Protein is a polymer which is formed by one or more long chains molecular units called amino acids. It performs a variety of functions in essentially all biological processes. Important functions are mechanical support, DNA replication, metabolic reactions, growth regulations, immune protection, transportation of molecules, storage of molecules, etc.
It is an organic material of protoplasm made up of carbon(C), hydrogen (H), oxygen (O) and nitrogen (N) atoms. Some proteins may contain sulfur (S) and phosphorous (P). Those atoms form amino acids, the organic acids, which contain at least a free amino group ((-NH2) and a carboxyl group (-COOH).
Peptide linkage (CO-NH) is formed by the combination of the –NH2 group of one amino acid with the –COOH group of another. This linkage combines with two amino acids. In this way, several peptide linkages join a number of amino acids and form the protein molecules. Thus amino acids are considered as the units of the protein molecules.
Generally, protein is made by amino acids with the help of a peptide bond. In this case (-COOH) of one amino acid create bonds with the (–NH2 ) of another amino acid by realizing of H2O where peptide bond should be dipeptide, tripeptide, and polypeptide.
There are twenty amino acids in proteins. The green plants (autotrophic organisms) synthesize their amino acids. The animals (heterotrophic organisms) must obtain certain amino acids in their diet because they cannot be synthesized protein in the body. These types of amino acids are known as essential amino acids.
Generally, essential amino acids are those which are essential for body growth, nitrogen balance, wear, and tear, etc but are not synthesized in the body and thus obtained through the diet. In man, the following amino acids are essential:
Types of Protein
A. On the basis of the Constitution, protein may be three types:
1. Simple protein: Simple proteins are those proteins which are made up of only amino acids or their derivatives. They are known as hemoproteins. There are many simple proteins. Some of them are described below:
Albumins: The albumins are soluble in water and coagulated by heat. It includes egg albumin of egg white, serum albumin of plasma, lactalbumin of milk, myoalbumin of muscle, etc.
Globulins: The globulins are insoluble in water but soluble in a salt solution. They are heat coagulated. It includes ovoglobulin of egg-yolk, serum globulin of plasma, myosin of muscle, etc.
Glutelins: The glutelins are plant proteins. They are soluble in very dilute acids and alkalis but they are insoluble in neutral solvents. They are heat coagulated. It includes glutelin of corn, glutenine of wheat and oryzenin of rice, etc.
Prolamines: The prolamines are also plant-based proteins found particularly in seeds. They are soluble in 70-80% alcohol but are insoluble in water, neutral solvent or absolute alcohol. It includes zein of corn, hordein of barley, gliadin of wheat, etc.
Scleroproteins or Albuminoids: These are entirely animal-based proteins and are the chief constituents of exoskeleton structures. They are the least soluble of all the proteins. It includes keratin of hair, horn, hoofs, nails, elastin of connective tissue and ligaments; collagen of bones, cartilage, and tendons.
Protamines: The protamines are the simplest of the proteins. They are strongly basic and soluble in water. They are not coagulated by heat. It includes salmin and clupeine of salmon and herring fish sperm respectively.
Histones: The histones are soluble in water. They are not readily coagulated by heat. Histones are not basic proteins. It includes globin of hemoglobin.
2. Conjugated Proteins: The proteins in which simple proteins are remain combined with some non-protein substances are known as conjugated protein. Sometimes they are also known as heteroproteins. Some important conjugated proteins are described below:
Nucleoproteins: They are composed of simple basic proteins (protamine or histone) and non-protein substances nucleic acid. The nucleic acid is made up of nucleotides and each nucleotide is composed of pentose, nitrogenous base and phosphoric acid. The pentose, ribose etc is present in ribonucleic acid (RNA) whereas deoxyribose is present in deoxyribonucleic acid (DNA). Bases are of two types (a) purine base-adenine and guanine and (b) pyrimidine bases- cytosine, thymine, and uracil.
Phosphoproteins: These types of proteins remain combined with phosphoric acids. It includes caseinogens of milk, vitelline of egg-yolk, etc.
Metalloproteins: The proteins remain combined with metallic elements and forms metalloproteins. It includes a large group of enzyme proteins which contain metallic elements such as Fe, Co, Mn, Zn, Mg, etc. The heam proteins which contain iron are metalloproteins.
Chromoproteins: They are composed of simple proteins and remain united with a coloring matter, prosthetic groups. It includes hemoglobin, myoglobin, flavoprotein, cytochrome, etc. Here hemoglobin and myoglobin, which bind one and four heme groups respectively.
Glycoproteins: These are proteins which remain combined with carbohydrates. The carbohydrate is usually mucopolysaccharide consisting of hexosamine and glucuronic acid. It includes mucin, follicle stimulating hormone (FSH), luteinizing hormone (LH), etc.
Lipoproteins: The lipoproteins are formed by the combination of protein with a lipid. It includes cell membrane, brain, egg yolk, etc. contain abundant lipoproteins.
3. Derived Proteins: These types of proteins are obtained as intermediate products during hydrolysis of simple or conjugated proteins, known as derived proteins. Generally, they are hydrolyzed by acids, alkalis or enzymes. It includes proteins, metaproteins, proteases, peptones, and polypeptides.
B. According to quality, proteins are of two types:
1. First class proteins: They are generally obtained from animal sources such as milk, eggs, meat, fish, etc. Thus animal proteins are first class proteins or proteins of high biological value. They contain more essential amino acids which are the basis of the qualitative aspect of protein materials.
2. Second class proteins: Plant proteins are generally second class protein.
C. On the basis of shape, Proteins are of two types, such as:
1. Scleroproteins: They are also called fibrous proteins which form long protein filaments. They are a rod or thread-like structures and are resistant to proteolytic enzymes. They are water-insoluble. They have important roles such as protection and support. They help to form connective tissue, bone matrix, tendons, and muscle fibers. These types of proteins are found in animals include elastin, collagen, actin, fibroin, myosin, keratin in hair, claws, feathers, etc..
Role of Scleroproteins or Fibrous Proteins
2. Globular proteins:Globular proteins are also known as spheroproteins. They have compact and relatively spherical or globe-like structures and soluble in water. They have metabolic functions and are made up of polypeptides that form spherical or oval structures. These types of proteins include myoglobin, insulin, hemoglobin, immunoglobin, albumin, and hormones like oxytocin, etc.
Role of Globular Protein
They play structural functions. Generally, globular proteins can function as:
D. On the basis of nature of Molecules, proteins are of two types:
1. Acidic proteins: They are found as anion which contains acidic amino acids such as blood groups.
2. Basic proteins: They are found as cations which contain basic amino acids such as lysine, arginine, etc.