Collagen_ECM_SJM_2_year

List the principal extracellular matrix (ECM) components of connective tissue and name the cell types that synthesize them in cartilage, bone, and general connective tissue.

Components: fibrous proteins (collagen, elastin), proteoglycans/GAGs, glycoproteins (fibronectin, laminin), water/ions. Chondrocytes (cartilage), osteoblasts (bone), fibroblasts (general CT).

Explain how the ECM’s composition (fibers + ground substance) determines tissue properties (rigid bone vs pliable cartilage) and its roles in development and morphogenesis.

High mineralized collagen → rigid bone; collagen + proteoglycan–rich, highly hydrated matrix → compressible cartilage. ECM provides scaffolding, growth factor gradients, and mechanotransduction for morphogenesis.

Quantify collagen’s abundance in mammals, identify the predominant collagen type in humans, and give turnover times for liver, bone, and skin.

Collagen ≈25–30% of total protein. Type I is predominant. Turnover: liver fast (days–weeks), bone slower (months), skin intermediate (weeks–months).

Describe the canonical collagen primary sequence (Gly-X-Y); explain how glycine and proline residues stabilize the triple helix.

Repeating Gly–X–Y (X/Y often Pro/Hyp). Gly at every third position allows tight packing; proline/hydroxyproline impose rigidity and enable interchain H-bonds for helix stability.

Specify the enzymatic and cofactor requirements for prolyl and lysyl hydroxylation and connect vitamin C deficiency to scurvy pathogenesis.

Prolyl/lysyl hydroxylases require O2, Fe2+, α‑ketoglutarate, and ascorbate to restore Fe2+. Vitamin C deficiency → underhydroxylated collagen → unstable helices, poor cross-linking → scurvy (bleeding gums, poor wound healing).

Explain enzymatic O-linked glycosylation of hydroxylysine in collagen and how it promotes proper fibrillogenesis.

Hydroxylysine residues receive galactose or galactosyl‑glucose in the ER/Golgi, guiding fibril diameter and spacing; defects disturb fibrillogenesis and tissue mechanics.

Summarize serine/threonine phosphorylation in collagen chains and its effect on triple-helix stability.

Some collagens are phosphorylated on Ser/Thr, modulating secretion and assembly; excessive phosphorylation can destabilize triple-helix folding.

Detail triple helix assembly: C-terminal disulfide nucleation, interchain H-bonding, left-handed α-chains forming a right-handed superhelix.

C-propeptide alignment and disulfide bonds nucleate trimerization; three left-handed α-chains wind into a right-handed triple helix stabilized by H-bonds involving Hyp.

Outline procollagen secretion and extracellular processing by N- and C-procollagen peptidases; define ‘tropocollagen’ and clinical use of propeptides as formation markers.

Procollagen is secreted, N- and C-propeptides are cleaved to yield tropocollagen; released propeptides (e.g., P1NP) are biomarkers of collagen formation/remodeling.

Explain the quarter-stagger arrangement of tropocollagen during fibrillogenesis and why it optimizes tensile resistance.

Molecules align with -67 nm D-periodicity and quarter-stagger offsets, creating gap/overlap regions that distribute load and resist crack propagation.

Describe lysyl oxidase–mediated cross-link formation (allysine/hydroxyallysine) and distinguish intra- vs intermolecular cross-links; name urinary markers of collagen degradation.

Lysyl oxidase deaminates Lys/Hyl → allysine/hydroxyallysine → Schiff base/aldol cross-links (intra/interfibrillar). Urinary pyridinoline/deoxypyridinoline reflect bone collagen degradation.

List key physical parameters of tropocollagen (dimensions, mass, handedness, residues per turn).

Length -300 nm, diameter -1.5 nm, right-handed triple helix; -3.3 residues per turn; -1000 aa per α-chain; -300 kDa per molecule.

Classify collagens by family (fibril-forming, FACIT, network-forming, basement membrane, beaded filament, anchoring fibrils, transmembrane, unique) and give one example each.

