Protein name is derived from a greek word which means '' holding first place".
(Berzelius and Mulder)
- Essential elements in protein are C, H, O, N,
- Most of the proteins contain sulphur. In some proteins iodine, iron and phosphorus are present.
- After water, proteins are most abundant compounds in protoplasm. (7-14%)
- Proteins are polymers of amino acid (Fisher and Hofmeister). There are approximately 300 amino acids known to exist but only 20 types of amino acids are used in formation of proteins.
- Each amino acid is amphoteric compound because it contains one weak acidic group –COOH and a weak alkaline group –NH2.
- In protoplasm free amino acid occurs as ions (at iso electric point).
Iso electric point is that point of pH at which amino acids do not move in electric field.
- Out of 20 amino acids, 10 amino acids are not synthesized in body of animals so they are must in diet. These are called Essential amino acids. e.g. Threonine, Valine, Leucine, Isoleucine, Lysine, Methionine, Phenylalanine Tryptophan, arginine, Histidine. Arginine and Histidine are semi essential.
- 10 amino acids are synthesized in animal body so these are called Non essential amino acids. for e.g. Glycine, Alanine, Serine, Cysteine, Aspartic acid, Glutamic acid, Asparagine, Glutamine, Tyrosine, Proline.
Classification of Amino Acids –
(A) Amino acids can be classified on the basis of their group –
(i) Non-polar R group – Glycine, Alanine, Valine, Leucine, Isoleucine, Proline, Methionine, Phenyl alanine, Tryptophan.
(ii) Polar but uncharged R group – Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine.
(iii) Positively charge polar R-group – Lysine, Arginine, Histidine (Basic Amino acid).
(iv) Negatively charged polar R-group – Aspartic acid, Glutamic acid (Acidic Amino acid).
Except glycine, each amino acid has two enantiomeric isomers.
- Eukaryotic proteins have L-amino acid while D-amino acid occurs in bacteria and antibodies.
- Amino acids join with peptide bond to form protein.
- Peptidyl transferase enzyme catalyses the synthesis of peptide bond.
- Property of protein depends on sequence of amino acid and configuration of protein molecules.
Special points on Amino acid :
Glycine is the simplest and tryptophan is complex Amino acid.
- Cysteine, Cystine, Methionine are the sulphur containing Amino acids.
- Phenyl alanine, Tyrosine, Tryptophan Amino acids are aromatic Amino acids.
- Serine & Threonine are alcoholic amino acids.
- Histidine, Proline & hydroxyproline are heterocyclic amino acids.
All the amino acids are laevo-rotatory, except Glycine which is non-rotatory.
Amino acids which participate in protein synthesis are called protein Amino acids and which do not participate are called non-Protein. eg. GABA, Ornithine, Citrulline.
Proline, Hydroxy proline contain imino group instead of amino group so they are also called Amino acids.
Configuration of Protein Molecule –
- Primary configuration or structure – A straight chain of amino acids linked by peptide bonds form primary structure of proteins. This structure of protein is most unstable. Newly formed proteins on ribosomes have primary structure .
(2) Secondary configuration – Protein molecules of sec. structure are spirally coiled. In addition to peptide bond, amino acids are linked by hydrogen bonds between oxygen of one amide group and hydrogen of another amide group. This structure is of two types –
(i) α–Helix – Right handed rotation of spirally coiled chain with approximately amino acids in each turn. This structure has intramolecular hydrogen bonding i.e. between two amino acids of same chain e.g. Keratin, Myosin, Tropomyosin.
(ii) β-Helix or pleated sheath structure – Protein molecule has zig – zag structure. Two or more protein molecules are held together by intermolecular hydrogen bonding. e.g. fibroin (silk).
(3) Tertiary Structure – Protein of tertiary structure are highly folded to give a globular appearance. They are soluble in water (colloid solution). This structure of protein has following bonds –
(i) Peptide bond = strongest bond in proteins.
(ii) Hydrogen bonds
(iii) Disulphide bond – These bonds form between – SH group of amino acid. (Methionine, Cysteine). These bonds are second strongest bond and stabilize tertiary structure of protein.
(iv) Hydrophobic bond – Between amino acids which have hydrophobic side chains for e.g. Aromatic amino acid.
(v) Ionic bond – Formation of ionic bond occurs between two opposite ends of protein molecule due to electrostatic attraction.
Majority of proteins and enzymes in protoplasm exhibit tertiary structure.
Quaternary Structure – Two or more polypeptide chains of tertiary structure unite by different types of bond to form quaternary structure of protein. Different polypeptide chains may be similar (lactic - dehydrogenas) or dissimilar types (Haemoglobin, insulin).
Quaternary structure is most stable structure of protein.
