Protein Chapter-1

1. Introduction to Proteins

• Definition: Proteins are macromolecules consisting of one or more long chains of amino acid residues.
• Structure: Proteins are composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and occasionally sulfur (S) atoms. They are synthesized based on the genetic code.

2. Amino Acids

• Basic Structure: Each amino acid contains:
• A central alpha carbon (Cα)
• An amino group (−NH2)
• A carboxyl group (−COOH)
• A hydrogen atom (H)
• A distinctive side chain (R group), which determines the properties and functions of the amino acid.

         Reference: https://www.astrochem.org/sci/Amino_Acids.php

• Classification: Amino acids are categorized based on the properties of their side chains:
• Nonpolar, aliphatic: Glycine, alanine, valine, leucine, isoleucine, methionine.
• Aromatic: Phenylalanine, tyrosine, tryptophan.
• Polar, uncharged: Serine, threonine, cysteine, proline, asparagine, glutamine.
• Positively charged (basic): Lysine, arginine, histidine.
• Negatively charged (acidic): Aspartate, glutamate.

3. Protein Structure Levels

• Primary Structure: The linear sequence of amino acids in a polypeptide chain, determined by mRNA.
• Secondary Structure: Local folding patterns within the polypeptide stabilized by hydrogen bonds.
  • Alpha Helix: Right-handed coil stabilized by hydrogen bonds between the backbone carbonyl oxygen and amide hydrogen atoms four residues apart.
  • Beta Sheet: Strands connected laterally by at least two or three backbone hydrogen bonds, forming a sheet-like array.
• Tertiary Structure: The overall three-dimensional shape of a polypeptide, stabilized by interactions between R groups, including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges (covalent bonds between cysteine residues).
• Quaternary Structure: The structure formed by multiple polypeptide chains (subunits), which may be identical (homomers) or different (heteromers). Hemoglobin is an example of a protein with quaternary structure.

Reference:https://www.thoughtco.com/protein-structure-373563

4. Protein Functions

• Enzymatic Activity: Catalysts that accelerate biochemical reactions by lowering activation energy. Examples include:
  • Hydrolases: Break bonds by hydrolysis (e.g., proteases).
  • Ligases: Join two molecules together (e.g., DNA ligase).
  • Oxidoreductases: Catalyze oxidation-reduction reactions (e.g., oxidases).
• Structural Role: Provide cellular and tissue structure, as in:
  • Collagen: Found in connective tissues.
  • Keratin: Key component of hair, nails, and the epidermis.
• Transport and Storage: Transport molecules or store ions and molecules.
  • Hemoglobin: Transports oxygen in the blood.
  • Ferritin: Stores iron in the liver.
• Signal Transduction: Function as receptors and hormones in cellular communication.
  • Insulin: Regulates glucose metabolism.
  • G-protein-coupled receptors (GPCRs): Transmit signals across cell membranes.
• Immune Response: Antibodies (immunoglobulins) identify and neutralize pathogens.

5. Protein Synthesis

• Transcription: Synthesis of mRNA from a DNA template in the nucleus. Key steps include:
  • Initiation: RNA polymerase binds to the promoter region of the gene.
  • Elongation: RNA polymerase synthesizes mRNA by adding nucleotides complementary to the DNA template.
  • Termination: Transcription ends when RNA polymerase reaches a terminator sequence.
• Translation: The mRNA is translated into a polypeptide chain at the ribosome.
• Initiation: The ribosome assembles around the mRNA and the first tRNA.
• Elongation: tRNAs bring amino acids to the ribosome, where they are added to the growing polypeptide chain.
• Termination: The process ends when a stop codon is reached.

 Reference: https://www.geeksforgeeks.org/protein-synthesis-diagram/

6. Protein Digestion and Absorption

• Digestion: Begins in the stomach with pepsin and continues in the small intestine with proteases (e.g., trypsin, chymotrypsin, carboxypeptidase).
• Absorption: Amino acids and small peptides are absorbed by enterocytes in the small intestine via active transport and facilitated diffusion.

7. Protein Metabolism

• Anabolism: Biosynthesis of proteins from amino acids.
  • Transamination: Transfer of an amino group from one amino acid to a keto acid, forming a new amino acid.
  • Aminoacyl-tRNA Synthetase: Enzyme that attaches amino acids to their corresponding tRNA.
• Catabolism: Breakdown of proteins into amino acids and further into ammonia, urea, and keto acids.
• Deamination: Removal of an amino group from an amino acid.
• Urea Cycle: Converts toxic ammonia to urea for excretion.

  Reference:https://www.osmosis.org/notes/Protein_Metabolism

8. Protein Requirements and Sources

• Daily Requirements: Varies by individual factors. The RDA is approximately 0.8 grams per kilogram of body weight for adults.
• Sources:
  • High-Quality Proteins (Complete Proteins): Provide all essential amino acids in sufficient quantities (e.g., meat, eggs, dairy).
  • Lower-Quality Proteins (Incomplete Proteins): Lack one or more essential amino acids (e.g., most plant-based sources like grains, nuts, and legumes).

9. Protein Deficiency

• Kwashiorkor: Severe protein deficiency characterized by edema, an enlarged liver, and dermatosis.
• Marasmus: Severe calorie and protein deficiency, leading to significant wasting and stunted growth.

10. Protein Quality

• Biological Value (BV): Measures the proportion of absorbed protein incorporated into body proteins. High BV proteins provide a greater proportion of utilized amino acids.
• Protein Digestibility Corrected Amino Acid Score (PDCAAS): Evaluates protein quality based on amino acid requirements and digestibility.
• Net Protein Utilization (NPU): Ratio of amino acids converted to proteins to the amino acids supplied.

11. Protein and Health

• Muscle Maintenance and Growth: Essential for muscle protein synthesis, repair, and maintenance. Influenced by factors like exercise and protein intake.
• Immune Function: Essential for the synthesis of immunoglobulins and other immune system components.
• Hormone Production: Many hormones, such as insulin and growth hormone, are protein-based.

12. Protein Supplements

• Types:
  • Whey Protein: Fast-digesting, high in essential amino acids, especially leucine.
  • Casein Protein: Slow-digesting, provides a steady release of amino acids.
  • Plant-Based Proteins: Derived from soy, pea, rice, etc., suitable for vegetarians and vegans.
• Uses: Often used to supplement dietary protein intake for muscle building, recovery, and other health benefits.