Fibril-forming: I, II, III; FACIT: IX, XII; Network-forming: IV; Basement membrane: IV; Beaded filament: VI; Anchoring fibrils: VII; Transmembrane: XIII, XVII; Unique: VIII, X.

Define collagenopathies and provide clinical features of Ehlers–Danlos syndrome due to cross-linking defects (e.g., lysyl oxidase issues).

Collagenopathies: disorders from collagen gene or processing defects. EDS (cross-linking defects): hyperextensible skin, joint hypermobility, easy bruising, vessel fragility.

Differentiate glycation from enzymatic glycosylation in collagen and describe how excess glycation alters mechanics and cell recognition (e.g., diabetes, inflammation).

Glycation: non-enzymatic AGE formation on Lys/Arg → additional cross-links, stiffness, impaired remodeling and altered integrin binding. Enzymatic glycosylation is controlled and functional.

Describe collagen turnover dynamics and the concept of fibrosis when synthesis outpaces degradation; relate to wound healing timelines.

Turnover is tissue-specific; in fibrosis, persistent TGF‑β signaling → excess collagen deposition > degradation. Wound healing: initial type III → later type I remodeling over weeks–months.

Compare cysteine proteases (e.g., cathepsin K) and matrix metalloproteinases (MMPs) in collagen degradation; indicate their activation mechanisms.

Cathepsin K: lysosomal, acidic pH, osteoclast resorption lacuna. MMPs: Zn2+ endopeptidases activated by proteolysis or oxidation; regulated by TIMPs and transcriptional control.

Explain cathepsin K’s role in osteoclast-mediated bone resorption and the concept of ‘cathepsin cannibalism’ involving cathepsin S.

Cathepsin K cleaves triple helical type I collagen in acidic lacunae. ‘Cannibalism’: cathepsin S can degrade cathepsin K, modulating resorptive capacity.

List major MMP subclasses (gelatinases, collagenases) and their substrates; outline regulation via gene expression, zymogen activation, and TIMPs.

Collagenases (MMP‑1,‑8,‑13) cleave fibrillar collagens; Gelatinases (MMP‑2,‑9) digest denatured collagen/gelatin; Regulation: transcription, pro-peptide removal (e.g., plasmin), and TIMP inhibition.

Describe elastin’s building blocks (tropoelastin, fibrillin) and the unique desmosine cross-links that enable elasticity.

Elastic fibers: elastin core (from tropoelastin) + fibrillin microfibrils scaffold. Desmosine/isodesmosine cross-links (derived from 4 Lys) provide reversible stretch and recoil.

Summarize tropoelastin’s amino acid composition and asymmetric shape and why monomers self-associate at the cell surface.

Rich in Gly, Val, Ala, Pro; hydrophobic domains drive coacervation and self-assembly on microfibrils; asymmetric, flexible structure aids alignment before cross-linking.

Explain elastin network assembly via random head-to-tail and lateral associations and subsequent lysyl oxidase cross-linking.

Tropoelastin monomers associate head-to-tail and laterally, then lysyl oxidase forms desmosine cross-links, stabilizing an insoluble elastic network.

Describe the composite elastic fiber (elastin core + fibrillin microfibrils) and how random-coil domains permit reversible recoil.

Fibrillin microfibrils organize deposition; elastin’s random-coil hydrophobic domains collapse/extend with strain, enabling entropic recoil.

Contrast how collagen and elastin fibers contribute to tissue mechanics (strength vs elasticity) and why limited direct interactions exist between them.

Collagen resists tensile loads (stiff/strong); elastin stores/release elastic energy (compliance). Limited interaction preserves independent mechanical roles and prevents damping of recoil.

Define a proteoglycan and explain how its high negative charge creates hydrated, compressible matrices that also act as ion filters.

Proteoglycan = core protein + GAG chains. Sulfated carboxylate groups attract counter-ions and water → swelling pressure, compressive resilience, and Donnan ion-selectivity.

List the building blocks of glycosaminoglycans (GAGs), including typical modifications (sulfation, epimerization) and their enzymatic basis.

