Brent Cornell

    
Amino acids are covalently joined via peptide bonds to form long chains called polypeptides

The order of the amino acid sequence is called the primary structure and determines the way the chain will fold

• Different amino acid sequences will fold into different configurations due to the chemical properties of the variable side chains

Amino acid sequences will commonly fold into two stable configurations, called secondary structures

• Alpha helices occur when the amino acid sequence folds into a coil / spiral arrangement
• Beta-pleated sheets occur when the amino acid sequence adopts a directionally-oriented staggered strand conformation

Both α-helices and β-pleated sheets result from hydrogen bonds forming between non-adjacent amine and carboxyl groups

• Where no secondary structure exists, the polypeptide chain will form a random coil

Secondary Structure – Alpha Helices versus Beta Pleated Sheets

The overall three-dimensional configuration of the protein is referred to as the tertiary structure of the protein

The tertiary structure of a polypeptide chain will be determined by the interactions between the variable side chains

• These interactions may include hydrogen bonds, disulphide bridges, ionic interactions, polar associations, etc.

The affinity or repulsion of side chains will affect the overall shape of the polypeptide chain and are determined by the position of specific amino acids within a sequence

• Hence, the order of the amino acid sequence (primary structure) determines all subsequent levels of protein folding

Protein Folding:  Primary → Secondary → Tertiary

    
Certain proteins possess a fourth level of structural organisation called a quaternary structure

Quaternary structures are found in proteins that consist of more than one polypeptide chain linked together 

Alternatively, proteins may have a quaternary structure if they include inorganic prosthetic groups as part of their structure

Not all proteins will have a quaternary structure – many proteins consist of a single polypeptide chain

Quaternary Structure of a Protein

An example of a protein with a quaternary structure is haemoglobin (O₂ carrying molecule in red blood cells)

• Haemoglobin is composed of four polypeptide chains (two alpha chains and two beta chains)
• It is also composed of iron-containing haeme groups (prosthetic groups responsible for binding oxygen)