Peptide Bonds and Structures

Peptide Bond Formation

• A peptide bond is formed between the carboxyl group of one amino acid and the amino group of another amino acid.
• This reaction removes a molecule of water (dehydration reaction).
• Peptide bonds are not broken by conditions that denature proteins, such as heating.

Structure of a Peptide Chain

• The peptide chain has two ends: the N-terminus (containing an amino group) and the C-terminus (containing a carboxyl group).
• Each amino acid in a polypeptide is called a “residue”.

Types of Peptides

• Oligopeptides contain 2-10 amino acids.
  • Dipeptide: two amino acids joined by one peptide bond.
  • Tripeptide: three amino acids joined by two peptide bonds.
• Polypeptides contain many amino acids (up to 50).
• A polypeptide with more than 50 amino acids is known as a protein.

Biologically Active Peptides

• Glutathione (3 amino acids): acts as an antioxidant and a cofactor in enzymatic reactions (helps detoxify chemicals).
• Thyrotropin Releasing Hormone (TRH) (3 amino acids): a hypothalamic hormone that regulates the release of thyroid hormones.
• Angiotensin II (8 amino acids): increases blood pressure (acts on the adrenal gland).
• Bradykinin (9 amino acids): a hypotensive/vasodilator that acts on smooth muscle.
• Oxytocin (9 amino acids): a uterus-contracting hormone that also stimulates lactation.

Biologically Active Polypeptides

• Insulin (30 and 21 amino acids): a pancreatic hormone needed for carbohydrate metabolism. Composed of two polypeptide chains.
• Glucagon (29 amino acids): a pancreatic hormone that opposes the action of insulin.
• Corticotropin (or Adrenocorticotropic hormone (ACTH)) (39 amino acids): secreted from the anterior pituitary gland in response to stress. Stimulates the adrenal cortex to produce glucocorticoids like cortisol.

Protein Classification

Based on Chemical Structure

• Simple proteins: composed only of amino acid residues.
• Conjugated proteins: combined with non-protein parts (e.g., nucleoprotein, phosphoprotein, lipoprotein, metalloprotein).
• Derived proteins: derivatives or degraded products of simple and conjugated proteins (e.g., peptones, peptides).

Based on Shape (Axial Ratio)

• Axial ratio: the ratio of length over width.
  • Fibrous proteins: axial ratio greater than 10. Polypeptides arranged in long strands or sheets, water insoluble, strong but flexible (e.g., keratin, elastin, collagen).
  • Globular proteins: axial ratio not more than 4. Compactly folded and coiled into spherical or globular forms, water soluble (e.g., albumin, globulin, hemoglobin, histones).

Fibrous Proteins

Collagen

• The most abundant protein in mammals (25-35% of total body protein).
• Can be converted to gelatin by boiling.
• Found in connective tissues like tendons, ligaments, and skin.
• A glycoprotein containing glucose and galactose.
• Rich in glycine, proline, hydroxyproline, hydroxylysine, and alanine.
• Hydroxylation of proline and lysine requires ferrous ions and vitamin C as cofactors.
• Basic structural unit is tropocollagen, consisting of three polypeptide chains (triple helix).
• Over 90% of collagen in the human body is type I.

Disorders of Collagen

• Scurvy: caused by vitamin C deficiency, impacting hydroxylation reactions of lysine and proline. Unstable tropocollagen leads to degradation.
• Osteogenesis imperfecta: a genetic disorder causing fragile bones, often due to glycine replacement with a bulky amino acid, impacting triple helix formation.

Elastin

• Similar to collagen, but cannot be converted to gelatin by boiling.
• Main component of elastic fibers, giving tissues stretchability and elasticity.
• Found in tissues like aorta, blood vessels, ligaments, and lungs.
• Basic subunit is tropoelastin.
• Rich in proline and lysine, but low in hydroxyproline.

α-keratin

• Key structural element of hair, nails, horns, claws, hooves, and the outer layer of skin.
• Rich in cysteine residues with high levels of disulfide bonds contributing to the strength of keratin fibers.

Protein Classification Based on Function

• Structural components: keratin (hair and nails), collagen (bone).
• Catalytic function: enzymes for almost all reactions in the body.
• Transport: hemoglobin for oxygen and carbon dioxide transport in blood.
• Regulatory function: hormones and receptors.
• Blood clotting: thrombin, fibrinogen, and other protein factors.
• Defense: antibodies against infections.
• Contractile: actin and myosin for muscle contraction.

This quiz covers the formation, structure, and types of peptides, including oligopeptides and polypeptides. Learn about the significance of peptide bonds in biological functions and the role of various peptides, such as glutathione, in enzymatic reactions. Test your knowledge on the basics of peptides and their importance in biochemistry.