KHULNA UNIVERSITY OF ENGINEERING & TECHNOLOGY
DEPARTMENT OF LEATHER ENGINEERING
LECTURE NOTE
ON
COLLAGEN
COURSE NO: LE 2205
COURSE TITLE: LEATHER AND POLYMER CHEMISTRY
PREPARED BY:
RAJAN KUMAR RAHA
ASSISTANT PROFESSOR
DPT. OF LE, KUET
Collagen:
Derived from Greek word “kolla” meaning Glue Producer”
Any of a group of fibrous proteins that occur in vertebrates as the chief constituent of connective tissue
fibrils and in bones and yield gelatin and glue upon boiling with water are called collagens.
Collagen is the most abundant protein in the human body and is the substance that holds the whole body
together. It is found in the bones, muscles, skin and tendons, where it forms a scaffold to provide strength
and structure
Characteristics of the fibrous proteins
Most of the polypeptide chain is parallel to a single axis
Secondary structure is simple one type only
Fibrous proteins are often mechanically strong & highly cross linked
Fibrous proteins are usually insoluble in water
They usually play a structural role
Extended protein structure
Functions in structure of the body or cell (tendons, bones, muscle, ligaments, hair, skin)
General properties of collagen:
Collagen is a generic name for a family at least 28 distinct types, each serving different functions
in animals, importantly as connective tissue.
Collagens are formed by α-amino acids and β-amino acids.
Hydroxyproline is almost uniquely present in collagen compared to other proteins and, therefore,
offers the basis of measuring the collagen content in any skin or skin derivatives.
Tryptophan is absent, therefore making collagen deficient as a foodstuff.
It is most abundant Fibrous protein (structural protein) in vertebrates
25% or more(up to 35%) of total body protein
Major component of connective tissue
Provides an extracellular framework for Strength & Flexibility
30 distinct types Polypeptide chains (each encoded by separate gene)
Distribution of collagen varies in different tissues.
bones tendons skin liver
4%
70%
90%
85%
Functions of Collagen:
It imparts strength, support, shape and elasticity to the tissues.
It accounts for 6% of the weight of strong, tendinous muscles
It provides flexibility, support, and movement to cartilage.
It encases and protects delicate organs like kidneys and spleen.
It fills the sclera of the eye in crystalline form.
Teeth (dentin) are made by adding mineral crystals to collagen.
Collagen contributes to proper alignment of cells for cell proliferation and differentiation.
When exposed in damaged blood vessels, it initiates thrombus formation
Types of collagen in skin structure:
Type 1 collagen
Basic structural unit is a triple helix molecule
Molecules packed together side by side
Cross linking gives the strength
Found in the supporting element of high tensile strength
Type III collagen
Also called reticulin
Differs from type 1 in amino acid composition
Forms fine fiber
Can form mixed fiber with type 1
It is a major component of grain enamel
Type Iv collagen
Found in the basement membrane
Form a structure between epidermis and grain
Network structure
Attacked by enzyme dispase, helps in unhairing
Type VII collagen
Not fiber forming collagen
Simple dimeric type of structure
It acts as a link between type Iv collagen and grain layer
Fig: Collagen distribution in skin structure
Amino acids in skin collagen and their importance in leather production:
Read from Tanning chemistry, the science of leather by Anthony D Covington (Page: 3)
Modified amino acids in collagen:
Hydroxylated residues found in collagen
Biosynthesis of collagen:
STEP 1: Synthesis of a-chains of pre-procollagen by DNA transcription in nucleus followed by mRNA
translation on ribosomes and its transfer to endoplasmic reticulum.
STEP 2: Hydroxylation of proline residues to obtain hydroxyproline (an aminoacid unique to collagen).
A reaction that substitutes a hydroxyl group, OH, for a hydrogen atom, H, in the proline
Hydroxylation is catalyzed by the enzyme prolyl-4-hydroxylase
Vitamin C and a-keto glutarate are essential for enzyme action
STEP 3: Hydroxylation of lysine residues to obtain hydroxylysine
Hydroxylysine is needed to permit the cross-linking of the triple helices into the fibers
The enzyme peptidyl proline hydroxylase is essential
STEP 4: Glycosylation of some hydroxlysine residues
Glucose and Galactose are added by enzymes Galactosyl transferase and Glycosyl transferase
STEP 5: Assembly of the three alpha chains to from procollagen
Formation of disulphide bonds between parts of the polypeptide chains known as registration
peptides at the C terminal
Three chains associate, align and the triple helix forms in a zipper-fashion giving procollagen.
