Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. In this lesson, we will learn about
WHAT ARE PROTEINS?
They may be defined as the high molecular weight mixed polymers of α-amino acids joined together with peptide linkage (-CO-NH-). Proteins are the chief constituents of all living matter. They contain carbon, hydrogen, nitrogen, and sulfur and some contain phosphorus also.
Plants can synthesize all of the amino acids; animals cannot, even though all of them are essential for life.
WHAT IS THE STRUCTURE OF PROTEINS?
The function of proteins depends on their structure. They are bio-polymers consisting of one or more strings of amino acid residues joined head-to-tail via peptide bonds. Each string folds into a 3-dimensional structure. There are four levels of protein structure:
The primary structure is held together by covalent bonds, which are made during the process of translation. The process by which the higher structures form is called protein folding and is a consequence of primary structure. Although any unique polypeptide may have more than one stable folded conformation, each conformation has its own biological activity and only one conformation is considered to be the active or native conformation.
If a region of a protein has any secondary structure, it is either an alpha helix or beta-sheet. The string is folded further into larger three-dimensional structures that are held together by hydrogen bonds, hydrophobic interactions and/or disulfide bonds.
Proteins are generally large molecules, sometimes having molecular masses of up to 3,000,000. Such long chains of amino acids are almost universally referred to as proteins but shorter strings of amino acids are referred to as polypeptides, peptides or very rarely oligopeptides.
Proteins may only exist in their active or native state, in a small range of pH values and under solution conditions with a minimum quantity of electrolytes, as many proteins will not remain in solution in distilled water. A protein that loses its native state is said to be denatured. Denatured proteins generally have no secondary structure other than a random coil. A protein in its native state is often described as folded.
WHAT ARE THE DIFFERENT TYPES OF PROTEINS?
1. Contractile proteins
Actin and myosin of the skeletal system are two examples of contractile proteins. These are responsible for muscle contraction and movement. Actin controls muscle contraction as well as cellular movement and division processes. Myosin supplies energy to the tasks carried out by actin.
2. Transport proteins
They bind and carry specific molecules or ions from one organ to another. For example, hemoglobin is responsible for transporting oxygen through the blood via red blood cells; cytochromes operate in the electron transport chain as electron carrier protein; lipoproteins in blood plasma carry lipids from the liver to other organs. There are other kinds of transport proteins in the plasma membranes and intracellular membranes of all organisms that bind and transport glucose and amino acids across the membrane.
3. Structural proteins
These proteins serve as supporting filaments, cables or sheets to give biological structures strength or protection. Collagen is a fibrous protein that forms the major component of tendons and cartilage. Leather is almost pure collagen. Elastin present in ligaments is also a structural protein. Keratin is present in hair, fingernails, and feathers; fibroin in silk fibers and spider webs; resilin in the wing hinges of some insects – all are collagen with high elasticity.
4. Storage proteins
These serve as biological reserves of metal ions and amino acids that are used by organisms. Nutrient and storage proteins are found in plant seeds, egg whites, and milk. For example, casein and ovalbumin are the storage proteins that store amino acids in animals – casein the major protein of milk and ovalbumin the major protein of egg white; prolamin gliadin (a component of gluten) is the storage protein in wheat, and ferritin is a storage protein that stores iron (component of hemoglobin).
5. Defense proteins
These are specialized proteins that defend the body against antigens or foreign invaders and thus protect the body from injury. Immunoglobulins or antibodies are specialized proteins made by the lymphocytes or vertebrates; they identify and defend against bacteria, viruses and other foreign intruders in blood. Fibrinogen and thrombin are blood-clotting proteins that prevent blood loss when the vascular system is injured.
6. Regulatory proteins
These help to regulate the cellular or physiological activity. Hormones are an example of regulatory proteins. For example, insulin, oxytocin, and somatotropin. Insulin regulates glucose metabolism, oxytocin stimulates contractions during childbirth, and somatotropin is a growth hormone that incites protein production in muscle cells.
