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quaternary structure

In some proteins several polypeptide chains fit together to form a larger molecule. The quaternary structure refers to the way the different polypeptide chains fit together.

tertiary structure

The final 3D structure of a protein. This structure is produced when the secondary structure of the polypeptide chain is folded.

Disulfide bridge

A covalent S-S bond that joins two cysteine amino acids together, also called an SS-bond or disulfide bond.

Proteins: Protein structures

Proteins have a primary, secondary, tertiary and sometimes a quaternary structure, and each aspect of the structure is important for the protein to carry out its functions. The bonds which hold amino acids together are peptide bonds. However the complex three dimensional shapes of proteins which enable them to carry out their functions in the cells and the body are created and held together by hydrogen bonds, ionic bonds and disulfide bonds.

Structure of proteins

The primary, secondary tertiary and quaternary structure of proteins

  • The primary structure of a protein is the order of the amino acids joined together to form the polypeptide chain.
  • The secondary structures of proteins - α-helices and β pleated sheets – are held together by hydrogen bonds between polar molecules in the backbone of the polypeptide chain. Hydrogen bonds are relatively weak but there are many of them.
  • The tertiary structure of a protein is produced when the secondary structure of the polypeptide chain is folded up - for example to form a globular protein such as an enzyme. The folds are held in place by
    • Hydrogen bonds – there are huge numbers of these relatively weak bonds.
    • Ionic bonds – strong bonds form between acid and basic (i.e. has the properties of a base, can accept a proton H+ ion) amine groups of some amino acids in the polypeptide chains.
    • Disulfide bonds - bonds formed between amino acids containing sulfur groups e.g. cysteine or methionine, when they occur close together in polypeptide structures. A covalent bond is formed between the sulfur-containing groups to form a sulfur bridge (also known as a disulfide bridge).
  • In some proteins several polypeptide chains fit together to form a large biologically active molecule e.g. haemoglobin. The quaternary structure refers to the way the different polypeptide chains fit together and are held in place by hydrogen bonds along with some ionic bonds and disulfide bonds.
3D shape of proteins

Hydrogen bonds and sulfur bridges create the complex 3-D shapes of proteins

The structure and functions of proteins are affected by both temperature and pH. Understanding the structure of proteins enables you to understand how temperature and pH have their effect.