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Chemistry of life

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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 disulphide bonds.

Primary structure

The primary structure of a protein is the order of the amino acids joined together to form the polypeptide chain.

3D Structure 1

Secondary structure

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.

3D Structure 2

Tertiary structure

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).
3D Structure 3

Quaternary structure

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 disulphide bonds.

3D Structure 4

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.