1. Collagen and Elastin.pdf

Fibrous Proteins Collagen and elastin BY: DR. SAFINAZ HAMDY EL KHOULANY LECTURER OF MEDICAL BIOCHEMISTRY AND MOLECULAR BIOLOGY  EXTRACELLULAR MATRIX (ECM) GROUNG SUBSTANCE OR CONNECTIVE TISSUE Definition: It is a gel like substance present in the...

Fibrous Proteins Collagen and elastin BY: DR. SAFINAZ HAMDY EL KHOULANY LECTURER OF MEDICAL BIOCHEMISTRY AND MOLECULAR BIOLOGY  EXTRACELLULAR MATRIX (ECM) GROUNG SUBSTANCE OR CONNECTIVE TISSUE Definition: It is a gel like substance present in the extracelular space  Components of ECM : 1- Sructural proteins: 2- Glycoproteins 3- Proteoglycans -Collagen (less CHO): (CHO up to 95%): -Elastin -Fibronectin -Mainly GAGs -Fibrillin -Laminin  Components of ECM : 1- Sructural proteins: 2- Glycoproteins 3- Proteoglycans -Collagen (less CHO): (CHO up to 95%): -Elastin -Fibronectin -Mainly GAGs -Fibrillin -Laminin They are folded or coiled Collagen is in the form of fibres that are tough and have high tensile strength Elastin has rubber like properties for elasticity of elastic tissues  COLLAGEN  Collagen is the most abundant protein in the human body.  It I the major protein in the E.C.M  COLLAGEN STRUCTURE  A typical collagen molecule is a long, rigid structure  It is formed of three polypeptides “α chains” which are wound around one another in a rope-like triple helix  The three polypeptide α chains are held together by hydrogen bonds between the chains.  COLLAGEN STRUCTURE  Each α - chain is approximately 1000 A.As  Variations in the amino acid sequence of the α chains produces different α – chain  These α chains are combined to form the various types of collagen found in the tissues (different combinations).  There are more than 25 collagen types  COLLAGEN STRUCTURE  Each α - chain is approximately 1000 A.As  Variations in the amino acid sequence of the α chains produces different α – chain 1 2  These α chains are combined to form the 3 various types of collagen found in the tissues (different combinations). 4 7  There are more than 25 collagen types 9 12  AMINO ACID SEQUENCE  Collagen is rich in proline and glycine  There are three main A.As: (repeating sequence, –Gly–X–Y–): X is frequently proline Y is often hydroxyproline (but can be hydroxylysine)  Glycine, the smallest amino acid, is found in every third position of the polypeptide chain. Hyp is hydroxyproline Hyl is hydroxylysine  HYDROXYPROLINE AND HYDROXYLYSINE  Collagen contains hydroxy -proline (hyp) and hydroxylysine (hyl), which are not present in most other proteins.  These residues result from the hydroxylation of some of the proline and lysine residues after their incorporation into polypeptide chains.  The hydroxylation is, thus, an example of post translational modification.  Hydroxy -proline is important in stabilizing the triple-helical structure of collagen because it maximizes interchain hydrogen bond formation.  BIOSYNTHESIS OF COLLAGEN Cells resposible for synthesis:  Fibroblasts  Osteoblasts (in bone)  Chondroblasts (in cartilage) In synthesis, posttranslational modifications occur (hydroxylation and glycosylation) occur Collagen is then secreted into the extracellular matrix In ECM, enzymic modification and the mature collagen monomers aggregate and become cross-linked to form collagen fibers.  PPTC BIOSYNTHESIS OF COLLAGEN Pro α chains Procllagen  Pre-pro- α chain formation (Signal sequence present) Tropocollagen  Pro α chain formation (Signal sequence cleaved)  Hydroxylated pro- α chain formation Collagen posttranslational  Glycosylated pro- α chain formation modifications  Pro-collagen formation (moved from RER to Golgi and secreted out of the fibroblast)  Tropocollagen molecule formation outside fibroblasts propeptides cleaved from ends and becomes insoluble presence of propeptide does not allow assembly intracellularly  Collagen fibril formation catalyzed by lysyl oxidase which covalently links α chains by crosslinking  Collagen fiber formation fibrils aggregate to form final bundles  PPTC