What are Proteins
Before learning about the monomers of proteins, let us see what proteins are. Proteins are the natural polymers that play a vital role in life processes. Proteins make more than 50% of dry weight of cells and are present in large amounts than any other biomolecule. Therefore, they differ greatly from other major types of biomolecules including lipids, carbohydrates, and nucleic acids. Most importantly, proteins are the most extensively studied biomolecules owing to their structure, functions, physiochemical properties, modification and their applications, especially in most advanced areas in science such as genetic engineering, eco-friendly material, novel composites based on renewable sources. Proteins as biomolecules are responsible for carrying out many major functions in biological systems, including enzyme catalysis (by enzymes), defense (by immunoglobulins, toxins and cell surface antigens), transport (by circulating transporters), support (by fibres), motion (by forming muscle fibres such as collagen, keratin and fibrin), regulation (by osmotic proteins, gene regulators, and hormones), and storage (by ion binding). Proteins are important renewable resources produced by animals, plants and microorganisms such as viruses and bacteria. Some important plant-based proteins include zein, soy proteins and wheat proteins. Casein and silk fibroin are some proteins found in animals. Examples of major bacterial proteins include lactate dehydrogenase, chymotrypsin, and fumarase.
Proteins are formed by joining of a large number of monomer units. Proteins contain one or more polypeptides. Each polypeptide chain is formed by joining of a large number of amino acids through chemical bonds known as peptide bonds. The gene coding for that specific protein determines the sequence of amino acids. Once a polypeptide chain is formed, it folds up to give a specific three-dimensional structure, which is unique to that particular polypeptide chain. The conformation of a polypeptide chain is determined mainly by the amino acid sequence and multiple, weak interactions among the parts of the polymer chain. These weak interactions can be disrupted by applying heat or adding a chemical that ultimately changes the conformation of polypeptide 3-D structure. This disruption process is known as denaturation of proteins. Denaturation will ultimately stop the functional activity of proteins. Hence, the structure of the protein is very important to maintain their roles.
Protein Structure
Proteins structure can be discussed in terms of four levels of structures; primary, secondary, tertiary, and quaternary. The primary structure of a protein is its amino acid sequence. There are two types of secondary structures; α-helix and β-sheet. The tertiary structure of proteins is determined by the three-dimensional structure, which can be either globular or fibrous. Tertiary structure is more complex and compact. Quaternary structure of a protein is far more complex due to its higher degree of folding patterns. Most of the proteins with quaternary structure, contain subunits, which are held together by non-covenant bonds. For example, haemoglobin has four subunits.
What are the Monomers of Proteins
A monomer is the main functional and structural unit of a polymer. They are the building blocks of polymers. The monomer of a protein is an amino acid. A large number of amino acid molecules join together by peptide bonds to form polypeptide chains. Two or more polypeptide chains are joined together to form large proteins. Amino acid sequence determines the structure and function of a protein.
General structure of an Amino acid
There are 20 different amino acids that form all the proteins in the biological system by arranging in different sequences. The sequence of amino acids is known as the primary structure of a protein. When considering the chemical formula of an amino acid molecule, it contains three groups; amino group (-NH2), carboxylic acid group (-COOH) and side-chain (R group), which is specific to each amino acid. The simplest amino acid contains a hydrogen atom as the R group known as glycine.
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