Structural Levels of Proteins and Denaturation

Primary protein

Is a sequence of a chain of amino acids

Secondary protein

Occurs when the sequence of amino acids are linked by hydrogen bonds

The polypeptide chains are folded regularly

Hydrogen bonding causes protein chains to fold and align to produce orderly patterns

The 2 prominent structure of secondary proteins are

• Alpha helix

• Beta pleated sheet

Alpha helix

Single protein chain twisted to resemble a coiled helical spring

Alpha helix is stabilized by

hydrogen bonds between the electropositive nitrogen atom of a peptide linkage and the electronegative carbonyl atom of the 4th amino acid on the amino terminal side of the peptide bond

Left alpha helix

The n-terminal is on the left side

Right alpha helix

The n-terminal is on the right side

Beta pleated sheet

The polypeptide chain is extended into a zigzag arrangement

Polypeptide chains are arranged side by side in the form of pleats/sheets

The adjacent chains can be parallel or antiparallel

What stabilize the beta pleated sheet

Hydrogen bonds are formed between adjacent segments of polypeptide chains

Parallel beta sheet

Antiparallel beta sheet

Tertiary protein

Occurs when certain attractions between alpha helices and pleasted sheets

It is the overall three-dimensional arrangement of all atoms in a protein

Forces involve to stabilize the tertiary and quaternary protein structure

• Hydrophobic interactions (nonpolar groups cluster inside)

• Hydrogen bonds (between polar groups)

• Disulfide bridges (-S-S-) (covalent bonds between cysteine residues)

• Ionic bonds (between charged side chains)

Quaternary protein

Is a protein consisting of more than one amino acid chain

The arrangement of two or more polypeptide chains (subunits) into a functional protein.

• Example:

  • Hemoglobin (contains 4 subunits)

  • Collagen (triple-helix structure)

Denaturation

Is the structural changes in proteins that can cause loss of function due to:

• heat

• extreme pH

• organic solvents

• solutes

Hemoglobin

Transport protein that carries oxygen in the blood

Collagen

• Fibrous protein in connective tissue

• Found in tendons, bone, cartilage, and blood vessels

Ferritin

Protein that stores iron in the liver

Actin and Myosin

Proteins that control muscle contraction

Keratin

fibrous protein in hair, skin, and nail

Myoglobin

Protein that stores oxygen in tissues

Insulin

• Protein hormone synthesized in the pancreas

• Controls blood glucose levels

Types of Denaturation

• Flocculation

• Coagulation

Flocculation

clumping together of the dispersed chain of the denatured protein

this denaturation is reversible

Coagulation

Occurs when the flocculated protein is heated further. The clumped chains become insoluble not only at its isoelectric pH but also over the entire pH range

this denaturation is irreversible