PROTEINS: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future pharmacists and healthcare professionals!
Amino acids are the building blocks of all proteins. There are 20 amino acids commonly found in protein. These 20 amino acids are linked together through peptide bonds, forming peptide chains and proteins. The chains containing less than 50 amino acids are called peptides, while those containing greater than 50 amino acids are called proteins. An amino acid chain with more than 25 (10-50) amino acids is called a polypeptide, e.g., insulin, hormones, etc. A protein consists of a long polypeptide chain or several polypeptide subunits, e.g., interferon, clotting factors, antibodies, enzymes, etc.
Dpharmguru’s exam insights:
In my years of teaching biochemistry, I have observed that students often get confused between the different levels of protein structure. Remember: “Primary structure is the amino acid sequence, secondary structure is folding (α-helix and β-pleated sheet), tertiary structure is 3D folding, and quaternary structure is multiple polypeptide chains.” This is a fundamental concept that appears in almost every biochemistry exam!
AMINO ACIDS
Definition
Any organic molecule with at least one carboxyl group (organic acid) and at least one amino group (organic base) is called an amino acid. The amino acids are of α-form, i.e., the amine group (-NH₂) is attached to the C atom next to the carboxyl group (-COOH). Each amino acid has four different groups attached to a-carbon:
- Amino group (-NH₂)
- Carboxyl group (-COOH)
- Hydrogen atom (-H)
- Side chain (R)
Classification of Amino Acids
1. On the Basis of Chemical Nature
- Nonpolar (Hydrophobic) Amino Acids: Have hydrophobic side chains that do not readily interact with water. Examples: Alanine, Valine, Leucine, Isoleucine, Methionine, Proline, Phenylalanine, Tryptophan.
- Polar (Hydrophilic) Amino Acids: Have polar side chains that can interact with water. Examples: Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine.
- Positively Charged (Basic) Amino Acids: Have side chains with a positive charge at physiological pH. Examples: Lysine, Arginine, Histidine.
- Negatively Charged (Acidic) Amino Acids: Have side chains with a negative charge at physiological pH. Examples: Aspartic acid, Glutamic acid.
2. On the Basis of Nutritional Requirements
| Essential Amino Acids | Non-Essential Amino Acids |
|---|---|
| Valine | Glycine |
| Isoleucine | Tyrosine |
| Tryptophan | Proline |
| Methionine | Cysteine |
| Leucine | Aspartic acid |
| Phenylalanine | Alanine |
| Threonine | Serine |
| Lysine | Hydroxyproline |
| Histidine | Cystine |
| Glutamic acid | |
| Arginine |
Dpharmguru’s exam insights:
Remember the essential amino acids with the mnemonic: “PVT TIM HALL” — Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine. This is a classic exam question!
Biological Role of Amino Acids
- They are the building blocks of proteins.
- Energy is produced from the metabolism of amino acids during long-term starvation.
- They are used as neurotransmitters and precursors for the biosynthesis of neurotransmitters and hormones.
- They provide the monomer units for polypeptide chains of protein.
- Niacin, Serotonin and melatonin are synthesised from Tryptophan.
- Melanin, thyroid hormone, catecholamines are synthesised from Tyrosine.
- Nitric oxide, a smooth muscle relaxant, is synthesised from Arginine.
- They act as precursors for haem, creatine, porphyrins, purines and pyrimidines.
PROTEINS
Definition
Proteins are high molecular organic compounds that consist of the long polypeptide chain of amino acid monomers that are joined together by the peptide bonds between the carboxyl and amino groups. Protein mainly consists of 20 different kinds of amino acid molecules that combine in different combinations or number to form different polypeptide chains.
Chemically a protein molecule consists of:
- Nitrogen (15-17%)
- Carbon (50-55%)
- Hydrogen (6-7.5%)
- Oxygen (21-24%)
- Sulphur (0.2-2.2%)
- Phosphorus (0.1-1%)
Classification of Proteins
1. Simple Proteins
- Globular Proteins: Spherical or oval-shaped, soluble in water. Examples: Albumins, Globulin, Glutelins, Prolamines, Protamines, Globins, Histones.
- Fibrous Proteins: Fibre-like in shape, water-insoluble. Examples: Collagens, Elastin, Keratins.
2. Conjugated Proteins
These proteins are made up of amino acids and non-protein moiety (prosthetic or conjugating group). Examples: Nucleoproteins, Glycoproteins, Lipoproteins, Phosphoproteins, Chromoproteins, Metalloproteins.
3. Derived Proteins
- Primary Derived Proteins: Denatured or coagulated products of proteins. Examples: Coagulated, Proteans, Metaproteins.
