Biochemistry Fibrous Proteins PDF

1A BIOCHEMISTRY FIBROUS PROTEINS DR. JANDOC OUTLINE I. COLLAGEN A. Types of collagen a. Fibril-f...

1A BIOCHEMISTRY FIBROUS PROTEINS DR. JANDOC OUTLINE I. COLLAGEN A. Types of collagen a. Fibril-forming collagens b. Network forming collagen c. Fibril-associated collagens B. Structure of collagen a. Amino acid sequence b. Triple-helical structure c. Hydroxyproline and hydroxylysine A. TYPES OF COLLAGEN d. Glycosylation  More than 25 types C. Biosynthesis of collagen  3 polypeptide α-chains held together by hydrogen bonds a. Formation of pro-α chains  Type I = 2 α1 and 1 α2 b. Hydroxylation  Type II = 3 α1 c. Glycosylation  Three groups: based on location and functions in the body d. Assembly and secretion  FIBRIL-FORMING COLLAGENS e. Extracellular cleavage of procollagen  Types I, II, and III molecules  Type I f. Formation of collagen fibrils  supporting elements of high tensile strength g. Cross-link formation  tendon and cornea D. Degradation of collagen  Type II E. Collagen diseases: Collagenopathies  Cartilaginous structures a. Ehlers-Danlos syndrome  Type III b. Osteogenesis imperfecta II. ELASTIN  Distensible tissues such as blood vessels A. Structure of elastin  In electron microscope B. Role of α1-antitrypsin in elastin degradation  Characteristic banding pattern a. α1-antitrypsin  Regular staggered packing of the individual b. Role of α1-AT in the lungs molecules in the fibril c. Emphysema resulting from α1-AT deficiency I. COLLAGEN  Most abundant protein in the body  Long, rigid structure  3 polypeptides form a helix  Dispersed as gel  Extracellular matrix, vitreous humor of the eye  NETWORK-FORMING COLLAGENS  Tight bundle  Types IV and VII  Tendons  Type IV  Parallel fibers that provide great strength  Major part of basement membrane  Stacked  Mechanical support for adjacent cells  Cornea of the eye  Semipermeable filtration barrier (lungs and  To transmit light with a minimum scattering kidney)  At an angle  Form a 3-dimensional mesh rather than fibrils  Bone  FIBRIL-ASSOCIATED COLLAGENS  Resist mechanical shear from any direction  Types IX and XII bind to the surface of fibrils  Component of the extracellular matrix Trans 1 | Raff 1 of 5 BIOCHEMISTRY GLOBULAR PROTEIN B. STRUCTURE OF COLLAGEN  Amino acid sequence  Glycosylation  Rich in proline and glycine  Enzymatic glycosylation of the hydroxyl group of  Proline hydroxylysine  Facilitate formation of helix = ring  kink  Most commonly glucose and galactose  Glycine  Found in every third position of the polypeptide C. BIOSYNTHESIS OF COLLAGEN chain  Precursors are performed in the fibroblasts  Repeating sequence  Osteoblasts of bone  - Gly – X – Y -  Chondroblasts of cartilage  X = proline  Formation of pro-α chains  Y = hydroxyproline (or hydroxylysine)  Prepro-α chains contain specific AA sequence at their N- terminal ends  Acts as SIGNAL that targets the polypeptide being synthesized for secretion from the cell  Facilitate binding of ribosomes to the rER  Direct the passage of the prepro-α chain into the rER lumen  Rapidly cleaved in the rER to yield a pro-α chain  Triple-helical structure  Hydroxylation  Fibrous and elongated  - Gly – X – Y –  AA side chains on its surface  Proline and lysine in the Y position can be hydroxylated  Bond formation between the R-groups of  REQUIREMENTS: neighboring collagen monomers  Hydroxylating enzymes  Results in aggregation  Prolyl hydroxylase  Hydroxyproline and Hydroxylysine  Lysyl hydroxylase  Not present in most proteins  Oxygen  Hydroxylation of proline and lysine  Ferrous form of Iron  Posttranslational modification  Hydroxyproline  Reducing agent (Vitamin C)  Important in stabilizing the triple-helical structure  Ascorbic acid deficiency (SCURVY)  Maximizes interchain hydrogen bond formation  Lack of prolyl and lysyl hydroxylation  Impaired interchain H-bond formation and formation of a stable helix  Collagen fibrils do not crosslink  Decrease tensile strength  often show bruises  subcutaneous extravasation of blood due to capillary fragility  Glycosylation  Modify hydroxylysine with glucose or glucosyl-galactose RAFF 2 of 5 BIOCHEMISTRY GLOBULAR PROTEIN  Assembly and secretion D. DEGRADATION OF COLLAGEN  After hydroxylation and glycosylation  Normal collagens  Pro-α chains form procollagen  Highly stable  Central region flanked by nonhelical AA and  Half-life = as long as several years carboxyterminal extensions called PROPEPTIDES  Dynamic and constantly remodelled in response to growth  Begins with interchain disulfide bond formation or injury of tissue between C-terminal of the pro-α chain  Proteolytic enzymes (COLLAGENASES)  Move to Golgi complex, packaged in secretory  Breakdown of collagen vesicles, fuse with cell membrane releasing it to the  Metalloproteinase family extracellular space  Type I collagen  