BIOSYNTHESIS OF COLLAGEN Pro α chains Procllagen  Pre-pro- α chain formation (Signal sequence present) Tropocollagen  Pro α chain formation (Signal sequence cleaved)  Hydroxylated pro- α chain formation Collagen  Glycosylated pro- α chain formation  Pro-collagen formation (moved from RER to Golgi and secreted out of the fibroblast)  Tropocollagen molecule formation outside fibroblasts - propeptides cleaved from ends and becomes insoluble - presence of propeptide does not allow assembly intracellularly  Collagen fibril formation catalyzed by lysyl oxidase which covalently links α chains by crosslinking  Collagen fiber formation fibrils aggregate to form final bundles 1. Formation of pro α chains: For proteins (like collagen) that normally function outside of cells, the newly synthesized polypeptide precursors of α chains (prepro-α chains) contain a special amino acid sequence at their N-terminal ends. This sequence acts as a signal that directs the passage of the prepro-α chain into the lumen of the RER. The signal sequence is rapidly cleaved in the RER to yield a precursor of collagen called a pro-α chain 1. Formation of pro α chains: 2. Hydroxylation: - Occur for Proline and lysine residues found in the Y-position of the –Gly–X–Y– sequence - This forms hydroxyproline and hydroxylysine residues. - Enzymes: Hydroxylating enzymes, prolyl hydroxylase and lysyl hydroxylase - Requirements of hydroxylation: -Molecular oxygen -Fe2+ - Reducing agent vitamin C (ascorbic acid) Vitamin C (ascorbic acid) importance - Without vitamin C (ascorbic acid) which the hydroxylating enzymes, prolyl hydroxylase and lysyl hydroxylase, are unable to function. - Ascorbic acid deficiency (scurvy) lack of prolyl and lysyl hydroxylation Hydroxy -proline Interchain H-bond formation and formation of a stable triple helix.is impaired is important in stabilizing the triple- Collagen fibrils cannot be cross-linked helical structure of collagen because it Ddecreasing the tensile strength of the assembled fiber maximizes interchain hydrogen bond Bruises on the limbs as a result of subcutaneous extravasation of blood due to capillary fragility formation. 3. Glycosylation: Some hydroxylysine residues are modified by glycosylation with glucose or galactose  4. Assembly of the 3 pro α chains Into procollagen - It is a precursor of collagen that has a central region of triple helix flanked by the nonhelical amino- and carboxyl-terminal extensions called propeptides. -This is by formation of interchain and intrachain disulfide bonds between these extensions of the pro-α chains. This brings the three α chains into an alignment favorable for helix formation.  5. Secretion of procollagen - The procollagen molecules move through the Golgi apparatus, where they are packaged in secretory vesicles. - The vesicles fuse with the cell membrane, causing the release of procollagen molecules into the extracellular space.  6- Tropocollagen formation: After their release, the procollagen molecules are cleaved by N- and C-procollagen peptidases, which remove the terminal propeptides, Formation of the triple-helical Tropocollagen molecules. 7. Formation of collagen fibrils: Individual Tropocollagen molecules spontaneously associate to form collagen fibrils. They form an ordered, overlapping, parallel array, with adjacent collagen molecules arranged in a quarter staggered pattern, each overlapping its neighbor by a length approximately three-quarters of a molecule  8- Formation of collagen fibers (Cross-link formation): H - The fibrillar array of collagen molecules serves as a substrate for lysyl oxidase. (OO) - lysyl oxidase is a Cu2+-containing extracellular enzyme -It oxidatively deaminates some of the lysyl and hydroxylysyl residues in collagen. Formation of reactive aldehydes (allysine and hydroxyallysine) These can condense with lysyl or hydroxy - lysyl residues in neighboring collagen molecules Formation of covalent cross-links Formation of mature collagen fibers  DEGRADATION OF COLLAGEN  Normal collagens are highly stable molecules, having half-lives as long as several years.  