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UNIT I:
Protein Structure and Function
Fibrous Proteins
Overview
• Collagen & elastin are examples of common, wellcharacterized fibrous proteins that serve structural
functions in the body
• E.g., collagen & elastin found as components in skin,
connective tissue, blood vessel walls, & sclera and
cornea of eye.
• Each fibrous protein exhibits special mechanical
properties, resulting from unique structure, obtained by
specific aa’s combined into regular 2º structural
elements.
• In contrast to globular proteins, whose shapes result
from complex interactions b/w 2º, 3º, and sometimes 4º
elements.
II. Collagen
• Is most abundant protein in human body
• A typical molecule is long, rigid, in which 3 polyps (αchains) wound around one another in a rope-like triplehelix.
• Although found throughout the body, their types and
organization dictated by structural role collagen plays in
a particular organ.
– In some tissues, collagen may be dispersed as a gel that give
support to structure, as in extracellular matrix or the vitreous
humor of eye.
– In other tissues, collagen may be bundled in tight, parallel fibers
that provide great strength, as in tendons.
– In cornea, collagen is stacked so as to transmit light with a
minimum of scattering.
– Collagen of bone occurs as fibers arranged at an angle to each
other to resist mechanical shear from any direction
The cornea is made up of many layers of collagen
arranged in a very regular pattern. These layers
of collagen are called the stromal lamellae
The molecular arrangement of
collagen and hydroxyapatite
crystals in compact bone.
(a) Collagen fibers overlap
adjacent fibers as they
repeat every 680 Å. Hole
zones are areas of low
density bone collagen and
overlap zones are areas of
very high density bone
collagen. (b) Hydroxyapatite
crystals are arranged in
layers within each fiber,
resembling overlapping
bricks
Figure 4.1
Triple-stranded helix of collagen.
A. Types of collagen
• Collagen superfamily of proteins include > 20 collagen
types, as well as additional proteins that have collagenlike domains.
• The 3 polyp. α-chains are held together by H-bonds b/w
the chains.
• Variations in aa sequence of α-chains result in structural
components that are ~ the same size (~ 1000 aa’s long),
but slightly different properties.
• These chains are combined to form various types of
collagen found in tissues. E.g., most common collagen,
type I, contains 2 chains α1 and 1 chain α2 (α12α2),
whereas type II collagen contains 3 α1 chains (α13).
• The collagens can be organized into 3 groups, based on
location and function in the body.
Figure 4.2
The most abundant
types of collagen.
Figure 4.3
Collagen fibrils at right have a characteristic banding pattern, reflecting the
regularly staggered packing of the individual collagen molecules in the fibril.
B. Structure of collagen.
1. Amino acid sequence: collagen is rich in Pro & Gly, both
important in formation of triple-stranded helix.
• Pro facilitates formation of helical conformation of each
α-chain because its ring causes “kinks” in peptide chain.
• Gly, smallest aa, is found in every 3rd position of the
polyp chain. It fits into restricted spaces where the 3
chains of the helix come together
• Gly residues are part of a repeating sequence, -Gly-XY-, X: frequently Pro, Y: often hydroxyproline or
hydroxylysine
• Thus, most α-chain can be regarded as a polyp whose
seq can be represented as (-Gly-X-Y-)333.
Figure 4.5
Amino acid sequence of a portion of the a1-chain of collagen. [Note: Hyp is
hydroxyproline and Hyl is hydroxylysine.]
2. Triple helical structure: unlike most globular
proteins that are folded into compact structures,
collagen, a fibrous protein, has an elongated,
triple helical structure that places many of its
aa’s side chains on surface of the triple helical
molecule
- This allows bond formation b/w exposed Rgroups of neighboring collagen monomers 
aggregation into fibers
3. Hydroxyproline & hydroxylysine:
- Collagen contains hyp & hyl, not present in most
other proteins
- These residues result from hydroxylation of
some Pro & Lys residues after incorporation into
polyp chains
- Hydroxylation is an e.g. of post-translational
modification
- Hydroxylation of Pro is important in stabilizing
triple-helical structure of collagen, it maximizes
interchain H-bond formation
Figure 4.6
Hydroxylation of prolyl
residues of pro-α-chains of
collagen by prolyl
hydroxylase.
3. Glycosylation:
- Hydroxyl group of hyl residues of collagen may
be enzymatically glycosylated
- Most commonly, glucose & galactose are
sequentially attached to polyp chain prior to
triple-helix formation
Figure 4.7
Formation of a
collagen fibril.
Figure 4.7
Formation of a collagen fibril. (Continued from
the previous page)
C. Biosynthesis of collagen
• Polyp precursors of collagen molecule are
formed in fibroblasts (or in related osteoblasts of
bone and chondroblasts of cartilage) and
secreted into extracellular matrix.
