Polypeptides(Proteins)

Presented byDevansh GuptaM.Sc. Polymer

ScienceSemester 1

ContentsIntroductionWhat are amino acids?Polypeptide FormationProperties Of Peptide BondDelocalization Of Additional Electron

Pair Of C=O Bond Over Peptide GroupFibrous Proteins

1) Collagen2) Elastin3) Keratin

IntroductionProteins are large, complex biomolecules or

macromolecules consisting of one or more chains of amino acids.

Here these amino acid molecules are linked together with the help of peptide bond.

It perform multiple functions within organisms, including catalyzing metabolic reactions, DNA replication, responding to stimuli and transporting molecules from one location to another.

Short chains of amino acids are called peptides.Proteins are polypeptide molecules that contain

many peptide sub units.

CO

OH

NH

H

DefenseEnzyme

Support

Transportation

Contractile

Regulation

Proteins can be functionally classified.

Hemoglobin

AntibodyProtease

Keratin & Collagen

Actin & Myosin

InsulinCarries O2

Fights Viruses

Degrades Protein

Forms Hair and Nails

Contracts Muscle Fibers

Controls Blood Glucose

Amino Acid

Amino acids are organic molecules. Each Amino acid has an amino group, carboxyl group,

R group and hydrogen attached to alpha (α) carbon. Here it contains both acidic and basic group that’s why

it act as a amphoteric. Crystalline solids with high MP’s. Highly-soluble in water.

Structure Of Amino Acid In Water

How Polypeptides Are Made?

Proteins(Polypeptides) are formed through formation of peptide bonds between amino acids.

An amino group from one residue forms an amide bond with the carboxyl of a second residue.

Here condensation polymerization take place and water molecule is eliminated.

Peptide linkages

Polypeptide Chain

Here in above polypeptide chain the carboxyl group of the amino acid n has formed a peptide bond, C-N, to the amino group of the amino acid n + 1.

Each peptide unit is a planar, rigid group with known bond distances and bond angles.

The peptide group is planar because the additional electron pair of the C=O bond is delocalized over the peptide group such that rotation around the C-N bond is prevented by an energy barrier.

The main-chain part, which is identical in all residues, contains a central a carbon atom attached to an NH group, a C'=O group, and an H atom.

The side chain R, which is different for different residues, is bound to the a carbon atom.

Delocalization Of The Additional Electron Pair Of C=O Bond

CollagenCollagen is a most abundant fibrous

protein with a large and complex structure.

25% or more(up to 35%) of total body protein.

Major component of connective tissue.It provides the structure, firmness and

strength to all of our internal organs. As the age of human being increases,

collagen decreases in the body causing wrinkles and joint weakening.

Structure of Collagen This structure

show them in a monomer form but when they become polymers; Hydrogen bonding takes place, between the Oxygen and the Hydrogen.

Collagen fibres, with the main amino acids.

Bonds which Open upHydrogen Bonding

Between Oxygen and Hydrogen

Collagen is a rod shaped molecule 3000Å long and 15Å in diameter.

Made up of 3 helical polypeptide chains each containing approx. 1000 amino acid residues.

These amino acids are arranged in a repetitious tripeptide sequence, Gly-X-Y, in which X can be any amino acid but is frequently a Proline and Y is frequently a Hydroxyproline or Hydroxylysine.

Hydrogen bonds between the residues stabilise the 3d structure of the tropocollagen.

Covalent bonds form between tropocollagen molecules which stabilise the collagen fibre.

Steric repulsion between proline and hydroproline side chain stabilise the whole helix of collagen.

Functions Of CollagenIt imparts strength, support, shape and

elasticity to the tissues. It provides flexibility, support, and movement

to cartilage. It encases and protects delicate organs like

kidneys and spleen. 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.

Synthesis of Collagen

Collagen is initially synthesized as a larger precursor polypeptide, procollagen.

Then numerous lysyl and prolyl residues of procollagen are hydroxylated by lysyl hydroxylase and prolyl hydroxylase, enzymes that require ascorbic acid as catalyst.

Hydroxylysyl and hydroxyprolyl residues provide additional hydrogen bonding that stabilizes the mature protein.

After that glucosyl and galactosyl transferases attach glucosyl or galactosyl residues to the hydroxyl groups of specific hydroxylysyl residues.

Registration peptides on amino and carboxyl ends of the alpha chain which serves as extensions.

The central portion of the precursor polypeptide then associates with other molecules to form the characteristic triple helix.

This process is accompanied by the removal of the globular amino terminal and carboxyl terminal extensions of the precursor polypeptide by selective proteolysis (Procollagen peptidase/Procollagen aminopeptidase/Procollagen carboxypeptidase).

Certain lysyl residues are modified by lysyl oxidase, a copper-containing protein that converts ε-amino groups to aldehydes.

Then it form a Schiff base with the ε-amino group of an unmodified lysyl residue, which is subsequently reduced to form a C-N single bond.

These covalent bonds cross-link the individual polypeptides and imbue the fiber with exceptional strength and rigidity.

Summary of Collagen Synthesis

Glucosyl & Galactosyl added by glycosyl transfarase & galactosyl transfarase.

Properties Of CollagenIt remains soft and flexible under low stress

and become stiff when stress is increased.Its melting point depends on the animal that it

came from, for example human collagen is 48°C. Collagen from pigs and cows melt at 38°C, and collagen from fish melts under 18°C

It’s boiling point is between 198-228°C depending on the type of collagen.

