Proteins and Junction Complexes - MBBS 1st Year Physiology & Biochemistry

PART 1: PROTEIN STRUCTURE

1. Primary Structure

• Adjacent amino acids are joined by peptide bonds (amide linkages between α-COOH of one and α-NH₂ of the next)
• Peptide bonds have partial double-bond character - they are rigid, planar, and almost always in trans configuration
• Linkage of 50+ amino acids = polypeptide / protein
• Each amino acid component is called a residue
• Abnormal primary structure (as in genetic diseases) = misfolding = loss of function (e.g., sickle cell disease)

• Resistant to heat and urea
• Requires strong acid/base + elevated temperature to break
• -C=O and -NH groups are uncharged (do not accept/release protons at pH 2-12)

2. Secondary Structure

A. Alpha (α)-Helix

• Right-handed coil; 3.6 amino acids per turn
• Stabilized by H-bonds between C=O of residue n and N-H of residue n+4
• H-bonds are parallel to the backbone axis
• Proline disrupts the helix (introduces a kink); Glycine adds flexibility
• Charged/bulky R-groups and β-carbon branching (e.g., valine) disfavor α-helix

B. Beta (β)-Sheet

• Two or more peptide strands aligned laterally
• H-bonds are perpendicular to the backbone
• Can be parallel (N-termini on same side) or antiparallel (N-termini alternating)
• Pleated appearance due to alternating position of α-carbons above/below the plane
• R groups extend above and below the sheet alternately

C. Beta (β)-Bends (Reverse turns)

• Reverse the direction of the polypeptide chain
• Usually 4 amino acids; often contain Proline (kink-former) and Glycine (smallest R group)
• Connect antiparallel β-strands
• Stabilized by H-bonds between 1st and 4th residue in the bend
• Found on the protein surface; often contain charged residues

3. Tertiary Structure

• Proteins are classified as fibrous (structural, water-insoluble, e.g., collagen, keratin) or globular (functional, often water-soluble, e.g., hemoglobin, enzymes)
• Motifs (super-secondary structures): recurring arrangements of secondary structures (e.g., helix-turn-helix, β-barrel)
• Domains: independently folding, functional units within a polypeptide

4. Quaternary Structure

• Stabilized by same non-covalent forces as tertiary structure
• Example: Hemoglobin = 2α + 2β subunits (tetramer)
• Allows cooperative behavior (e.g., O₂ binding in hemoglobin)
• Homodimer = two identical subunits; Heterodimer = two different subunits

PART 2: JUNCTION COMPLEXES

1. Junctions that fasten cells together (mechanical stability)
2. Junctions that permit transfer of molecules between cells (communication)

1. Tight Junction (Zonula Occludens)

• Located at the apical margins of epithelial cells (intestinal mucosa, renal tubules, choroid plexus)
• Made of ridges (half from each cell) that adhere so strongly they almost obliterate the intercellular space
• Transmembrane proteins: Occludin, Claudins, Junctional Adhesion Molecules (JAMs)
• Two main functions:
  • Paracellular barrier - regulate/restrict passage of ions and solutes between cells (degree of "leakiness" depends on claudin composition)
  • Fence function - prevent lateral diffusion of membrane proteins, maintaining distinct apical vs. basolateral domains (essential for directional epithelial transport)
• Also important for endothelial barrier function and endothelium-dependent vasodilation

2. Adherens Junction (Zonula Adherens)

• Continuous structure located basal to the zonula occludens in epithelial cells
• Major site of attachment for intracellular microfilaments (actin cytoskeleton)
• Contains cadherins (Ca²⁺-dependent adhesion molecules)
• Helps hold cells together and transmit mechanical forces

3. Desmosome (Macula Adherens)

• Patch-like thickenings of apposed membranes of two adjacent cells
• Intracellularly connected to intermediate filaments (keratin in epithelial cells)
• Intercellular space contains filamentous material including cadherins and other transmembrane proteins
• Provides strong mechanical links - especially important in tissues subject to mechanical stress (skin, cardiac muscle)

4. Hemidesmosome

• Looks like half a desmosome
• Attaches cells to the underlying basal lamina (not to adjacent cells)
• Connected intracellularly to intermediate filaments
• Contains integrins (not cadherins) - key difference from desmosomes

5. Focal Adhesions

• Also attach cells to their basal lamina
• Associated with actin filaments inside the cell
• Labile structures that play an important role in cell movement

6. Gap Junction (Nexus)

• Intercellular space is reduced to ~4 nm (vs ~20 nm normally)
• Formed by connexons from each cell aligning to form a dodecameric gap junction
• Each connexon = 6 connexin proteins surrounding a central channel
• Each connexin traverses the membrane 4 times
• Pore diameter: 0.8-1.4 nm; allows passage of molecules < 1000 Da
  • Permits passage of: ions, sugars, amino acids, cAMP, IP₃, and other intracellular signaling molecules
• Functions:
  • Rapid electrical coupling between cells (e.g., cardiac muscle, smooth muscle, CNS)
  • Exchange of chemical messengers and metabolites
  • Connexins can also form hemichannels (connexin semichannels) for regulated passage between cytoplasm and ECF
• At least 20 different connexin genes in humans
• Mutations in connexins cause selective diseases: arrhythmias, female infertility, cataracts, hearing loss (Connexin 26 mutations = autosomal recessive deafness), abnormal bone development

