Amino acids

• Amino acids


  The building blocks of proteins
• Common amino acids


  • Amino acids for which at least one codon exists in the genetic code
• Derived amino acids


  Usually formed by enzymatic modification of one of the common amino acids after it has been incorporated into a protein
• Amino acid structure


  • Contain amine [-NH2], carboxyl [-COOH], and side chain [R group]
  • Major elements are carbon, hydrogen, nitrogen, oxygen
• Stereochemistry


  Amino acids represent nonsuperimposable mirror images (enantiomers), only L-enantiomers are found in proteins
• Aliphatic amino acids


  • Characterized by aliphatic side chains (open-chain structures of carbon and hydrogen)
  • Hydrophobic in nature
• Aromatic amino acids


  • Contain an aromatic ring in their side chain
• Sulfur-containing amino acids


  • Methionine initiates protein synthesis and is a precursor for S-adenosylmethionine
  • Cysteine forms disulfide bridges that stabilize protein structure
• Hydroxy amino acids


  • Contain a hydroxyl (-OH) group in their side chain
  • Play important roles in protein structure and function, found in enzyme active sites
• Amide amino acids


  • Have a carbonyl group (C=O) attached to a nitrogen atom in their side chain
  • Polar and capable of forming hydrogen bonds
• Acidic amino acids


  • Have acidic side chains due to carboxylic acid group
  • Negatively charged at physiological pH
  • Participate in electrostatic interactions crucial for protein folding and function
  • Aspartic acid and glutamic acid are neurotransmitters
• Basic amino acids


  • Have basic (alkaline) side chains due to amino group
  • Positively charged at physiological pH
  • Involved in various biological processes like collagen formation and gene expression
• Imino acids


  Derivatives of amino acids where the amino group (-NH2) is replaced by an imino group (-NH)
• Classification of amino acid functional groups and polarity

  • Aliphatic
  • Aromatic
  • Sulfur-containing
  • Hydroxy
  • Amide
  • Acidic
  • Basic
  • Imino
• Amino acid classification based on nutritional requirements

  • Essential (require supplementation during increased demand)
  • Non-essential
• Amino acid classification based on metabolic fate

  • Glucogenic (yield pyruvate or TCA cycle intermediates)
  • Ketogenic (yield acetoacetate or precursors)
• Variations in primary structure

  • Hypervariable regions tolerate different amino acid residues
  • Invariant regions have exactly the same sequence
• Developmental variation


  • Hemoglobin isoforms change from fetal to adult forms after birth
  • Fetal hemoglobin has higher oxygen affinity
• Tissue-specific isoforms


  • Proteins that differ in primary structure and properties between tissues but retain the same function
• Modified amino acids


  Undergo post-translational modifications like addition of chemical groups, oxidation, etc.
• Glycosylation


  • Addition of carbohydrates to proteins
  • N-linked oligosaccharides protect cells
  • O-linked oligosaccharides attached to serine or threonine
• Fatty acylation


  • Addition of lipids to proteins
  • Palmitoyl and myristoyl groups attach to membrane proteins
• Regulatory modifications


  • Phosphorylation, acetylation, and ADP-ribosylation can alter protein structure and activity