The primary structure is the sequence of amino acids, which form a chain connected by . The amino acid sequence of a protein determines the higher levels of structure of the molecule. A single change in the primary structure (the amino acid sequence) can have a profound biological change in the overall structure and function.
The secondary structure is the way a small part, saptially near in the linear sequence of a protein folds up into:
the first structure described by Linus Pauling. It has a rod shape. The peptide is coiled around an imaginary cylinder and stabilized by hydrogen bonds formed between components of the peptide bonds.
sheets: the amino acids adopt the conformation of a and the structure is stabilized by hydrogen bonds between amino acids in different polypeptide strands. Note that some of the strands are parallel and some are antiparallel.
Other parts of the structure are not highly stable, and adopt a random coil formation.
The tertiary structure refers to the way random coils, alpha helixes and beta pleated sheets fold in respect to each other, i.e. it refers to the protein as a whole. Amino acids which are very distant in the primary structure might be close in the tertiary one because of the folding of the chain. They are still held by hydrogen bonds like the primary and secondary but in this stage sulphur bridges and ionic bonds are involved.
The quaternary structure is only present when a protein consists of two or more polypeptides. It refers to the way these polypeptides are arranged to form the biologically active protein.
A good example is the quaternary structure of haemoglobin, made up of two alpha and two beta polypeptide chains. It is found in red blood cells and contains four tightly packed polypeptide chains whose spatial arrangement is vital for the efficient functioning of the molecule.
Many structures in organisms consist of a protein combined with another molecule to form a conjugated protein. The non protein component to which the protein is attached is called the prosthetic group. Egg yolk, haemoglobin and other pigments are all examples of conjugated proteins.
If a protein is combined with a carbohydrate, the resulting compound is called a glycoprotein. Usually the protein forms the core of the molecule and the carbohydrate consists of a branched polysaccharide chain projecting from it.
The function of a protein is directly related to its shape. In general proteins fall into two groups; globular and fibrous.
Globular proteins have no systematic structures. There may be single chains, two or more chains which interact in the usual ways or there may be portions of the chains with helical structures, pleated structures, or completely random structures. Globular proteins are relatively spherical in shape as the name implies. Common globular proteins include haemoglobin, insulin, serum globulins in blood, and many enzymes. They are important in buffering and are generally water soluble. Among the many other classes of globular proteins, special mention should be made of antibodies. Each of us can make hundreds of thousand of different antibodies, which bind to particular disease-causing agents and toxins.
Fibrous proteins are insoluble and consist of long, parallel polypeptide chains cross linked at many points along their life. They are essential for many structures in the body. E.g. keratin is found in the skin and in hairs, feathers, nails, hooves and horns. Collagen is another fibrous protein. It is the most abundant protein in vertebrates, making up a third of their total protein mass. The human body is mainly held together by collagen as it is found in bones, cartilage, tendons, ligaments, connective tissue and skin. Collagen fibres have a tensile strength greater than steel which makes it very strong. Careful analysis of collagen had shown that they consist of three polypeptide chains coiled round each other in a triple helix. The resulting structure is like a plaited rope and has great strength.
In conclusion proteins are probably the most important class of biochemical molecules, although of course lipids and carbohydrates are also essential for life. Proteins are the basis for the major structural components of animal and human tissue.