Significance of Structure of Protein –
- The most important constituents of animals are protein and their derivatives. Proteins form approximately 15% of animal protoplasm. The physical and biological properties of proteins are dependent upon their secondary and tertiary configurations. Protein is electrically charged because it has and &COO& ionic components. In an acidic medium the & COO- group of protein converts in COOH and the protein itself becomes positively charged. In contrast, in an alkaline medium the group of protein changes to & NH2 + H2O and as a result it becomes negatively charged. Therefore, at a specific pH a protein will possess an equal number of both negative and positive charges and it is at this specific pH a protein becomes soluble.
- If the pH changes towards either acidic or alkaline side, then the protein begins to precipitate. This property of protein has great biological significance. The cytoplasm of cells of organisms has an approximate pH of 7 but the pH of proteins present in it is about 6 and thus, the proteins are present in a relatively alkaline medium. Therefore, the proteins are negatively charged and also are not in a fully dissolved state. It is because of this insolubility, proteins form the structural skeleton of organismal cells. Similarly, the pH of nucleoplasm is about 7 but the pH of proteins, namely, histones and protamines, in it is relatively more. Therefore, as a result they are positively charged and do not remain fully dissolved in the nucleoplasm forming minute organelles, the most important being the chromosomes.
- As has been described above, the structural units namely amino acids of proteins contain both a carboxyl group (& COOH) or acidic group and an amino group (NH2) or alkaline group attached to the same carbon atom. Therefore, proteins depending upon the pH of the medium can exhibit both alkaline and acidic properties. Such compounds which exhibit both acidic and alkaline properties are called amphoteric compounds or zwitter ions. In the protoplasm, this dual property of proteins is utilized for neutralisation of strong acids and alkalis since the protein acts an an ideal buffer in either of the situations.
- Besides changes in pH, salts, heavy metals, temperature, pressure, etc. also cause precipitation of proteins. Because of these changes, the secondary and tertiary configuration of proteins is destroyed and many times the tertiary structured globular proteins become converted to secondary configuration fibrous proteins . Such alternations in the physical state of proteins is called denaturation. If the change in the medium of proteins is mild and for a short period, then denaturation of the proteins is also temporary, however, if the change in medium is strong and prolonged then denaturation is permanent and the protein becomes coagulated.
For example, the white or albumen of egg is a soluble globular protein but on heating it permanently coagulates into fibrous insoluble form. It is clear, that strong alternations result in the denaturation of proteins and they lose their biological properties and significance. It is this reason, that cells of organisms are unable to bear strong changes and they ultimately die.
TYPES OF PROTEINS :
1. Simple Proteins - proteins which composed of only amino acid.
(i) Fibrous Protein – e.g. Collagen, Elastin, Keratin
(ii) Globular Proteins – e.g. Albumin, Histones, Globin, Protemines, Prolamines (Glaidin, Gluten, Zein), Gluteline – slimy part of gluten of wheat.
2. Conjugated Proteins – Simple protein + non protein part (Prosthetic group)
(i) Nucleoproteins – e.g. Chromatin, Ribosomes etc.
(ii) Chromoprotein – Prosthetic group is a porphyrin pigment e.g Haemoglobin, Haemocyanin, Cytochromes etc.
(iii) Lipoprotein eg. Cell membrane, Lipovitelline of Yolk.
(iv) Phospho proteins – Casienogen, Pepsin, Ovovitelline, Phosvitin.
(v) Lecitho protein – Fibrinogen.
(vi) Metallo protein – Cu-tyrosinase, Zn-Carbonic anhydrase, Mn-Arginase, Mo-Zanthine Oxidase Mg- Kinase.
(vii) Glycoproteins and Mucoproteins – Glyco proteins have less than 4% Carbohydrates in their structure. They are most specific type proteins. e.g. α, β γ-globuline of blood group proteins, mucin, Erythropoetin etc.
Muco proteins have more than 4% Carbohydrate e.g. Mucoids of synovial fluid, Osteomucoprotein of bones, Tendomucoprotein of tendons, Chodro mucoprotein of cartilage.
3. Derived Protein – These are formed by denaturation or hydrolysis of protein.
(i) Primary derived proteins are denaturation product of normal proteins e.g. Fibrin, Myosin
(ii) Secondary derived proteins are digestion products of proteins e.g. Proteoses, peptones, Peptides.
Special Point on Protein :
Monomeric protein : Protein composed of one polypeptide chain.
Oligomeric/Polymeric/Multimeric protein : Protein composed of more then one polypeptide chains.
Peptide : A molecule shorter than 20 Amino acids.
Polypeptide : It usually has more than 20 Amino acids.
Protein : It contains minimum 50 Amino acids or more than 50 Amino acids.