Primary Structure of Proteins

Shape of Proteins

Protein conformation determines the function of the protein

4 levels of protein structures

1. Primary

2. Secondary

3. Tertiary

4. Quaternary

Primary Structure

Sequence of amino acid residue joined by peptide bonds

Primary structure of Lysozyme

• 129 amino acid residues long 

• Antibacterial enzyme found in tears and saliva 

• N- terminus

• C- terminus

• 3 letter code

• 1 letter code

Essential amino acids

Cannot be synthesized in vivo so must be obtained from the diet

Non-essential amino acids

Can be synthesized in vivo

Secondary structure

results when a polypeptide coils or folds

2 types of secondary structure

• a (alpha) helix

• B (beta) pleated sheet

a (alpha) helix

• hydrogen bonding occurs between the C=O of one amino acid residue and he N-H of another

• hydrogen bonding between every fourth amino acid residue 

  • holds spiral shape of an ‘a’ helix

B pleated sheet

• B pleated sheet polypeptides turn back upon themselves

• Hydrogen bonding occurs between extended lengths 

Fibrous proteins

are structural proteins

• with a helices and/or B pleated sheets that hydrogen bond to each other

Tertiary structure

• is the overall three-dimensional shape of a polypeptide

• results from interactions between amino acid residues and R groups

• when a helices and B pleated sheets fold and interact with one another

Tertiary structure 

• strong disulfide linkages maintain the tertiary shape;

  • cysteine - cysteine (SH group)

• hydrogen bonds

• ionic bonds

• covalent bonds also contribute

• hydrophobic interactions

Quaternary structure 

• when two or more polypeptide chains join in aggregates

• proteins contain more than one polypeptide chain

Quaternary structure 

• such as in the protein molecule haemoglobin and insulin

• often quaternary proteins are complexed with a different molecule, often a metal

• haemoglobin contains iron