CLINICAL ASPECTS OF BIOCHEMISTRY

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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
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X-RAY DIFFRACTION PATTERNS
Myoglobin
Silk fibroin
?
DNA!
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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)
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COLLAGEN FIBRES
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COLLAGEN SECONDARY STRUCTURE
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PART OF COLLAGEN SEQUENCE (a1)
-Gly-Pro-Met-Gly-Pro-Ser-Gly-Pro-Arg- 13
Gly-Leu-Hyp-Gly-Pro-Hyp-Gly-Ala-Hyp- 22
Gly-Pro-Glu-Gly-Pro-Glu-Gly-Pro-Hyp- 31
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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

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SOME COLLAGEN TYPES
Type I tendon and bone
a1(I)2a2 Type II cartilage
a1(II)3 Type III
cardiovascular system a1(III)3
fetal dermis TypeIV
basal membranes a1(IV)3
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Modified amino acids in Collagen
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Tropocollagen a(I) chain structure
polar N-terminal region - (Gly - X - Y)n - polar
C-terminal region
Non-helical
95 of sequence
Non-helical
n 330 X is often Pro Y is often hydroxy Pro
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Carbohydrate attachment in collagen
NH3

CH2
1-2 1 Glucose - galactose
- O - CH a b
CH2
hydroxylysine

CH2 ____________ ________


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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)
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Organization of a collagen fibre
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Collagen denaturation and gel formation
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Thermal stability of Collagen
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Crosslinks in Collagen
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Further crosslinking of collagen (one of
several possible routes)
Histidino-hydroxymerodesmosine
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Crosslinks between tropocollagen molecules
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Conversion of Procollagen to Collagen
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ASSEMBLY OF TROPOCOLLAGEN
N
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Collagen biosynthesis
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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

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COLLAGENASES

• Two types
• 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.

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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