The Extracellular Matrices Part II. 2. Elastin fibers. 3. Proteoglycans (PG) and glycosaminoglycans (GAG). 4. Cell-adhesion molecules (CAM). 1 Elastin fibers ? A network of randomly coiled macromolecules. No periodicity. Highly extensible chains. ? Rubber-like elasticity is complicated by hydrophobic bonding effects. ? Interaction of hydrophobic (nonpolar) AA with water leads to hydrophobic bonding. Primarily entropic, not energetic, bonding between molecules. It forces nonpolar macromolecules, such as elastin, to adopt a compact, rather then extended, shape in hydrated tissue. ? Stretching of elastin fibers leads to large entropy loss due to reduction in chain configurations and increased “ordering” of water molecules against nonpolar AA. Spontaneous retraction. ? Elastic ligament of neck. Blood vessel wall. 2 The Hydrophobic bond 3 ?G = ?H ? T?S Equilibrium when ?G = 0. G is Gibbs’ free energy, the enthalpy is H = E + PV, T is absolute temperature and S is the entropy. The process goes spontaneously from left to right when ?G < 0. Find the position of thermodynamic equilibrium for a well-known example of insolubility: CH 4 in benzene → CH 4 in H 2 O The experimental data show (all units in calories per mol): ?G = ? H ? T ? S +2600 = ?2800 ? 298(?18) +2600 = ?2800 + 5400 Conclusion: Insolubility of paraffin in water due to entropy loss, not to enthalpy change! (Kauzmann) Historical models of cell membrane structure Image removed due to copyright considerations Image removed due to copyright consi erations 4 Cell membrane showing bilayer structure 5 Elastin fibers in the relaxed aorta. Elastin macromolecules are random coils tied together to form a 3-dimensional (insoluble) network. Images removed due to copyright considerations Images removed due to copyright consi erations Macromolecules coil upon themselves due to high content of nonpolar (hydrophobic) amino acids that mediate withdrawal from polar medium (aqueous buffer) and promote bonding within chains. These networks stretch extensively like all rubbers. 6 Proteoglycans (PGs) and glycosaminoglycans (GAGs) ? A proteoglycan is a polypeptide chain (proteo) with polysaccharide (glycan or GAG) side chains. ? Primary structure modeled as an alternating copolymer of two different glucose-like units, one of them an acidic sugar-like molecule, the other an amino sugar with a negatively charged sulfate group (except hyaluronic acid that is not sulfated). ? Electrostatic interactions between charged groups in GAG side chains of PG responsible for about 50% of stiffness of articular cartilage (Grodzinsky et al.). 7 Proteoglycans (PGs) and glycosaminoglycans (GAGs) Images removed due to copyright considerations Images removed due to copyright consi erations 8 9 Γλυκοσαμινογλυκανε? Glycosamino- glycans disaccharide repeat unit Image removed due to copyright considerations. Diagram of Chondroitin 4-Sulfate Image removed due to copyright considerations. Diagram of Dermatan Sulfate. Image removed due to copyright considerations. Diagram of Heparan Sulfate. 10 Γλυκοσαμινογλυκανε?Proteoglycans and glycosaminoglycans repeat unit of chondroitin 6-sulfate a proteoglycan Image removed due to copyright considerations. Cell-adhesion molecules ? Cell-matrix interactions involve binding of transmembrane proteins (integrins) on specific sites (ligands) in ECM molecules such as fibronectin, laminin and collagen. Example: integrins of contractile fibroblasts bind to fibronectin molecules that are attached to collagen fibers (fibronexus). ? Integrins connect with proteins in the cell cytoplasm and a signal is transmitted to or from the nucleus. 11 12 Μια ιντεγκρινη συνδεει το εσωτερικο του κυτταρου(κατω) με ΕΚΜ (πανω) An integrin “connects” the interior of the cell (cytoplasm) with the ECM outside it After Hynes, 1990. Cell Membrane Phospholipid molecule Bilayer structure of cell membrane viewed by electron microscopy duplicate of Intro s.14 – 2 redraws, 1 delete Cell membrane showing cell surface receptors (integrins) 13 Image removed due to copyright considerations. Cells pull matrix (thin silicone film), causing buckling (Harris et al.) Image removed due to copyright considerations Image removed due to copyright consi erations 14 Location of fibronectin binding sites (Hynes, 1990) Image removed due to copyright considerations Image removed due to copyright consi erations 15