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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
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