2.2.13 Fibrous Proteins Flashcards
Fibrous Proteins
Fibrous proteins are long strands of polypeptide chains that have cross-linkages due to hydrogen bonds
These proteins have little or no tertiary structure
Due to a large number of hydrophobic R groups, fibrous proteins are insoluble is water
Fibrous proteins have a limited number of amino acids with the sequence usually being highly repetitive
The highly repetitive sequence creates a very organised structures that are strong and this along with their insolubility property, makes fibrous proteins very suitable for structural roles
Examples of Fibrous Proteins
Keratin - makes up hair,nails, horns and feathers (it is a very tough fibrous protein)
Elastin - found in connective tissue, tendons, skin and bone (it can stretch and then return to it original shape)
Collagen - a connective tissue found in skin, tendons and ligaments
Globular vs Fibrous Tertiary Proteins
Shape
- g = roughly circular
- f = long strands
Amino Acid Sequence
- g = irregular and wide range of R groups
- f = repetitive with a limited range of R groups
Function
- g = physiological/functional
- f = structural
Example
- g = haemoglobin, enzymes, insulin, immunoglobulin
- f = collagen, keratin, myosin, actin, fibrin
Solubility
- g = (generally) soluble in water
- f = (generally) insoluble in water
Collagen
Collagen is the most common structural protein found in vertebrates
It provides structural support
In vertebrates it is the component of connective tissue which forms:
- tendons
- cartilage
- ligaments
- bones
- teeth
- skin
- walls of blood vessels
- cornea of the eye
Collagen is an insoluble fibrous protein
Structure of Collagen
Collagen is formed from 3 polypeptide chains closely held together by hydrogen bonds to form a triple helix (known as tropocollagen)
Each polypeptide chain is a helix shape (but not alpha-helix as the chain is not tightly wound) and contains about 1000 amino acids with glycine, proline and hydroxyproline being the most common
In the primary structure of collagen, almost every thirds amino acid is glycine
- this is the smallest amino acid with a R group that contains a single hydrogen atom
Glycine tends to be found on the side of the polypeptide chains, allowing the 3 chains to be arranged closely together forming a tight triple helix structure
Along with hydrogen bonds forming between the 3 chains, therefore are also covalent bonds present
Covalent bonds also form cross-link between groups of amino acids in interacting triple helices when they are arranged parallel to each other
- the cross-links hold the collagen molecules together to form fibrils
The collagen molecules are positioned in the fibrils so that there are staggered ends (this gives the striated effect send in electron micrographs)
When many fibrils are arranged together they form collagen fibres
Collagen fibres are positioned so that they are lines up with the forces they are withstanding
Function of Collagen
Collagen is a flexible structural protein forming connective tissues
The presence of the many hydrogen bonds within the triple helix structure of collagen results in great tensile strength
- this enables collagens to be able to withstand large pulling forces without stretching or breaking
The staggered ends of the collagen molecules within the fibrils provide strength
Collagen is a stable protein due to the high proportion of proline and hydroxyproline amino acids present
- these amino acids increase stability as their R groups repel each other
The length of collagen molecules mean they take too long to dissolve in water (making t insoluble in water)
Collagen vs Haemoglobin
Number of Polypeptide Chains
- c = 3 (triple helix)
- h = 4 (2 alpha-globin and 2 beta-globin)
Outline (Shape)
- c = long, thin
- h = spherical/round
Types of Protein
- c = fibrous
- h = globular
Main Function
- c = structural (connective tissue e.g. tendons, skin)
- h = variable
Prosthetic Group Present
- c = no
- h = yes (haem group)
Solubility
- c = insoluble in water
- h = soluble in water
