Proteins and Their Structure

Proteins

• Large molecules composed of amino acids
• Properties depend on the sequence of amino acids
• Fold into highly organized 3D structures
• The folding pattern is crucial to the function of the protein

4 Levels of Protein Structure

• Primary Structure: The unique and specific sequence of amino acids in a polypeptide chain

  • Joined by peptide bonds
  • Synthesized from mRNA using the genetic code
  • Begins at the amino end and terminates at the carboxyl end
  • Ultimately determines all properties of a protein

• Secondary Structure: Local structures, including folds, turns, α-helices, and β-sheets, held in place by hydrogen bonds

• Tertiary Structure:

  • Produces the 3D structure of the amino acid chain
  • The most stable structure in a given environment
  • Native structure is the biologically active form of a protein
  • Creates an active site for biological function
  • Ultimately dictated by the primary structure and how the amino acid chain folds

• Quaternary Structure:

  • Arrangement of polypeptide chains into a functional protein
  • Some proteins are assemblies of two or more chains
  • The way these chains are organized is called the quaternary structure
  • Hemoglobin, for example, consists of 4 subunits: 2 α chains (identical) and 2 β chains (also identical)

Amino Acids

• The building blocks of proteins
• 20 common amino acids
• Each amino acid consists of:
  • Central carbon atom
  • Amino group
  • Carboxyl group
  • Side [R] Chain
• Differences in side chains (R groups) distinguish the 20 different amino acids

Classification of Amino Acids

• Non-polar (hydrophobic) amino acids
• Polar (hydrophilic), uncharged amino acids
• Polar (hydrophilic), charged either positive or negative

Forces Stabilizing Tertiary Structure

• Hydrogen bonds - Very weak
• Salt bridges - Weak
• Hydrophobic interactions - Weak
• Disulfide bridges - Strong

Globular & Fibrous Proteins

• Globular Proteins:

  • 3D structure normally folds up in a ball
  • Hydrophilic R groups point outwards
  • Hydrophobic R groups point inwards
  • Soluble
  • Metabolic functions
  • Example: Hemoglobin, IgG

• Fibrous Proteins:

  • 2º structure does not fold up, they form fibers
  • Not surrounded by hydrophilic R groups
  • Insoluble
  • Structural functions
  • Example: Collagen, Keratin

Secondary Structures

• The two most important secondary structures are α-helices and β-sheets
• Both held together by hydrogen bonds between the N-H and C=O projecting from the main chain
• α-helix:
  • Rodlike structure
  • Stabilized by hydrogen bonds between NH and CO groups of the main chain
  • CO group of each amino acid is hydrogen bonded to the NH group of the amino acid
• β-sheets:
  • Stabilized by hydrogen bonds between NH and CO groups in different polypeptide strands
  • Hydrogen bonds most often form between the CO and NH groups of adjacent chains, either parallel or antiparallel, from different regions of the polypeptide

Hemoglobin

• Example of a protein with quaternary structure
• Consists of 4 subunits
• 2 α chains (identical) and 2 β chains (also identical)

Protein Structure Summary

• 1º: Linear sequence of amino acids and disulfide bonds
• 2º: Local structures: folds, turns, α-helices and β-sheets held in place by hydrogen bonds
• 3º: 3D arrangement of all atoms in a single polypeptide chain
• 4º: Arrangement of polypeptide chains into a functional protein

Explore the intricate world of proteins with this quiz covering their composition, structure, and function. From primary to tertiary structures, understand how the sequence of amino acids dictates the 3D conformation and biological activity of proteins. Test your knowledge on the essential properties and roles of proteins in biological systems.