However, connective tissue is dynamic and is constantly being remodeled, often in response to growth or injury of the tissue.  Breakdown of collagen fibers is dependent on the proteolytic action of collagenases COLLAGEN DISEASES: COLLAGENOPATHIES  Defects in any one of the many steps in collagen fiber synthesis can result in a genetic disease involving an inability of collagen to form fibers properly  More than 1,000 mutations have been identified in 22 genes coding for 12 of the collagen types.  Examples: 1. Ehlers-Danlos syndrome (EDS) 2. Osteogenesis imperfecta (OI)  1. EHLERS-DANLOS SYNDROME (EDS): Causes: 1- Deficiency of collagen-processing enzymes (for example, lysyl hydroxylase or procollagen peptidase) 2- Mutations in the amino acid sequences of collagen types I, III, or V. Collagen containing mutant chains is not secreted, and is either degraded or accumulated to high levels in intracellular compartments. The most clinically important mutations are found in the gene for type III collagen. Because collagen type III is an important component of the arteries, potentially lethal vascular problems occur. Although collagen type III is only a minor component of the collagen fibrils in the skin, patients with EDS also show, for unknown reasons, defects in collagen type I fibrils. This results in fragile, stretchy skin and loose joints  2. OSTEOGENESIS IMPERFECTA (OI) (BRITTLE BONE SYNDROME) It is characterized by : 1- bones that easily bend and fracture 2- retarded wound healing 3- rotated and twisted spine leading to a (kyphotic) appearance Type I OI (osteogenesis imperfecta tarda) The disease is the consequence of decreased production of α1 and α2 chains. It presents in early infancy with fractures secondary to minor trauma Type II OI (osteogenesis imperfecta congenita) It is the most severe. Patients die of pulmonary hypoplasia in utero or during the neonatal period. Most patients with severe OI have mutations in the gene for either the pro-α1 or pro-α2 chains of type I collagen. The most common mutations cause the replacement of glycine residues (in –Gly–X– Y–) by amino acids with bulky side chains. The resultant structurally abnormal pro-α chains prevent the formation of the required triple-helical conformation.  ELASTIN  Collagen fibers are tough and have high tensile strength.  Elastin is a connective tissue protein with rubber-like properties.  Elastin is also rich in proline and lysine, but contains only a little hydroxyproline and hydroxylysine. Elastic fibers  They are composed of elastin and glycoprotein (fibrillin)  Sites: the lungs, the walls of large arteries, and elastic ligaments.  They can be stretched to several times their normal length, but recoil to their original shape when the stretching force is relaxed. Elastin  Elastin is an insoluble protein polymer synthesized from a precursor, Tropoelastin, which is a linear polypeptide Fibrillin  It is a glycoprotein secreted into the extracellular matrix by fibroblasts Tropoelastin is secreted by the cell into the extracellular space where it interacts with fibrillin, which function as a scaffold onto which tropoelastin is deposited Thus, Fibrillin is important for integrity of connective tissue Cell 1) It is the precursor of Elastin Tropoelastin It is a linear polypeptide It composed of about 700 amino acids that are primarily small and nonpolar (for example, glycine, alanine, and valine). Extracellular space specific glycoprotein Tropoelastin microfibrils, such as fibrillin 2) Tropoelastin is secreted by the 3) cell into the extracellular space Fibrillin functions as a scaffold onto which tropoelastin is deposited Lysine Tropoelastin 4) fibrillin Some of the lysyl side chains of the tropoelastin Lysine Tropoelastin are oxidatively deaminated by lysyl oxidase, forming allysine residues 5) Three of the allysyl side chains plus one unaltered lysyl side chain from the same or neighboring