BIOCHEMISTRY OF SKIN AND CONNECTIVE TISSUES

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1 BIOCHEMISTRY OF SKIN AND CONNECTIVE TISSUES Sri Widia A Jusman Department of Biochemistry & Molecular Biology FMUI 1

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3 SKIN Epidermis - horny layer (keratin-filled dead cells) - granular layer - spinous cells - basal layer (dividing cells) Basal lamina Dermis - collagen - elastic fibers - fibroblast 3

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5 KERATINS most important structural (intermediate filaments) proteins of epithelial tissues - intermediate filament φ 8-10 nm; microfilament φ 6 nm; microtubule φ 23 nm hair, fingernails, horny layer of the skin are formed from keratin cytoskeleton of dead cells contains long stretches of α-helix interrupted by short nonhelical segments 2 types - type I acidic keratins - type II basic keratins 15 variants each heterodimer type I forming a coiled coil with type II 5

6 KERATINS (cont) Contact between 2 α-helices are formed by hydrophobic amino acids side chains on 1 edge of each helix Different keratins expressed in different cell types - basal layer - K14 (type I) & K5 (type II) - spinous & granular layer K10 (type I) & K1 (type II) - single-layered epithelial cells K18,19,20 (type I) & K7, K8 (type II) - hair & nails various other keratin pairs Rich in cysteine, cystine (cysteine-cysteine) 6

7 helices NH2 COOH Non helical domains Non helical sequences Non helical domains Type I keratin COOH NH2 COOH NH2 Type II keratin 7

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9 Abnormalities of keratin structure cause skin diseases Epidermolysis bullosa Dominantly inherited skin blistering disease Caused by point mutation in the gene of K14 or K5 (expressed only in the basal cells of epidermis) Mild mechanical stress damage the dermal-epidermal junction - destroy the basal cell layer but the overlying cells intact base of epidermis fills with extracellular fluid blister forms Epidermolytic hyperkeratosis Dominantly inherited skin disease with scaling, hyperkeratosis, blistering Point mutation in the gene of K1 & K10 (expressed in the spinous and ganular cell layers) 9

10 COLLAGEN Is the most abundant protein in human body Most abundant in connective tissues Connective tissue consist mainly of extracellular matrix The mechanical properties of connective tissue determined by the composition of the extracellular matrix Collagen fiber are required for tensile strength 19 different types of collagen only type I, II, III, V, VI, XI form fibrils Collagen type I and III abundant in dermis 10

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13 Type I collagen Most abundant collagen in the body Amino acids composition - 33 % glycine - 10 % proline % 3-OH-proline Post-translational - 10 % 4-OH-proline modification - 1 % 5-OH-lysine - no tryptophan ( essential amino acid) Contains small amount of carbohydrate Heating convert collagen (insoluble) into gelatin (soluble) 13

14 Collagen fibrils Basic structural unit tropocollagen 3 twisted polypeptides Type I collagen - [α1(i)]2 α2(i) - 2 chains of α1(i) polypetide - 1 chain of α2(i) polypetide 3 helical polypeptides wound around each other in a right-handed triple helix Long ropelike tropocollagen form fibrils by aligning themselves in parallel form a characteristic staggered array 14

15 Collagen fibrils (cont) have >> tensile strength - 1 mm φ collagen fibrils able to carry weight ± 10 kg durable life spans weeks years very resistant to common protease, such as trypsin, pepsin degraded by intracellular collagenase 15

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18 Collagen was modified posttranslationally Polypeptides synthesized by ribosomes on RER called pre-procollagen Signal peptidase removed AA from N-end of pre-procollagen convert into procollagen Intrachain and interchain S-S bonds formed Some Pro and Lys residue become hydroxylated, some of the 5-OH-Lys become glycosylated Triple helix forms in the C N terminal direction Procollagen secreted Propeptide (170 AA at N-end and 220 AA at C-end) removed by extracellular protease, procollagen convert into tropocollagen Tropocollagen assemble into fibrils Crosslinking of lysil residu essential for fibrils strength 18

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22 Collagen metabolism is altered in aging and disease Collagen of old animals / humans more crosslinked than young subjects Hydroxylation of Pro & Lys residu require vitamin C, Cu ion - deficiency of vit C insufficient hydroxylation collagen denatures spontaneously in body temperature hemorrhagic tendency, loosening of teeth, poor wound healing, rupture of scar tissue - Menke s syndrome dietary deficiency of Cu kinky hair, growth retardation 22

23 Genetic defect of collagen structure & biosynthesis Osteogenic imperfecta - brittle bones, frequent fractures - caused by mutation of α1 and α2 gene of type I collagen - gly residue replace by another aa damage the triple helix Ehlers-Danloss syndrome - inherited stretchy skin and loose joints rubber man - mutation of type V collagen gene, defect type I, type III Chondrodysplasias - abn of type II, IX, X, XI - skeletal deformities, dwarfism Epidermolysis bullosa - defect of type VII collagen 23

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25 ELASTIN has specific aa composition - glycine (31 %), alanine (22 %), proline (11 %), OH-proline (1 %), no OH-lysine Synthesized as a soluble monomer tropoelastin Some pro residue hydroxylated to OH-pro After secreted from the cell certain lysyl residues oxidatively deaminated by lysyl oxidase formed desmosine (condensation of 3 lysine-derived aldehyde with unmodified lysine to form tetrafunctional crosslink) Exhibit a variety of random coil conformation permit elastin to stretch and subsequently recoil during its functions 25

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27 Formation of desmosin from lysine 27

28 Cross-link of lysine / lysine derivate forming desmosin in elastin 28

29 MELANIN Dark pigment of skin, hair, iris, retina Protect the skin absorbs uv light - uv light λ nm dangerous damage DNA causing sunburn and cancer A polymeric product heterogenous MW, poorly defined structure 29

30 Eumelanin A black-to- dark brown insoluble material found in human black hair or retina of the eye Highly polymeric crosslinked structure consist of hundreds monomeric units Pheomelanin A yellow-to-reddish-brown alkali soluble material found in red hair or red feathers 30

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32 Curly red lines indicate sites of attachment to extended polymer or proteins 32

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34 MELANIN (cont) Precursor is tyrosine oxidized to DOPA (diohphenylalanine) oxidized to dopaquinone by tyrosinase, Cu2+ dependent Oculocutaneous albinism defect of melanin synthesis due to - lacking tyrosinase or - defect of carrier for transport tyrosine across the melanosome membrane 34

35 PHENYLALANINE PHE hydroxylase TYROSINE TYROSINASE Cu2+ DOPA TYROSINASE Cu2+ DOPAQUINONE Nonenzimatik MELANIN Synthesis of melanin from phenylalanine / tyrosine 35

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