PROTEIN
Proteins are essential biomolecules that play a vital role in virtually all biological processes. Here’s a comprehensive overview of proteins, their types, structure, and functions:
1. What Are Proteins?
- Definition: Proteins are large, complex molecules made up of chains of amino acids. They are vital for the structure, function, and regulation of the body’s tissues and organs.
- Composition: Proteins are composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sometimes sulfur (S).
- Building Blocks: Proteins are made of amino acids (20 standard ones in humans).
2. Types of Proteins
Proteins can be categorized based on their function, structure, and origin:
a) Functional Classification
- Enzymes:
- Catalyze biochemical reactions (e.g., amylase, DNA polymerase).
- Structural Proteins:
- Provide support and shape (e.g., collagen, keratin).
- Transport Proteins:
- Carry molecules across the body (e.g., hemoglobin, albumin).
- Signaling Proteins:
- Transmit signals (e.g., insulin, growth hormone).
- Defensive Proteins:
- Protect the body from pathogens (e.g., antibodies).
- Contractile Proteins:
- Aid in movement (e.g., actin, myosin).
- Storage Proteins:
- Store essential nutrients (e.g., ferritin, casein).
b) Classification Based on Origin
- Animal Proteins: Found in meat, fish, eggs, and dairy.
- Plant Proteins: Found in legumes, nuts, seeds, and grains.
c) Classification Based on Composition
- Simple Proteins: Composed only of amino acids (e.g., albumin).
- Conjugated Proteins: Contain a non-protein component (e.g., glycoproteins, hemoglobin).
3. Structure of Proteins
Proteins have a hierarchical structure that determines their function:
a) Primary Structure
- Definition: The sequence of amino acids in a polypeptide chain.
- Importance: Determines the protein’s overall shape and function.
b) Secondary Structure
- Definition: Local folding of the polypeptide chain into specific structures.
- Key Structures:
- Alpha Helix (α-helix): A coiled structure stabilized by hydrogen bonds.
- Beta Sheet (β-sheet): A sheet-like structure formed by hydrogen bonds between parallel or antiparallel chains.
c) Tertiary Structure
- Definition: The three-dimensional folding of the entire polypeptide chain.
- Stabilizing Forces:
- Hydrogen bonds
- Ionic bonds
- Disulfide bridges
- Hydrophobic interactions
d) Quaternary Structure
- Definition: The arrangement of multiple polypeptide chains (subunits) into a single functional protein.
- Example: Hemoglobin, composed of four subunits.
4. Protein Functions
Proteins have diverse roles in the body, including:
- Catalysis: Enzymes speed up biochemical reactions.
- Structure: Provide mechanical support (e.g., collagen in connective tissues).
- Transport: Facilitate movement of substances (e.g., oxygen by hemoglobin).
- Communication: Signal transduction and hormone function.
- Defense: Antibodies protect against pathogens.
- Energy Source: Proteins can be broken down for energy when necessary.
5. Protein Synthesis
- Steps:
- Transcription: DNA is transcribed into messenger RNA (mRNA) in the nucleus.
- Translation: mRNA is translated into a polypeptide chain at the ribosome, using transfer RNA (tRNA) and ribosomal RNA (rRNA).
6. Protein Denaturation
- Definition: Loss of a protein’s structure (and function) due to external factors like heat, pH changes, or chemicals.
- Examples: Cooking an egg denatures the albumin protein, turning it solid.
7. Protein Deficiency
- Symptoms: Weakness, fatigue, muscle loss, weakened immunity, and delayed wound healing.
- Diseases:
- Kwashiorkor: Severe protein deficiency in children.
- Marasmus: Deficiency in protein and overall calories.
8. Dietary Proteins
- Complete Proteins: Contain all nine essential amino acids (e.g., meat, fish, eggs, quinoa).
- Incomplete Proteins: Lack one or more essential amino acids (e.g., rice, beans). Complementary proteins (e.g., rice + beans) can provide all essential amino acids.
9. Techniques for Protein Study
- X-ray Crystallography: Determines protein structure.
- Mass Spectrometry: Identifies and quantifies proteins.
- Electrophoresis: Separates proteins based on size/charge.