It has a degradation temperature of 438 °C Its relative molecular mass is 28,000amu.Bond dissociation enthalpy of Hydrogen bond

is −393.7 ± 1.5 kJ/mol.

Applications of CollagenHistory

From the Greek word for glue, kola, the word collagen means "glue producer".

Collagen adhesive was used by Egyptians about 4,000 years ago, and Native Americans used it in bows about 1,500 years ago. The oldest glue in the world, carbon-dated as more than 8,000 years old, was found to be collagen.

If collagen is partially hydrolyzed, the three tropocollagen strands separate into globular, random coils, producing gelatin, which is used in many foods, including flavored gelatin desserts. Besides food, gelatin has been used in pharmaceutical, cosmetic, and photography industries.

Collagens are widely employed in the construction of artificial skin substitutes used in the management of severe burns & beauty treatments.

Collagen is also sold commercially as a joint mobility supplement. This lacks supportive research as the proteins would just be broken down into its base amino acids during digestion, and could go to a variety of places besides the joints depending upon need and DNA orders. 

Collagen is also used in Wound Healing medicines.

Collagen is used as temporary thermoplastic glues in musical instruments like violin and guitar .

ElastinElastin is found in skin and tissues of the

body.It is a protein in connective tissue that

is somewhat elastic in nature and allows many tissues in the body to regain their shape after stretching or contracting.

It is a protein that helps to keep our skin flexible but tight.

 It also helps keep skin smooth as it stretches to accommodate normal activities like flexing a muscle or opening and closing the mouth to talk or eat.

Fiber DistributionLungsElastic ligamentsSkinBladderElastic cartilageBlood vessels

Structure Of ElastinElastin is an cross-linked insoluble

protein which is mainly made up of amino acids like valine, glycine, alanine and proline.

Elastin consists of linear polypeptide tropoelastin subunits whose molecular weight is more than 72,000 amu.

It does not contains hydroxylysine or covalently linked sugar residues & 1% of it is composed of hydroxyproline.

It interconverts between number of conformations, both disordered (on left) and b-spiral (bottom left).

After cross-linking, when elastin is stretched or compressed it becomes unstable and regain it’s disordered conformational structure.

Synthesis Of Elastin

Properties Of ElastinIt shows flexible elastic nature under low stress

and become rigid when stress is increased.It forms in the body till the age of 12-14 years

after that it can’t be formed in the body.Its melting point depends on the animal that it

came from, for example human elastin is 45°C. Elastin from pigs and cows melt at 35°C, and Elastin from fish melts under 15°C.

It’s boiling point is between 155-165°C.It has a degradation temperature of 295°C. Its relative molecular mass is 72,000amu.

Applications Of ElastinUsed for skin-care productsAnti-aging creamsSkin renewingFace treatmentBody lotionUsed as a coating on collagen patches for

injuries.The addition of an active form of elastin to

the skin’s surface has the potential to reduce the requirement for drugs or cosmetic surgery procedures, making it a highly cost effective and safe alternative.

Keratin Keratin is a fibrous protein and the major

component in the skin, hair, nails, horns, hoofs, wool and feathers, the hair is totally made up of keratin associated proteins.

It serves important structural and protective functions, particularly in the epithelium.

It acts both as an external protective protein in the cuticle and as an internal structural protein in the cortex.

Some keratin have also been found to regulate key cellular activities, such as cell growth and protein synthesis.

Structure Of KeratinStructure of keratin was first

determined by Hanukoglu and Fuchs.Fibrous keratin molecule supercoil to

form a very stable helical filament consisting of multiple copies of the keratin monomer.

Keratin monomer consist of amino acids such as glycine, alanine and serine which are responsible for the formation of inter and intra molecular hydrogen bonds.

It also contains large amount of sulphur containing amino acids like cysteine which is required for the disulphide bridges that provides additional strength and rigidity by permanent and thermally stable crosslinking.

According to the structure keratin protein can be divided in to two types.

1) Alpha keratin2) Beta keratin

Alpha keratin, which is found in the baleen plates of whales, horns, skin, hair and the wool are primarily fibrous and helical in structure.

While beta keratin is in harder sheet form and can be found in nails and in the scales and claws of reptiles, their shells and in the feathers, beaks, claws of birds and quills of porcupines.

Properties Of KeratinKeratin is completely insoluble in cold or hot

water.It cannot be attacked by proteolytic enzymes.The length and diameter of keratin fibers

depends on their water content, complete hydration (16%) increses their length by 10 to 12%.

Due to both of the bonds hydrogen bond and disulphide bond keratin fiber bears an outstanding mechanical properties.

It has a degradation temperature of 400°C.Degrades with the evolution of CO2 and Sulphur

dioxide.

Applications Of KeratinBeing rich in reactive chemical groups which

make it more hydrophilic and capable of interacting with incorporating drugs, it is used as a solvent for this type of drugs for controlled drug delivery system.

For the production of biodegradable films and coatings.

To study the in vitro skin permeability for drugs avoiding the use of animal or human skin.

Keratin powder used as an absorbent for wound dressing due to release of keratin derivatives to the wound.

In hair care products.For the treatment for damaged hair .

That’s All