Quick Comparison Table

High-Yield Points for Exams

1. Peptide bond is rigid and planar due to partial double-bond character; always in trans configuration
2. α-helix: H-bonds parallel to backbone; 3.6 residues/turn; Proline breaks it
3. β-sheet: H-bonds perpendicular to backbone; can be parallel or antiparallel
4. Tight junctions = claudins + occludin + JAMs - control paracellular permeability
5. Desmosomes use cadherins but link to intermediate filaments (not actin)
6. Hemidesmosomes use integrins (not cadherins) and also link to intermediate filaments
7. Gap junctions: connexon = 6 connexins; allow molecules < 1000 Da; responsible for electrical coupling in heart
8. Connexin 26 (GJB2) mutation = most common cause of hereditary non-syndromic deafness

• Lippincott Illustrated Reviews: Biochemistry, 8th ed. - Protein Structure chapter
• Ganong's Review of Medical Physiology, 26th ed. - Intercellular Connections & Gap Junctions

Junction Complexes - MBBS 1st Year Physiology

Overview

1. Junctions that fasten cells together - provide structural stability (tight junctions, adherens junctions, desmosomes, hemidesmosomes)
2. Junctions that permit transfer of molecules - allow communication (gap junctions)

Diagram 1 - Position of all junctions in a polarized epithelial cell (Ganong's)

Diagram 2 - 3D detailed view of junctional complexes (Junqueira's Histology)

Diagram 3 - TEM (Electron Microscope) view of junctional complex

1. Tight Junction (Zonula Occludens)

• Made of ridges (half from each adjacent cell) that interdigitate and almost obliterate the intercellular space
• Three families of transmembrane proteins:
  • Claudins - main barrier proteins (determine leakiness)
  • Occludin
  • Junctional Adhesion Molecules (JAMs)
• Linked intracellularly to actin filaments via scaffolding proteins (ZO-1, ZO-2, ZO-3)

2. Adherens Junction (Zonula Adherens)

• Transmembrane proteins: E-cadherin + catenin complexes (Ca²⁺-dependent)
• Intracellularly linked to actin filaments (major attachment site for actin cytoskeleton)

• Creates strong cell-to-cell adhesion
• Stabilizes and strengthens the nearby tight junction
• Mechanically couples the cytoskeletons of adjacent cells
• Transmits contractile forces between cells

3. Desmosome (Macula Adherens)

• Apposed membrane thickenings (plaques) on the cytoplasmic face of each cell
• Transmembrane proteins: Desmogleins and Desmocollins (members of the cadherin family)
• Intracellularly linked to intermediate filaments (keratins) - both parallel to and radiating away from the membrane
• Intercellular space filled with filamentous material linking the two plaques

• Provides the strongest mechanical anchoring between cells
• Distributes mechanical stress across the entire tissue (like rivets)
• Especially important in tissues under physical stress: skin, cardiac muscle, uterus

4. Hemidesmosome

• Looks like half a desmosome but is functionally different
• Transmembrane proteins: Integrins (not cadherins - key exam difference!)
• Intracellularly linked to intermediate filaments
• Extracellularly binds to laminin in the basal lamina

5. Focal Adhesion

• Contains integrins
• Linked intracellularly to actin filaments (not intermediate filaments)
• Labile (transient, dynamic) structures

6. Gap Junction (Nexus / Communicating Junction)

• Intercellular space reduced to ~4 nm (vs normal ~20 nm)
• Each gap junction = two connexons (one from each cell) aligned face-to-face → dodecameric structure
• Each connexon = 6 connexin proteins surrounding a central channel
• Each connexin has 4 transmembrane domains
• Pore diameter = 0.8-1.4 nm → passes molecules up to ~1000 Da

• Electrical coupling - rapid propagation of action potentials (cardiac muscle, smooth muscle, CNS)
• Metabolic coupling - exchange of nutrients and metabolic intermediates
• Developmental signaling - coordination of cell growth and differentiation
• Connexin semichannels - connexons alone (hemichannels) can also allow regulated release of small molecules to the ECF

• Connexin 26 (GJB2) mutation → most common cause of autosomal recessive non-syndromic deafness
• Connexin 30 (Cx30) defect → Clouston syndrome (skin disorder)
• Connexin deletions → cardiac arrhythmias, sudden cardiac death
• Various connexins → cataracts, peripheral neuropathy, female sterility

Master Comparison Table

High-Yield Exam Points

1. The apico-basal order from top to bottom: Tight junction → Adherens junction → Desmosome → Gap junction → Hemidesmosome (mnemonic: "TADGH" - Tight, Adherens, Desmosome, Gap, Hemi)
2. Only gap junctions allow direct cell-to-cell transfer of molecules
3. Desmosomes and hemidesmosomes both anchor to intermediate filaments - but desmosomes use cadherins while hemidesmosomes use integrins
4. Tight junctions control paracellular permeability - leakiness is determined by claudin subtype
5. Connexin 26 mutation = most common inherited cause of deafness
6. Gap junctions pass molecules up to 1000 Da through a pore of 0.8-1.4 nm
7. The term "zonula" = belt/band (encircles the whole cell); "macula" = spot/patch (discrete points)