Transcript collagen
CLINICAL ASPECTS OF BIOCHEMISTRY PROTEINS AND DISEASE - LECTURES 1 & 2 MIKE WALLIS STRUCTURAL/FIBROUS PROTEINS Two main groups (a) keratin, myosin, fibrinogen - mainly alpha helix (b) collagen, elastin, resilin, silk fibroin - not alpha helix X-RAY DIFFRACTION PATTERNS Myoglobin Silk fibroin ? DNA! COLLAGEN Major component of connective tissue: tendons, bone, cartilage etc ~25% of body protein in vertebrates Relatively little in insects or protozoa Extracellular - part of the extracellular matrix Fibres with characteristic striations; strong but not elastic At least 25 different types (not all form fibres) COLLAGEN FIBRES COLLAGEN SECONDARY STRUCTURE PART OF COLLAGEN SEQUENCE (a1) -Gly-Pro-Met-Gly-Pro-Ser-Gly-Pro-Arg13 Gly-Leu-Hyp-Gly-Pro-Hyp-Gly-Ala-Hyp22 Gly-Pro-Glu-Gly-Pro-Glu-Gly-Pro-Hyp31 TROPOCOLLAGEN • Soluble form of collagen; the 'subunit' of a collagen fibre • Yields increased by extracting from lathyrogen-treated animals • Lathyrogens (e.g. b-aminopropionitrile, H2N.CH2,CH2.CN) cause lathyrism in cattle etc. • Tropocollagen has Mr of ~300,000; dimensions 3000Å x 14Å • Tropocollagen contains 3 polypeptide chains, each of ~1000 aas SOME COLLAGEN TYPES Type I tendon and bone [a1(I)]2a2 Type II cartilage [a1(II)]3 Type III cardiovascular system fetal dermis [a1(III)]3 TypeIV basal membranes [a1(IV)]3 Modified amino acids in Collagen Tropocollagen a(I) chain structure polar N-terminal region - (Gly - X - Y)n - polar C-terminal region Non-helical n = ~330 X is often Pro Y is often hydroxy Pro ~ 95% of sequence Non-helical Carbohydrate attachment in collagen NH3+ | CH2 1-2 1 | Glucose - galactose - O - CH a b | CH2 hydroxylysine | CH2 ____________ |________ COLLAGEN DISEASES POINT MUTATIONS AND DELETIONS IN TYPE I COLLAGEN LEAD TO OSTEOGENESIS IMPERFECTA (OI) Especially Gly X mutations Dominant, variable severity, usually lethal in homozygote ~70% of OI is due to Type I collagen defects ~1100 other mutations of 6 types of collagen cause human diseases, including: Chondrodysplasias Some osteoporosis and osteoarthritis Some kidney disease and vascular disease (transgenic models have been developed for many of these diseases) Organization of a collagen fibre Collagen denaturation and gel formation Thermal stability of Collagen Collagen source Pro + Hypro (%) Tm Body temp Calf skin 23.3 39 37 Shark skin 19.1 29 24-28 Cod skin 15.5 16 10-14 Crosslinks in Collagen Further crosslinking of collagen (one of several possible routes) Histidino-hydroxymerodesmosine Crosslinks between tropocollagen molecules Conversion of Procollagen to Collagen ASSEMBLY OF TROPOCOLLAGEN CHAIN SELECTION REGISTRATION NUCLEATION PROPAGATION N Collagen biosynthesis FIBROBLAST 1. 2. 3. Secretion Procollagen synthesis Hydroxylation & glycosylation PROCOLLAGEN 4. Hydrolytic processing TROPOCOLLAGEN 5. Self assembly 6. Cross linking COLLAGEN FIBRE MATURE FIBRE HYDROXYLATION OF PROLINE AND LYSINE • Requires Fe2+, O2, ascorbic acid (vitamin C) • 2 microsomal enzymes, prolylhydroxylase and lysylhydroxylase • Hydroxylation of Pro in position Y: Gly - X - Y - Gly - X - Y • Lack of vitamin C leads to scurvy - many symptoms are due to defective collagen hydroxylation COLLAGENASES Two types: 1. E.g. Clostridium histolyticum (causes gas gangrene) X - Gly - Pro - Y 2. Tissue collagenase "Remodelling"/growth of vertebrate tissues. Very specific - cleaves at residue ~750 only. Tm of collagen important. Important for metastatic invasion. COLLAGEN DEFECTS 1. Mutations in collagen genes - Osteogenesis imperfecta etc. 2. Lack of vitamin C - scurvy 3. Defective procollagen conversion: Dermatosparaxis (cattle) Ehlers-Danlos syndrome (human) 4. Defective cross-linking - lathyrism 5. Too much cross-linking - ageing of skin etc 6. Too much collagen - fibrosis