Introduction to Proteins.

 

Definition:

These are high molecular weight biomolecules made up of chains of amino acids linked together with “peptide bonds”, found in all living organisms.

Biological Functions of Proteins:

  1. Proteins, which are composed of amino acids, serve in many roles in the body (e.g., as enzymes, structural components, hormones, and antibodies).

  2. They are structural components of hair and nail, collagen of bone etc e.g. keratin.

  3. Through Proteins genetic information is expressed.

  4. Gaseous transportation e.g. Hemoglobin in RBCs of Blood.

  5. Homeostatic control of the volume of the circulating blood and that of the interstitial fluids through the plasma proteins.

  6. Blood clotting through thrombin, fibrinogen and other protein factors.

  7. Defense against infections through protein antibodies.

  8. Hereditary transmission by nucleoproteins of the cell nucleus.

  9. Ovalbumine, glutelin etc. are storage proteins.

  10. Myosin acts as a contractile protein important for muscle contraction.

Classification of Proteins:

  • Depending on their chemical nature the proteins are classified as follows,

  1. Simple Proteins:

    • They are composed of only amino acid residue. 

    • On hydrolysis these proteins yield only constituent amino acids.

    •  It is further divided into:

    • Fibrous protein: Keratin, Elastin, Collagen

    • Globular protein: Albumin, Globulin, Glutelin, Histones.;

  2. Conjugated proteins:

    • They are combined with non-protein moiety. Eg. Nucleoprotein, Phosphoprotein, Lipoprotein, Metalloprotein etc.

    • The non protein moiety is called the “Prosthetic group”.

    • Nucleoprotein: Combination of proteins with RNA or DNA.

    • Chromoproteins: Combination of proteins with colored components e.g. Globin with Heme (Red Color) forms Hemoglobin.

    • Glycoproteins: Combination of proteins with carbohydrates.

    • Lipoproteins:  Combination of proteins with lipids.

    • Metalloproteins:  Combination of proteins with metals.

  3. Derived proteins:

    • They are derivatives or degraded products of simple and conjugated proteins. They may be :

      • Primary derived protein: Proteans, Metaproteins, Coagulated proteins

      • Secondary derived proteins: Proteosans or albunoses, peptones, peptides.

Structure of Proteins:

  • The linear sequence of amino acids in a polypeptide chain determines the three-dimensional configuration of a protein, and the structure of a protein determines its function.

  • The structure of proteins can be divided into four levels of organization:

    • Primary Structure.

    • Secondary Structure.

    • Tertiary Structure.

    • Quaternary Structure.

  • Primary Structure:

    • The primary structure of a protein consists of the amino acid sequence along the polypeptide chain.

    • Amino acids are joined by peptide bonds.

    • The charges on a polypeptide chain are due only to the N-terminal amino group, the C-terminal carboxyl group, and the side chains on amino acids.

    • The primary structure determines the further levels of organization of protein molecules.

  • Secondary Structure:

    • In a protein molecule, the polypeptide is present in different geometric configurations.

    • These arrangements are due to hydrogen bonding between carboxyl groups and amino groups of the peptide bonds.

    • The atoms of the side chains are not involved.

    • This is called the secondary structure of the protein.

    • It is of two types,

      • Alpha-helix

      • Beta-helix

  • Alpha-helix:

    • The α-helix is a right-handed coiled strand.

    • The side-chain substituents of the amino acid groups in an α-helix extend to the outside.

    • Hydrogen bonds form between the oxygen of the C=O of each peptide bond in the strand and the hydrogen of the N-H group of the peptide bond four amino acids below it in the helix.

    • The side-chain substituents of the amino acids fit in beside the N-H groups.

    • The sheet conformation consists of pairs of strands lying side-by-side.

    • The two strands can be either parallel or antiparallel.

    • Hair protein Keratin is made up of Alpha helix.

  • Beta Helix:

    • In this arrangement, the polypeptide chains are stretched out beside one another and then bonded by intermolecular H-bonds.

    •  In this structure, all peptide chains are stretched out to nearly maximum extension and then laid side by side which is held together by intermolecular hydrogen bonds. 

    • The structure resembles the pleated folds and therefore is known as β – pleated sheet.

  • Tertiary Structure:

    • This structure arises from further folding of the secondary structure of the protein, due to sidechain interactions.

    • H-bonds, electrostatic forces, disulphide linkages, and Vander Waals forces stabilize this structure.

    • The tertiary structure of proteins represents overall folding of the polypeptide chains, further folding of the secondary structure.

    • It gives rise to two major molecular shapes called fibrous and globular.

    • The main forces which stabilize the secondary and tertiary structures of proteins are hydrogen bonds, disulphide linkages, van der Waals and electrostatic forces of attraction.

  • Quaternary Structure:

    • Quaternary structure refers to the interaction of one or more subunits to form a functional protein, using the same forces that stabilize the tertiary structure.