STEP 6: Secretion of procollagen molecules by exocytosis into the extra cellular space.
STEP 7: Cleavage of registration peptides in the extra cellular space, by procollagen peptidases.
The resulting molecule is collagen.
STEP 8: Self-assembly or polymerization of collagen molecules forms collagen fibrils.
STEP 9: Cross-linkage between adjacent collagen molecules that stabilizes the fibrils.
Types of crosslinks based on the number of molecules involved
Bivalent (reducible): linking the N or C terminal (i.e. telopeptides) of one molecule to the helical
region on another
Trivalent (stable or mature crosslink): linking the N or C terminals of two molecules to the helical
region of the third
Formation of crosslink between Lys, His, and hydroxylysin residues in collagen molecule:
Aldol condensation cross-links in collagen
Crosslink between collagen molecules (CM)
Bivalent crosslink
Trivalent crosslink
Structure of Collagen:
The basic structural unit of a collagen molecule is a triple helix. Triple helical structure may occur
throughout the molecule or only in a part of it.
Structure Of type I mature Collagen:
Triple helical structure occurs throughout the molecule.
This triple helix is composed of 3 polypeptide chains twisted around each other.
Which generally consists of two identical chains (α1) and an additional chain that differs slightly
in its chemical composition (α2)
Each polypeptide chain is in turn a left hander helix with 3 amino acids per turn totally containing
approximately 1000 amino acids per chain.
Each alpha chain has an unusual abundance of 3 amino-acids glycine, proline and
hydroxyproline.
Glycine occurs at every third position in the amino acid sequence which can be represented as
(Gly-X-Y)n.
X and Y are other amino acids of which proline and hydroxyproline occupy 100 positions each.
Glycine occupies the crowded center of the triple helix as it has a small side chain [H atom]
where as Hydroxyproline and proline point outwards imparting rigidity to the triple helix.
The alpha chains are wound around each other in a right handed super helix to form a rod like
molecule 1.4nm wide and 300nm long
The triple helices are stabilized by Hydrogen bonds, covalent cross-links, electrostatic and
hydrophobic interactions and van der waals forces.
The covalent cross links within and between the helices are formed by copper dependent enzyme
lysyl oxidase between the lysine and hydroxylysine residues.
These triple helical molecules pack together side by side to form elongated fibrils.
Fibrils are displaced longitudinally from each other by 67 nm [one quarter of its length] to form a
quarter staggered arrangement.
Fibrils bundle up to form fibers making up tissues.
Fig: schematic representation of collagen structure (1)
Fig: schematic representation of collagen structure (2)
What makes collagen a strong tensile protein?
The triple helix secondary structure.
The locked peptide structure due to the presence of β-amino acids
The assembly of tropocollagen subunits into fibers.
The chemical cross-linking between fibrils to form fibers.
Isoelectric Point of collagen:
Read from Tanning chemistry, the science of leather by Anthony D Covington (Page: 8-9)
Role of water in the collagen structure:
Read from Tanning chemistry, the science of leather by Anthony D Covington (Page: 12)
Hydrothermal stability of collagen:
Read from Tanning chemistry, the science of leather by Anthony D Covington (Page: 23-26)
Natural degradation of collagen:
Break down of the collagen matrix element is a key component of any normal tissue that is undergoing
morphogenesis and growth. But it is vital that this process is kept under rigid control.
- - - - - - (EQUILIBRIUM BETWEEN DEGRADATION & SYNTHESIS) - - - -
Although several enzymes are involved in the destruction of matrix components collage breakdown is
mediated primarily by the COLLAGENASES (Type of MMP) these are specialized enzymes that have
evolved specifically to hydrolyze collagens, because their triple helical collagen structure is resistant to
most common proteinases.
The collagenases belong to a family of enzymes called Matrix Metallo Proteinase (MMPs) that consists of
at least 13 members with closely related domain structures and discrete functions. MMPs are usually
secreted by the connective tissue cells (predominantly fibroblasts) but are also produced by some
leucocytes (PMNs, Macrophages).
Mechanism of action:
Extracellular degradation of collagen fibers is mediated by MMPs and can be thought of as a
multistep process. Collagen degradation pathway is a stepwise process. Activation of collagenase
(A) initiates degradation of collagen by collagenase (B). Once the initial break is made in the
collagen fibril by collagenase, gelatinase activity further cleaves collagen into smaller peptides
(C).
Fig: Collagen degradation pathway