polypeptides form a desmosine cross-link 5) Three of the allysyl side chains plus one unaltered lysyl side chain from the same or neighboring polypeptides form a desmosine cross-link Lysine Tropoelastin fibrillin Allysine Lysine Tropoelastin Allysine Allysine Lysine 5) Three of the allysyl side chains plus one unaltered lysyl side chain from the same or neighboring polypeptides form a desmosine cross-link Lysine Tropoelastin fibrillin Allysine Lysine Tropoelastin Allysine Allysine Lysine  This produces elastin—an extensively interconnected, rubbery network that can stretch and bend in any direction when stressed, giving connective tissue elasticity  DISORDERS OF ELASTIN  Marfan syndrome  Alpha 1- Antitrypsin deficiency  Fibrillin-1 protein is encoded by the FBN1 gene, located on chromosome 15.  Mutations in the FBN1 gene cause Marfan syndrome and are also sometimes associated with adolescent idiopathic scoliosis.  With this disease, abnormal fibrillin protein is incorporated into microfibrils along with normal fibrillin, inhibiting the formation of functional microfibrils  Reduced or abnormal fibrillin-1 leads to tissue weakness  MARFAN SYNDROME  It is an autosomal dominant disorder  It is characterized by impaired structural integrity in the skeleton, the eye, and the cardiovascular system.  Skeleton : - People tend to be tall (grow to above-average height) and thin, with long arms, legs, fingers and toes. -have flexible joints -Have scoliosis. MARFAN SYNDROME  Lungs: Emphysema from the destruction of the connective tissue of alveolar walls  The cardiovascular system : The most serious complications involve the heart and aorta, with an increased risk of: 1- aortic aneurysm. 2- Mitral valve prolapse  The eyes: Lens dislocation  Patients with OI, EDS, or Marfan syndrome may have blue sclera due to tissue thinning that allows underlying pigment to show through  Antitrypsin (α1-AT, A1AT, currently also called α1-antiproteinase): It is present in the blood and other body fluids It was originally named α1-antitrypsin because it inhibits the activity of trypsin (a proteolytic enzyme synthesized as trypsinogen by the pancreas It inhibits a number of proteolytic enzymes (proteases or proteinases) that hydrolyze and destroy proteins, especially One of these is neutrophil elastase––a powerful protease that is released into the extracellular space, and degrades elastin of alveolar walls, as well as other structural proteins in a variety of tissues Most of it is synthesized and secreted by the liver. The remainder is synthesized by several tissues, including monocytes and alveolar macrophages, which may be important in the prevention of local tissue injury by elastase  Role of Alpha 1-AT in the lungs In the normal lung, the alveoli are chronically exposed to low levels of neutrophil elastase released from activated and degenerating neutrophils. This proteolytic activity can destroy the elastin in alveolar walls if unopposed by the action of α1-AT Because lung tissue cannot regenerate, emphysema results from the destruction of the connective tissue of alveolar walls.  Causes of alpha 1 antitrypsin defeciency 1- Genetic cause: different mutations in the gene for α1-AT are known to cause a deficiency of this protein, The most widespread is single base mutation (GAG → AAG) , resulting in the substitution of lysine for glutamic acid at position 342 of the protein) The polymerization of the mutated protein in the endoplasmic reticulum of hepatocytes causes decreased secretion of α1-AT by the liver. The accumulated polymer may result in cirrhosis (scarring of the liver).  Causes of alpha 1 antitrypsin defeciency 2- Smoking: Smoking oxidation and subsequent inactivation of methionine residue of alpha 1 antitrypsin No inactivation of elastase of neutrophils. Destruction of Elastin in alveolar wall Smokers with α1-AT deficiency, therefore, have a considerably elevated rate of lung destruction and a poorer survival rate than nonsmokers with the deficiency.