    • It is the spatial arrangement of subunits in a protein that consists of more than one polypeptide chain.

    • e.g. Hemoglobin (𝝰2𝞫2).

Peptides and Polypeptides:

  • A peptide is a condensation product of two or more amino acids.

  • A dipeptide is a condensation product of two amino acids.

  • A tripeptide is a condensation product of three amino acids and so on.

  • The term peptide is usually used for a product containing 2-20 amino acids e.g. Oxytocin, Vasopressin.

  • The term polypeptide is usually used for a product containing 20-50 amino acids, e.g. Insuline.

  • The products containing more than 50 amino acids are called “Proteins”.

Qualitative Tests of Proteins:

  1. Heat Test:

    • When a protein solution is heated it causes loss of its protein structure which results in denaturation losing its biological activity.

  2. Test with Trichloroacetic Acid (TCA):

    • TCA causes denaturation of protein and is used for deproteinization.

  3. Biuret Test:

    • Also known as Piotrowski's test, is a chemical test used for detecting the presence of peptide bonds.

    • In the presence of peptides, a copper(II) ion forms violet colored complexes in an alkaline solution.

    • Biuret reagent contains copper sulphate in an alkaline medium, it reacts with protein solution forming violet color.

  4. Xanthoprotic Test:

    • The xanthoproteic test is performed for the detection of aromatic amino acids (tyrosine, tryptophan, and phenylalanine) in a protein solution. 

    • The nitration of the aromatic amino acid chain occurs due to reaction with nitric acid, giving the solution yellow coloration.

  5. Millon’s Test:

    • Millon’s test is an analytical test used for the detection of the amino acid tyrosine.;

    • Millon’s test is based on the principle of nitrification of the phenol group in tyrosine, which then forms complexes with heavy metals like mercury. 

    • The reagent used for the test is called Millon’s reagent, and it consists of mercuric nitrate and mercurous nitrate that is dissolved in concentrated nitric acid.

  6. Precipitation Test:

    • Proteins get precipitated by various agents like salts, heavy metals, tannins, organic solvents etc.

Biochemical importance of Proteins:

  • Proteins are one of the important components of the diet.

  • They are required for normal growth and functioning of the body.

  • Nutritionally they are classified in two categories as,

    • Complete Proteins.

    • Incomplete Proteins.

  • Complete Proteins:

    • These proteins contain all essential amino acids in required quantities.

    • e.g. Milk proteins, Egg Proteins.

  • Incomplete Proteins:

    • These proteins do not contain all essential amino acids in required quantities.

    • e.g. Gelatin.

  • Proteins are mainly used for making new components and very rarely are used as a source of energy.

Deficiency Disorders of Proteins:

  • Proteins are required for several vital processes in the body, their deficiency leads to deficiency disorders which may be caused due to low dietary intake or malfunctioning in the body like malabsorption and faulty conversions.

  • Following are two main deficiency disorders of proteins.

    • Marasmus.

    • Kwashiorkor.

  • Marasmus:

    • Marasmus is a severe form of protein-energy malnutrition that results when a person does not consume enough protein and carbohydrates.

    • Without these vital nutrients, energy levels become dangerously low and vital functions begin to stop.

    • Causes:

  • Not having enough nutrition or having too little food: Mainly happens in poor people.

  • Symptoms:

  • Failure to grow, known as stunted growth.

  • Wasting, or a loss of body tissue and fat.

  • Bones become visible under their skin.

  • persistent dizziness

  • lack of energy

  • dry skin

  • brittle hair.

  • Treatment:

  • Diet rich in calories, proteins and other nutritional factors is the best treatment and a way to avoid Marasmus.

  • Kwashiorkor:

  • Kwashiorkor is a form of severe protein malnutrition characterized by edema and an enlarged liver.

  • It is caused by sufficient calorie intake, but with insufficient protein consumption, which distinguishes it from marasmus.

  • Kwashiorkor cases occur primarily in areas of poor food supply.

  • Causes:

    • Not having enough nutrition or having too little food: Mainly happens in poor people.

    • Premature termination of breastfeeding.

    • Over dilution of cow milk.

  • Symptoms:

  • Pitting edema: Swelling of ankles and feets.

  • Distended abdomen, 

  • An enlarged liver, 

  • Thinning of hair,

  •  Loss of teeth, 

  • Skin or hair depigmentation, 

  • Dermatitis.

  • Anorexia (Loss of appetite)

  • Irritability.

  • Treatment:

  • Diet rich in proteins and other nutritional factors is the best treatment and a way to avoid Kwashiorkor

Commonly Asked Questions.

  1. Define and classify proteins.

  2. Give qualitative analysis tests for proteins.

  3. Give the biological importance of proteins.

  4. What are peptides, polypeptides and proteins?

  5. What is a peptide bond?

  6. Write in detail different structures of proteins.

  7. Discuss protein deficiency disorders.

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