologically active conformation of a protein is referred to as tertiary structure. The structure reflects the overall shape of the molecule. The three - dimensional tertiary structure of a protein is stabilized by interactions between side chain functional group, covalent, disulfide bonds, hydrogen bonds, salt bridges, and hydrophobic interactions. 

d)Quaternary Structure

Quaternary structure refers to a complex or an assembly of two or more separated peptide chains that are held together by non-covalent or, in some case, covalent interactions. 

If the subunits are identical, it is a homogeneous quaternary structure; but if there are dissimilarities, it is heterogeneous. For instance, insulin consists of A and B chain which are different. Haemoglobin has 4 chains, two of them are α and two are β. These, the polymers may be dimers, trimers, tetramers and so on.