2.1 Amino acids and protein primary structure

Amino acids are the building blocks of proteins, with 20 standard types forming the basis of life. These molecules have a central alpha-carbon and four key groups, including a unique side chain that determines their properties and influences protein structure and function.

Proteins are formed through peptide bonds between amino acids, creating a polypeptide chain with distinct N and C termini. The primary structure, or linear sequence of amino acids, is crucial in determining protein function and higher-order structures, shaping overall protein behavior and interactions.

Amino Acid Fundamentals and Protein Structure

Common amino acids and properties

• 20 standard amino acids in proteins comprise building blocks of life
  • Essential amino acids cannot be synthesized by humans (lysine, methionine, tryptophan)
  • Non-essential amino acids can be synthesized by humans (alanine, glutamine, glycine)
• General structure of amino acids features central alpha-carbon with 4 key groups
  • Central alpha-carbon acts as backbone of amino acid structure
  • Amino group (-NH2) provides basic properties
  • Carboxyl group (-COOH) contributes acidic characteristics
  • Side chain (R-group) determines unique properties of each amino acid
• Classification based on side chain properties influences protein folding and function
  • Hydrophobic (nonpolar) amino acids cluster in protein core (leucine, isoleucine, valine)
  • Hydrophilic (polar) amino acids interact with aqueous environment (serine, threonine, asparagine)
  • Acidic amino acids carry negative charge at physiological pH (aspartic acid, glutamic acid)
  • Basic amino acids carry positive charge at physiological pH (lysine, arginine, histidine)
• Key properties affecting protein structure and function shape protein behavior
  • Size ranges from small (glycine) to large (tryptophan)
  • Charge varies from negative (aspartic acid) to positive (lysine)
  • Polarity influences solubility and interactions (serine vs leucine)
  • Aromaticity contributes to protein stability and interactions (phenylalanine, tyrosine, tryptophan)

Formation of peptide bonds

• Peptide bond formation occurs through condensation reaction between amino acids
  • Carboxyl group of one amino acid reacts with amino group of another
  • Water molecule released as byproduct of bond formation
• Peptide bond characteristics influence protein structure
  • Planar structure restricts rotation around bond
  • Partial double bond character contributes to rigidity
  • Trans configuration preferred due to steric hindrance
• Polypeptide chain formation builds protein backbone
  • N-terminus (amino end) starts the chain
  • C-terminus (carboxyl end) finishes the chain
  • Backbone consists of repeating -N-C$\alpha$-C- units forming protein skeleton
• Directionality of protein synthesis proceeds from N-terminus to C-terminus
  1. Ribosome initiates synthesis at N-terminus
  2. Amino acids added sequentially
  3. Chain elongation continues towards C-terminus
  4. Termination occurs at C-terminus

Protein primary structure

• Primary structure defines linear sequence of amino acids in polypeptide chain
• Importance in determining protein function shapes overall protein behavior
  • Dictates higher-order structures through amino acid interactions
  • Influences protein-protein interactions via surface properties
  • Determines enzyme active sites through specific amino acid arrangements
• Methods for determining primary structure enable protein sequencing
  • Edman degradation sequentially cleaves N-terminal amino acids
  • Mass spectrometry analyzes peptide fragments to deduce sequence
• Significance in evolutionary studies reveals protein relationships
  • Sequence homology between species indicates common ancestry
  • Protein family identification helps classify related proteins

Levels of protein organization

• Four levels of protein structure build upon each other
  • Primary structure amino acid sequence forms foundation
  • Secondary structure local folding patterns create regular structures
    • Alpha helices coil around central axis
    • Beta sheets form extended pleated structures
  • Tertiary structure overall 3D shape of single polypeptide determines function
    • Stabilized by various interactions create unique fold
      • Hydrogen bonds between backbone atoms
      • Ionic bonds between charged side chains
      • Hydrophobic interactions among nonpolar residues
      • Disulfide bridges covalently link cysteine residues
  • Quaternary structure arrangement of multiple polypeptide subunits forms complex proteins
• Relationship between structure levels demonstrates hierarchical organization
  • Each level builds upon the previous creating increasing complexity
  • Higher-order structures depend on primary sequence for proper folding
• Factors influencing protein folding shape final structure
  • Amino acid properties determine local interactions
  • Environmental conditions (pH, temperature, ionic strength) affect stability
  • Chaperone proteins assist in proper folding preventing aggregation