Proteins are polymers of the bifunctional monomer, amino acids.

The amino group from one amino acid attached to the carboxylic group of the adjacent amino acid, the resulting link between them is an amide link called (peptide bond).

The resulting structure is called (Dipeptide). Similarly we can have Tri, Tetra, Penta …

Peptides. Many amino acids joined in this fashion is called

(Polypeptide). In this reaction, water is released. In a reverse

reaction, the peptide bond can be cleaved by water (hydrolysis).

Peptide bonds can be hydrolyzed by boiling with strong acids and bases, and by certain enzymes (e.g. trypsin, chymotrypsin).

O O ║ ║ H2N C ------ HN C OH

C C

R1 R2

Peptide bond

A peptide is named from the free amine (NH3

+) using -yl endings for the names of the amino acids.

The last amino acid with the free carboxyl group (COO-) uses its amino acid name.

Example:

Glycine + Alanine Glycylalanine + H2O

Formation of peptide bond

A polypeptide containing 50 or more amino acids is called a protein.

The primary structure of a protein is the sequence of amino acids in the peptide chain.

Even a slight change in primary structure can affect a protein’s conformation and ability to function.

Insulin: Was the first protein to have its primary structure determined.

Amino acid sequence of Insulin (51 amino acids)

Alpha Helix The secondary structures of proteins indicate the

arrangement of the polypeptide chains in space. The alpha helix is a three-dimensional arrangement of the

polypeptide chain that gives a shape like a coiled telephone cord.

The coiled shape of the alpha helix is held in place by hydrogen bonds between the amide groups and the carbonyl groups of the a.as. along the chain.

Pleated Sheet Holds proteins in a parallel arrangement with hydrogen

bonds. Has R groups that extend above and below the sheet. Is typical of fibrous proteins such as silk.Triple Helix Consists of three alpha helix chains. Contains large amounts glycine, proline, hydroxy proline and

hydroxylysine that contain –OH groups for hydrogen bonding.

Is found in collagen, connective tissue, skin, tendons, and cartilage.

Gives a specific overall shape to a protein.

Involves interactions and cross links between different parts of the peptide chain.

Is stabilized byHydrophobic and hydrophilic

interactions Salt bridgesHydrogen bondsDisulfide bonds

The quaternary structure contains two or more tertiary subunits.

Hemoglobin contains two alpha chains and two beta chains.

The heme group in each subunit picks up oxygen for transport in the blood to the tissues.

Qaternary structure of the multisubunit protein (Hb)

Levels of structure in proteins

Fibrous proteins have high -helix or -sheet content. Most are structural proteins.

Examples include:Collagen:• Principal component of connective tissue (tendons,

cartilage, bones, teeth).• Proline content is unusually high• Proline and Hydroxy Proline together make 30% of

residues.Elastin:• Abundant in ligaments, lungs, artery walls, skin.• Provides tissues with ability to stretch in all directions

without tearing.Keratin: Found in hair, fingernails, claws, horns and beaks.

Water-soluble Most proteins which occur in the plasma

are of globular nature. they are approximately spherical in

shape, or consist of several different lobes (domains).

Hemoglobin is an example of a tetramer Globular protein.

Globular proteins:Globular proteins:A large number of atoms rolled into relative small volumes through A large number of atoms rolled into relative small volumes through foldingfolding

Keratin Structure

Is the pH-value of a solution at which the total net charge of a protein equals zero.

The charge is positive below pI,

while the charge is negative above pI

Splits the peptide bonds to give smaller peptides and amino acids.

Occurs in the digestion of proteins. Occurs in cells when amino acids are needed

to synthesize new proteins and repair tissues.

Denaturation involves the disruption of bonds in the secondary, tertiary and quaternary protein structures.

Denaturation of protein will not affect the primary structure.

Denatured proteins are functionally inactive. Heat and organic compounds break apart H

bonds and disrupt hydrophobic interactions. Acids and bases break H bonds between polar

R groups and disrupt ionic bonds. Heavy metal ions react with S-S bonds to form

solids. Agitation such as whipping stretches chains

until bonds break.