Biochemistry · MU Pleven

2. Folding of Proteins. Transformation of α Structures into β. Importance.

Covers how a polypeptide folds from its primary sequence through secondary, supersecondary, domain, tertiary and quaternary stages, and the forces and chaperones that stabilise it. It then explains the conversion of α-helical regions into β-sheets, its causes, and its roles in diseases such as prion disease, Alzheimer's and type II diabetes, with a comparison table of α-helix and β-sheet features and a table of disease examples. The note closes with why correct folding matters and brief sections on protein classification, post-translational modifications and bioactive peptides.

Inside: 10 sections · 7 labelled pictures · 4 tables · 22 flashcards · 6 min

Topic 2 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 10 sections, about 624 words (6 min of reading), with 22 flashcards.

In this note

  1. Folding of proteins.
  2. Classification & function of proteins
  3. Definition of Protein Folding
  4. The Process of Protein Folding
  5. Forces Involved in Protein Folding
  6. Role of Molecular Chaperones
  7. Transformation of α-Structures into β-Structures
  8. Importance of Proper Folding
  9. Post-translational modifications of proteins
  10. Biologically active peptides

Classification & function of proteins

Proteins are classified according to:

  • Shape
  • Globular proteins: protein axis correlation is lower than 1:30
  • Fibrillar proteins: protein axis correlation is higher than 1:30
  • Collagen: triple helical structure of repetitions of glycine-proline hydroxyproline
  • Keratins
  • Content
  • Glycoproteins
  • Lipoproteins
  • Nucleoproteins
  • Metaloproteins
  • Hemoproteins

The main functions of proteins are:

  • Enzymes: catalyse chemical reactions
  • Transport: in membranes or blood (hemoglobin, tranferin…)
  • Hormones: insulin, glucagon, ACTH, GH, FSH, LH, PLT
  • Protection: immunoglobulins, interferon
  • Regulation of gene transcription and translation
  • Contraction
  • Receptors
  • Structural

Definition of Protein Folding

Protein folding is the process by which a linear polypeptide chain (primary structure) acquires its specific three-dimensional conformation, becoming a biologically active, functional protein.

➤ Key Point:

The amino acid sequence itself (primary structure) contains all the information necessary for folding into the correct 3D structure — this was shown by Anfinsen's experiment on ribonuclease.

The Process of Protein Folding

Folding occurs co-translationally (during synthesis on ribosomes) and is a hierarchical process involving several stages:

Stage 1: Formation of Secondary Structure

  • Local folding into α-helices and β-sheets begins as soon as the polypeptide emerges from the ribosome.
  • Stabilized mainly by hydrogen bonds.

Stage 2: Supersecondary Structure (Motifs)

  • α-helices and β-sheets combine to form simple motifs (β–α–β loops, hairpins, etc.).

Stage 3: Domain Formation

  • Motifs assemble into compact, semi-independent folding units called domains.

Stage 4: Tertiary Structure Formation

  • Entire polypeptide chain folds into its native 3D conformation, forming the functional protein.
  • Stabilized by hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges.

Stage 5: Quaternary Assembly (if needed)

  • Multiple polypeptide chains (subunits) may combine to form oligomeric proteins (e.g., hemoglobin).

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