Protein Cycle: Translation, Folding, Processing

Protein Cycle

• Translation: synthesis of a polypeptide chain on a ribosome (RER) with its primary structure determined by an mRNA template
• Folding/Refolding: nascent polypeptide folds into its native conformation with the assistance of chaperones to prevent misfolding

Processing

• Accompanied by processing events such as:
  • Proteolytic cleavage of an N-terminal leader sequence
  • Formation of disulfide bonds (S-S)

Covalent Modification

• Removing unneeded polypeptides
• Attaching a fatty acid molecule, for example

Translocation

• Modified peptide is translocated to a membrane
• Examples: hemoglobin for the circulatory system

Activation/Catalysis

• Binding an allosteric effector triggers a catalytically active conformation
• Enzymes are involved in this step

Aging

• Proteins become damaged by chemical attacks, deamidation, or denaturation

Ubiquitination

• Proteins may be labeled by the covalent attachment of several ubiquitin molecules (Ub)

Degradation

• Ubiquitinated protein is degraded to its component amino acids (reused for new protein synthesis)
• Proteasome is involved in this step

Four Levels of Protein Structure

• Primary Structure: sequence of amino acids and disulfide links
• Secondary Structure: formed by hydrogen bonding, examples: alpha helix and beta-pleated sheet
• Tertiary Structure: complete 3-D conformation of a protein
• Quaternary Structure: association of two or more polypeptide chains

Protein Structure Examples

• Alpha Helix:
  • Produces a helical coiling of the peptide backbone
  • R-groups lie on the exterior of the helix and perpendicular to its axis
  • Approximately 11 amino acids long (3.6 per complete turn)
  • DELIMA (amino acids) favor alpha helix formation
  • GP favors disruption of the helix
• Beta-Pleated Sheet (B-Sheet):
  • Composed of 2 or more regions of stretches of at least 5-10 amino acids
  • Stabilized by H-bonding between amide N's and carbonyl C's
  • Found in both fibrous and globular proteins

This quiz covers the protein cycle involving translation on a ribosome, folding with the help of chaperones, and processing events like proteolytic cleavage. Test your knowledge on how polypeptides are synthesized, folded, and processed during protein synthesis.