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:

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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).

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2. Types of Proteins

Proteins can be categorized based on their function, structure, and origin:

a) Functional Classification

1. Enzymes:
   • Catalyze biochemical reactions (e.g., amylase, DNA polymerase).
2. Structural Proteins:
   • Provide support and shape (e.g., collagen, keratin).
3. Transport Proteins:
   • Carry molecules across the body (e.g., hemoglobin, albumin).
4. Signaling Proteins:
   • Transmit signals (e.g., insulin, growth hormone).
5. Defensive Proteins:
   • Protect the body from pathogens (e.g., antibodies).
6. Contractile Proteins:
   • Aid in movement (e.g., actin, myosin).
7. 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).

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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:
  1. Alpha Helix (α-helix): A coiled structure stabilized by hydrogen bonds.
  2. 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.

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4. Protein Functions

Proteins have diverse roles in the body, including:

1. Catalysis: Enzymes speed up biochemical reactions.
2. Structure: Provide mechanical support (e.g., collagen in connective tissues).
3. Transport: Facilitate movement of substances (e.g., oxygen by hemoglobin).
4. Communication: Signal transduction and hormone function.
5. Defense: Antibodies protect against pathogens.
6. Energy Source: Proteins can be broken down for energy when necessary.

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5. Protein Synthesis

• Steps:
  1. Transcription: DNA is transcribed into messenger RNA (mRNA) in the nucleus.
  2. Translation: mRNA is translated into a polypeptide chain at the ribosome, using transfer RNA (tRNA) and ribosomal RNA (rRNA).

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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.

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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.

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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.

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9. Techniques for Protein Study

• X-ray Crystallography: Determines protein structure.
• Mass Spectrometry: Identifies and quantifies proteins.
• Electrophoresis: Separates proteins based on size/charge.