GAGs are repeating disaccharides (hexosamine + uronic acid/galactose). Modifications via specific Golgi enzymes add sulfate groups and epimerize uronic acids (e.g., GlcA↔IdoA).

Name five major GAGs (hyaluronan, chondroitin sulfate, heparin, dermatan sulfate, keratan sulfate) and give typical tissue locations/functions.

Hyaluronan (synovial fluid/extracellular space—lubrication), Chondroitin sulfate (cartilage—compressive strength), Heparin (mast cells—anticoagulant), Dermatan sulfate (skin/tendons), Keratan sulfate (cornea/cartilage).

Describe the hyalectan (lectican) family architecture, including hyaluronan binding via link protein and assembly into massive ECM complexes.

Aggrecan/versican/brevican/neurocan have N‑terminal HA‑binding domains; link proteins stabilize binding; C‑terminal domains bind ECM proteins → large aggregates on HA backbones.

Summarize aggrecan’s composition (GAG content and types), cellular source, and its roles in cartilage hydration and chondrocyte–matrix interactions.

Aggrecan from chondrocytes carries numerous CS and KS chains; forms huge HA–link protein aggregates, retaining water for compressive resilience and modulating growth factor availability.

Detail the O-glycosidic linkage that attaches GAG chains to core protein (Ser–xylose–galactose–galactose) and where this occurs in the cell.

Linkage region: Ser–Xyl–Gal–Gal–GlcA; assembled in the Golgi before chain elongation and sulfation.

List steps of proteoglycan biosynthesis (RER → Golgi elongation → epimerization → sulfation → assembly on hyaluronan → secretion).

Core protein synthesized in RER → glycosylation/chain initiation in cis-Golgi → polymerization, epimerization, sulfation in medial/trans-Golgi → HA assembly at cell surface → secretion.

Explain age- and turnover-related changes in aggrecan structure and why they predispose to cartilage erosion.

With age: shorter GAG chains, lower sulfation; proteolysis by ADAMTS/MMPs reduces aggregate size → diminished water retention and load distribution → cartilage wear.

Describe intracellular degradation of proteoglycans and the lysosomal enzymes involved; connect enzyme deficiencies to oligosaccharidoses.

Proteoglycans are endocytosed and degraded by exo-/endo-glycosidases and sulfatases in lysosomes; enzyme deficits cause mucopolysaccharidoses with GAG storage.

List at least four diverse biological functions of proteoglycans (hydration, lubrication, growth factor sequestration, angiogenesis modulation, cell growth).

Hydration/compression resistance; joint lubrication; reservoir and presentation of growth factors (FGF, TGF‑β); modulation of cell migration/adhesion/angiogenesis.

Explain why proteoglycan gels are highly viscous and resist compression yet return to shape; relate to ionic interactions and bound water.

Fixed negative charges create osmotic swelling and electrostatic repulsion; water is tightly bound but mobile, enabling deformation and elastic recovery.

Compare collagen enzymatic cross-links to elastin’s desmosine cross-links in chemistry and mechanical consequence.

Collagen: allysine/hydroxyallysine-derived cross-links (LOX) → tensile strength. Elastin: desmosine/isodesmosine (four-Lys cross-links) → extensibility and recoil.

Explain the clinical signs of scurvy in terms of failed proline/lysine hydroxylation and unstable triple helices.

Defective hydroxylation → unstable collagen → capillary fragility (petechiae, bleeding gums), poor wound healing, bone pain in children.

Relate Ehlers–Danlos joint hypermobility and vessel fragility to disordered collagen cross-linking.

Impaired cross-linking or collagen III defects → weak connective tissue → hypermobile joints, skin laxity, easy bruising, risk of vascular/organ rupture.

Describe how urinary pyridinoline/deoxypyridinoline measurements reflect bone resorption activity.

They are mature collagen cross-link fragments released during osteoclastic degradation of type I collagen and excreted in urine—higher levels indicate increased bone resorption