Extracellular cleavage of procollagen molecules  Breakage point is specific  Procollagen molecules are cleaved by N- and C-  ¾ + ¼ length fragments procollagen peptidases  Further degraded into AA by  Remove terminal propeptidases, releasing triple- PROTEINASES helical tropocollagen molecules E. COLLAGEN DISEASES: Collagenopathies  Formation of collagen fibrils  Ehlers-Danlos syndrome  Tropocollagen associate to form fibrils  Generalized CT disorder  Ordered, overlapping, parallel array  Result from inheritable defects in the metabolism of  Adjacent molecules in staggered pattern fibrillar collagen molecules  Overlapping to approximately ¾ of a another  Deficiency of collagen-processing enzymes molecule  Lysyl hydroxylase, procollagen peptidase  Cross-link formation  Mutation in the AA sequence of Types I, III, or V  Lysyl oxidase  Type I = skin  Act on fibrillar array of collagen  Fragile, stretchy akin and loose joints  Oxidatively deaminate the lysyl or hydroxylysyl  Type III = most clinically important (in blood vessels) residues in neighboring collagen to form  Either degraded of accumulated to high levels in covalent cross-links intracellular compartments  Cu 2+ containing enzyme  Potentially lethal vascular problems  Cytochrome oxidase  Osteogenesis Imperfecta  Dopamine hydroxylase  Brittle Bone syndrome  Superoxide dismutase  Retarded wound healing  X-linked (Menkes disease) – Cu  Humped-back (kyphotic) appearance deficiency  Rotated and twisted spine  Wilson Disease – Cu overload  TYPE I  Osteogenesis imperfect tarda  Decreased α1 and α2 chain production  Present early in infancy  Fractures secondary to minor trauma  Suspected if prenatal ultrasound detects bowing or fractures of long bones  TYPE II  Osteogenesis imperfect congenital  Most severe  Die of pulmonary hypoplasia in utero or during neonatal period  Mutation in pro-α1 and pro-α2 chains of type I collagen  Replacement of Gly by AA with bulky side chains  Prevent formation of the required triple-helix II. ELASTIN  Rubber-like properties  Elastic fibers with elastin and glycoprotein microfibrils  Lungs, walls of large arteries, elastic ligaments  Stretchable  recoil to original shape A. STRUCTURE OF ELASTIN  Insoluble protein polymer  Synthesized from tropoelastin = linear polypeptide (700 AA) RAFF 3 of 5 BIOCHEMISTRY GLOBULAR PROTEIN  Small and nonpolar (glycine, alanine, valine)  Role of α1-AT in the lungs  Also rich in proline and lysine but little hydroxylysine and  Normal hydroxyproline  Alveoli is exposed to low levels of elastase (from  TROPOELASTIN activated and degenerating neutrophils)  Secreted by the cell into the extracellular space  Destroy Elastin in alveoli if unopposed by α1-AT  Interacts with glycoprotein microfibrils  Emphysema  FIBRILLIN  Lung tissue cannot regenerate  Act as scaffold where tropoelastin is deposited  Destruction of CT of alveolar walls  Allysine residue  Emphysema resulting from α1-AT deficiency  Oxidative deamination of lysyl side chain by  α1-antitrypsin mutation lysyl oxidase  single purine base mutation (GAG  AAG)  ELASTIN  position 342 = Glutamate  Lysine  3 allysine side chain + 1 unaltered lysyl side chain from  undergo polymerization in hepatocyte ER same or neighboring polypeptides form a desmosine cross-  decreased α1-AT secretion link  INHERITANCE  Extensively interconnected, rubbery network  Heterozygote  Give tissue its elasticity  Sufficient protection of alveoli from damage  Homozygote  Promote emphysema  α1-antitrypsin methionine  required for the binding of the inhibitor to target the protease  inactivation via inhalational trauma  Smoking  Causes oxidation  methionine residue inactivation  no neutralization of elastase  Elevated rate of lung destruction  poorer survival rate  Treatment  Augmentation therapy  Weekly intravenous α1-AT administration  Mutation in fibrillin-1 protein  Marfan syndrome  CT disorder  impaired structural integrity of skeleton, eye and cardiovascular system  abnormal fibrillin incorporated into microfibrils  BLUE SCLERA  Due to tissue thinning  Patients with OI, EDS or Marfan B. ROLE OF α1-ANTITRYPSIN IN ELASTIN DEGRADATION  α1-antitrypsin  A1AT, α1-antiproteinase  Inhibit proteolytic enzymes that hydrolyze and destroy proteins  Inhibits activity of the trypsin  Synthesized as trypsinogen in the pancreas  90% of the α1-globulin  Synthesized by the liver, some in tissues including monocytes and alveolar macrophages  Physiologic role of inhibiting neutrophil elastase  Degrade elastin of alveolar walls, as well as other structural protein  Prevent local tissue injury RAFF 4 of 5 BIOCHEMISTRY GLOBULAR PROTEIN II. ALPHA-KERATINS  Proteins that form TOUGH FIBERS  Hair, nail, outer epidermal layer  Constituent of intermediate filament  Rich in cysteine  Provide disulfide bond  Insoluble and resistant to stretching RAFF 5 of 5