Amino Acids and Their Properties

Amino Acids Overview

• Building blocks of proteins containing an amino group, carboxylic acid group, and a variable side chain attached to an ɑ-carbon.
• The unique side chain of each amino acid determines its properties and classification.

Standard Amino Acids

• There are 20 standard amino acids categorized into four groups:
  • Nonpolar amino acids
  • Polar neutral amino acids
  • Polar acidic amino acids
  • Polar basic amino acids

Nonpolar Amino Acids

• Characterized by a nonpolar side chain, making them hydrophobic.
• Contain one amino group and one carboxyl group.

Polar Neutral Amino Acids

• Have polar but neutral side chains at physiological pH, neither acidic nor basic.
• Include one amino and one carboxyl group.

Polar Acidic Amino Acids

• Contain one amino group and two carboxyl groups; one is part of the side chain.
• Carry a negative charge at physiological pH due to the side-chain carboxyl group.

Polar Basic Amino Acids

• Comprised of two amino groups and one carboxyl group; one amino group in the side chain.
• Possess a positive charge at physiological pH as the side chain accepts a proton.

Amino Acid Classification Summary

• Nonpolar amino acids: 1 amino group, 1 carboxyl group, hydrophobic.
• Polar neutral amino acids: 1 amino group, 1 carboxyl group, hydrophilic.
• Polar acidic amino acids: 1 amino group, 2 carboxyl groups, hydrophilic.
• Polar basic amino acids: 2 amino groups, 1 carboxyl group, hydrophilic.

Essential Amino Acids

• Must be obtained through diet as the body cannot synthesize these.
• Include Phenylalanine, Valine, Threonine, Tryptophan, Methionine, Leucine, Isoleucine, Lysine, Histidine, and Arginine (for children).

Functions and Sources of Essential Amino Acids

• Phenylalanine: Precursor to neurotransmitters; found in dairy products.
• Valine: Aids muscle growth and regeneration; found in meats, tofu.
• Threonine: Important for structural proteins like collagen; found in beef, soy.
• Tryptophan: Precursor to serotonin; found in chicken, eggs.
• Methionine: Vital for tissue growth; found in meat, fish.
• Leucine: Regulates blood sugar and stimulates healing; found in animal proteins.
• Isoleucine: Supports immune function; found in eggs, chicken.
• Lysine: Important for protein synthesis and calcium absorption; found in red meat, eggs.
• Histidine: Precursor for histamine; found in meat, dairy.
• Arginine: Supports growth in children; found in dairy products.

Dietary Proteins

• Complete Dietary Protein: Contains all essential amino acids in sufficient proportions; typically found in animal sources.
• Incomplete Dietary Protein: Lacks adequate essential amino acids; examples include gelatin and certain plant proteins.
• Limiting Amino Acids: Specific essential amino acids that are deficient in incomplete proteins.
• Complementary Dietary Proteins: Two incomplete proteins that combine to provide all essential amino acids; for instance, rice and beans.

Chirality and Molecular Handedness

• Alpha (ɑ-) Amino Acid: An amino acid where the amino group is bonded to the carbon adjacent to the carboxylic group.
• Fischer Projection: A method to represent 3D structures of amino acids, with specific rules for positioning groups indicating stereochemistry.

Peptide Bonds and Small Peptides

• Peptide Bond: An amide bond linking two amino acids, forming polypeptides.
• Polypeptide: A chain of amino acids.
• Amino Acid Residues: The remaining portion of an amino acid in a peptide after water release during bond formation.

Peptide Nomenclature

• C-terminal amino acid retains its full name; others acquire “-yl” suffixes.
• Sequential naming begins at the N-terminal amino acid.

Important Small Peptides

• Small Peptide Hormones: Like oxytocin and vasopressin, produced by the pituitary gland; involved in various physiological functions like uterine contractions and kidney water regulation.
• Small Peptide Neurotransmitters: Such as enkephalins, which modulate pain perception and mood.
• Glutathione: A tripeptide with antioxidant properties, protecting cells from oxidative damage.