• After enzymatic modification, mature collagen
monomers aggregate & become cross-linked to
form collagen fibrils
1.
-
-
Formation of pro-α-chains:
Collagen is one of many proteins that normally function
outside of cells
Like most proteins produced for export, newly
synthesized polyp precursors of α-chains contain a
special aa sequence at their N-terminus, acts as a
signal that polyp being synthesized is destined to leave
cell
Signal sequence facilitates binding of ribosomes to
rER, & directs passage of polyp chain into cisternae of
rER
Signal sequence is rapidly cleaved in ER to yield a
precursor of collage = pro- α-chain
2. Hydroxylation
- Pro- α-chains processed by a number of
enzymatic steps within lumen of rER
while polyp is being synthesized
- Pro & Lys residues found in Y-position of
–Gly-X-Y- sequence can be hydroxylated
to form hyp and hyl residues
- Hydroxylation reactions require
molecular oxygen and the reducing
agent vitamin C (ascorbic acid), without
which hydroxylating enzymes, prolyl
hydroxylase & lysyl hydroxylase, are
unable to function.
- In case of ascorbic acid
deficiency (so, lack of prolyl and
lysyl hydroxylation), collagen
fibers cannot be crosslinked 
greatly decreasing tensile
strength of assembled fiber
- One resulting deficiency disease
is scurvy.
- Patients with ascorbic acid
deficiency also often show
bruises on limbs as a result of
subcutaneous extravasation of
blood (capillary fragility)
Figure 4.8. The legs of a
46-year-old man with
scurvy.
3. Glycosylation: some hydroxylysine
residues are modified by glycosylation with
glucose or glucosyl-galactose
4. Assembly and secretion:
- After hydroxylation & glycosylation, pro-αchains form pro-collagen, a precursor of
collagen that has a central region of triple
helix flanked by non-helical amino- and
carboxyl-terminal extensions called
propeptides
- Formation of procollagen begins with formation
of interchain disulfide bonds b/w C-terminal
extensions of pro-α-chains, this brings three αchains into an alignment favorable for helix
formation
- Procollagen molecules translocated to Golgi,
packaged in secretory vesicles  vesicles fuse
with CM  release of procollagen molecules
into extracellular space
5. Extracellular cleavage of procollagen molecules:
- After release, pro-collagen molecules are
cleaved by N- and C-procollagen peptidases,
which remove terminal propeptides, releasing
triple-helical collagen molecules
6. Formation of collagen fibrils:
- Individual collagen molecules spontaneously
associate to form fibrils.
- They form ordered, overlapping, parallel array,
with adjacent collagen molecules arranged in a
staggered pattern, each overlapping its neighbor
by a length ~ three-quarters of a molecule.
7. Cross-link formation:
- Fibrillar array of collagen molecules serves as a
substrate for lysyl oxidase. This extracellular enz
oxidatively deaminates some lysyl and hyl
residues in collagen. Reactive aldehydes that
result (allysine and hydroxyallysine) can
condense with lysyl or hyl residues in neighboring
collagen molecules to form covalent cross-links
- This cross-linking is essential for achieving tensile
strength necessary for proper functioning of
connective tissue. Therefore, any mutation that
interferes with ability of collagen to form crosslinked fibrils almost certainly affects stability of
collagen
III. Elastin
• Elastin is a connective tissue protein with
rubber-like properties.
• Elastic fibers composed of elastin & glycoprotein
microfibrils are found in lungs, walls of large
arteries, & elastic ligaments. They can be
stretched to several times their normal length,
but recoil to their shape when stretching force is
relaxed
A. Structure of elastin
• Insoluble protein polymer synthesized from a
precursor, tropoelastin: a linear polymer ~ 700
aa’s that are primarily small & non-polar (e.g.,
Gly, Ala, Val)
• Elastin is also rich in Pro & Lys, but contains
only little hyp & no hyl
• Tropoelastin is secreted by the cell into
extracellular space. There it interacts with
specific glycoprotein microfibrils, e.g., fibrillin,
which function as scaffold onto which
tropoelastin deposited
• Mutations in fibrillin gene  Marfan’s syndrome
• Some lysyl side chains of tropoelastin polyps
oxidatively deaminated by lysyl oxidase 
allysine residues
• 3 allysyl side chains + 1 unaltered lysyl side
chain from same or neighboring polyps form a
desmosine cross-link
• This produces elastin- an extensively
interconnected, rubbery network that can stretch
and bend in any direction when stressed 
connective tissue elasticity
Figure 4.12. Desmosine cross-link in elastin.
Figure 4.13. Elastin fibers in relaxed and stretched conformations.