- Secondary Derived Proteins: Degraded products of proteins undergoing hydrolysis. Examples: Proteoses, Peptones, Polypeptides, Peptides.
Structure of Proteins
- Primary Structure: The sequence of amino acids in each protein is unique. Held together by covalent peptide bonds.
- Secondary Structure: Folding of the polypeptide chain due to hydrogen bonds. Includes:
- α-Helical Structure: Polypeptide chain folds around a long axis.
- β-Pleated Sheet Structure: Amino acid residues arranged in a zigzag manner.
- Tertiary Structure: 3-D folding of the chain; stabilised by salt bridges, hydrogen bonds, disulphide bonds.
- Quaternary Structure: Interaction and arrangement of multiple polypeptide chains to form a larger protein complex. Example: Haemoglobin (α₂β₂).
Dpharmguru’s exam insights:
Remember: Haemoglobin is an example of quaternary structure. Also, insulin is an example of a protein with tertiary structure. These examples are frequently tested in exams!
Biological Role of Proteins
- Enzymes: Act as biocatalysts regulating biological processes.
- Hormones: Some proteins function as hormones (e.g., insulin, growth hormone).
- Hereditary Characters: Chromatin material carries genetic information.
- Interferons: Regulatory glycoproteins used to treat viral diseases.
- Protection: Immunoglobulins (antibodies) neutralise antigens. Thrombin and fibrinogen help in blood clotting.
- Transport: Serum albumin transports fatty acids; haemoglobin transports oxygen.
- Toxins: Several proteins are poisonous (e.g., snake venom, ricin).
- Contraction: Actin and myosin are responsible for muscle contraction.
- Receptor Proteins: Present on cell surface, receive stimuli from the environment.
- Structural Material: Keratin (hair, nails), collagen (connective tissue, bone).
- Storage Proteins: Stored as reserve food (e.g., egg albumin, seeds).
Qualitative Tests for Proteins
- Ninhydrin Test: Proteins give blue to violet colour with ninhydrin solution.
- Biuret Test: Purple colour is obtained when treated with dilute copper sulphate in alkaline medium.
- Xanthoproteic Test: Yellow colour with concentrated nitric acid due to nitration of aromatic amino acids.
- Millon’s Test: Red precipitate with Millon’s reagent due to tyrosine.
- Sodium Nitroprusside Test: Red colour with cysteine or cystine.
- Sakaguchi Reaction: Red colour with α-naphthol-sodium hypochlorite due to arginine.
- Hopkins-Cole Reaction: Violet colour with glyoxalic acid due to tryptophan.
Dpharmguru’s exam insights:
Remember the protein tests: Biuret test is for peptide bonds, Xanthoproteic test is for aromatic amino acids, Millon’s test is for tyrosine, and Sakaguchi test is for arginine. These are frequently tested in practical exams!
DISEASES RELATED TO MALNUTRITION OF PROTEINS
1. Kwashiorkor
This disorder is characterised by oedema (fluid retention), therefore is also named edematous malnutrition. It occurs due to deficiency of dietary protein.
- Symptoms: Change in skin and hair colour, loss of muscle mass, retarded growth, swollen ankles and belly, damaged immune system.
- Treatment: Increasing protein and calories in diet, starting with carbohydrates, sugars, and fats.
2. Marasmus
This disorder occurs in any individual having severe malnutrition; however, children mostly suffer from marasmus.
- Symptoms: Underweight, loss of muscle mass and subcutaneous fat, dry skin and brittle hair, chronic diarrhoea, respiratory infections.
- Treatment: Dried skim milk powder with boiled water, later adding vegetable oil, casein, and sugar.
Dpharmguru’s exam insights:
Remember the difference between Kwashiorkor and Marasmus: Kwashiorkor has oedema (swollen belly) due to protein deficiency, while Marasmus has wasting (severe weight loss) due to overall calorie deficiency. This is a very common exam question!
REFERENCES AND FURTHER READING
- Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry (7th ed.). W. H. Freeman.
- Berg, J. M., Tymoczko, J. L., & Stryer, L. (2019). Biochemistry (9th ed.). W. H. Freeman.
- Satyanarayana, U., & Chakrapani, U. (2021). Biochemistry (5th ed.). Elsevier.
- Vasudevan, D. M., Sreekumari, S., & Vaidyanathan, K. (2022). Textbook of Biochemistry for Medical Students (9th ed.). Jaypee Brothers.
- National Institutes of Health (NIH). (2022). Protein and Amino Acid Resources. Retrieved from https://www.nih.gov.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
