Introduction
Proteins are complex nitrogen-containing organic compounds found in all living cells. The word protein comes from the Greek word proteios meaning “first”, reflecting their importance in life processes. Proteins are essential for cell structure, function, repair, growth and regulation of body tissues. They are made of long chains of amino acids linked by peptide bonds and folded into specific shapes to perform biological functions.
Definition of Proteins
Proteins are large biochemical molecules made of carbon, hydrogen, oxygen and nitrogen and sometimes sulfur. They consist of one or more polypeptide chains that are folded into a functional structure. Each polypeptide is a linear chain of amino acids connected by peptide bonds.
Introduction to Protein Structure
- Proteins are made of about 20 naturally occurring amino acids.
- Amino acids may be polar or non-polar and determine the final folding of the protein.
- Proteins can have one or multiple chains (heavy and light chains).
- They are hydrolysed by acids or enzymes to form amino acids.
- Proteins are amphoteric (react as acid and base) and can be easily denatured by heat, solvents or pH changes.
Biochemical Importance of Proteins
- Act as structural materials in cells and tissues.
- Enzymes that regulate biochemical reactions are proteins.
- Immunoglobulins (antibodies) act as defence proteins.
- Storage proteins like ferritin store iron.
- Transport proteins carry molecules across membranes or in body fluids.
Classification of Proteins
Proteins may be broadly classified into three categories:
- Simple proteins: yield only amino acids on hydrolysis (e.g., albumin, globulin).
- Conjugated proteins: contain a non-protein part like metal ions, lipids or carbohydrates (e.g., glycoproteins, phosphoproteins).
- Derived proteins: formed by partial hydrolysis or denaturation (e.g., proteoses, peptones).
Isolation of Proteins
Modern isolation techniques include ultracentrifugation, precipitation, electrophoresis and chromatography. For plant tissues, protein extraction is usually done using TCA (trichloroacetic acid), acetone or phenol.
General Method for Isolation
- Tissue is homogenized in cold TCA with beta-mercaptoethanol.
- The mixture is kept at –20°C to precipitate proteins.
- Precipitated proteins are collected by centrifugation.
- Pellet is washed with cold acetone and dried.
Chemistry of Proteins
- Proteins vary in length and complexity depending on amino acid sequence.
- They consist of heavy and light chains in some functional proteins.
- Hydrolysis yields amino acids.
- Proteins form colloidal solutions in water.
- Heat or extreme pH causes denaturation.
Methods of Protein Analysis
1. Biuret Method
- In alkaline medium, Cu2+ ions react with peptide bonds to form a violet-purple colour.
- Absorbance measured at 540 nm.
2. Lowry Method
- Combination of Biuret reagent and Folin-Ciocalteu reagent.
- Tyrosine and tryptophan residues produce a blue colour.
- Read between 500–750 nm depending on concentration.
3. Turbidimetric Method
- Proteins are precipitated by TCA and turbidity is measured.
Other Methods
- Kjeldahl method (nitrogen estimation).
- Dye-binding method.
- UV spectroscopy at 280 nm.
General Chemical Tests for Proteins
1. Biuret Test
Protein solution + NaOH + copper sulphate → violet colour due to complex formation.
2. Ninhydrin Test
Ninhydrin + amino acids → deep violet or purple colour (Ruhemann’s purple).
3. Xanthoproteic Test
Concentrated nitric acid + protein gives yellow colour which turns orange with alkali.
4. Sulphur Test
Protein + NaOH + lead acetate → black precipitate of lead sulphide.
5. Sakaguchi Test
Alpha-naphthol + NaOH + alkaline hypobromite → red colour indicating arginine.
Protein-Related Drugs
1. Gelatin
Gelatin is a protein obtained by boiling animal skin, bones and tendons. It contains high levels of glycine, proline and hydroxyproline.
Preparation
Collagen is hydrolysed using acid or alkali (Type A and Type B gelatin). Extracted gelatin is concentrated, gelled, dried and bleached.
Uses
- Used in capsules, culture media and photographic films.
- Used in food products like jellies and marshmallows.
- Used in medical applications such as sponges and films.
2. Casein
Casein is a phosphoprotein found in milk. It forms 80% of cow milk protein.
Isolation
- Milk is warmed and acidified to coagulate casein.
- Precipitate is washed with ethanol and ether-ethanol mix.
- Dried and weighed to obtain casein content.
Properties
- Yellowish powder, hydrophobic in nature.
- Poorly soluble in water.
- Isoelectric point at pH 4.6.
- Forms micelles with calcium and magnesium.
Chemical Nature
- Contains phosphate groups mainly on serine and threonine.
- Includes alpha-casein, beta-casein and kappa-casein.
Uses
- Main component of cheese.
- Food additive and nutritional supplement.
- Casein peptides used for blood pressure, cholesterol and bone health.
Detailed Notes:
For PDF style full-color notes, open the complete study material below: