Proteins

Amino acids, peptide bonds, primary, secondary, tertiary and quaternary structures

Proteins

Proteins are either fibrous or globular in structure

Fibrous proteins, such as keratin and collagen, are structural build muscles, hair, nails, ligaments, tendons, eyeballs, connective tissue etc.

Globular proteins usually metabolic and help carry out chemical reactions

Enzymes are globular proteins, as well as proteins embedded in cell membranes

Amino Acids and Peptides

All proteins begin as strings of amino acids, bonded together by peptide linkages

This is another example of dehydration synthesis

The bond forms between the amino group and the carboxyl group of adjacent amino acids

20 Amino Acids

Orange – non-polar Green – polar Purple – ionic and acidic Blue – ionic and basic

Frederick Sanger 1918-Portraits from the National Portrait Gallery in London

The First Amino Acid Sequence

Sanger wrote in his 1952 review: It has frequently been suggested that proteins may not be

pure chemical entities but may consist of mixtures of closely related substances with no absolute unique structure. The chemical results obtained so far suggest that this is not the case, and that a protein is really a single chemical substance, each molecule of one protein being identical to every other molecule of the same pure protein.

He used HCL to hydrolyze insulin into small peptides He then used a combination of chromatography and

electrophoresis to separate the small peptides and identify their chemical structure

He was the first person to think of proteins as stable chemicals and one the first of two Nobel prizes for this discovery

Polypeptides – Primary Structure

Amino acids form long chains called polypeptides There are 20 amino acids and we need to consume all 20 at once to

build proteins (8 are essential from the diet and 12 are manufactured) Any unused protein that we consume is deaminated (amino group is

removed) and converted into a lipid The peptide is considered to be the primary structure of a protein –

the sequence of amino acids and a non-functional chain

Secondary Structure α-helix

This is one possible secondary structure, the alpha helix (α-helix)

The polypeptide coils and is held in place with hydrogen bonds between the NH and C=O of the peptide bond

Seconday Structure β pleated sheet

Polypeptides can also form pleated sheets (p. 44)

Hydrogen bonds form between adjacent polypeptides at the NH and C=O

The secondary structure is dependent on the initial primary structure

the NH and C=O groups must line up correctly to form the hydrogen bonds

The alpha helix in keratin

Many alpha helicies bind together to form fibrils which build up the structure of hair

Tertiary Structure

Once the secondary structures are complete, proteins fold into 3 dimensional shapes based on intermolecular interactions, ionic bonds and covalent bonds defined by the primary structure

Covalent bonds between sulfhydryl groups form disulfide bridges

Hydrogen bonds, dipole-dipole interactions and hydrophobic interactions produce characteristic shapes

Ionic bonds form between positive and negative charged functional groups

Quartinary Structure

Multiple polypeptide chains can come together to form fully functional proteins, such as haemoglobin