While nucleic acids are the physical support of the genetic information, proteins encoded in the nucleotide sequences are the actual effectors of the metabolism.
Proteins are involved in most of the biological aspects of the cellular life:
1. Cellular organization, for example as constituent of cell wall, cell membranes or cytoskeletons
2. Biosynthesis and catabolism of other biomolecules: carbohydrate, lipids and other biopolymers (e.g. lignins)
3. Post-translational modifications (e.g. protease maturation, glycosylation, phosphorylation…) of other proteins
4. Recognition of other biomolecules for intra- and inter-cellular communication
5. Maintenance (replication and repair) and expression (transcription) of DNA molecules.
Each protein has a specific function in highly regulated networks. Hence cellular life is the result of the finely tuned expression of hundreds to thousand proteins.
Since the biological function of a given protein is highly dependent on its molecular structure, altering this structure will affect its function, by nature, by intensity (yield of activity) or both.
Protein engineering is the manipulation of the structures of proteins so as to produce desired properties, or the synthesis of proteins with particular structures, for fundamental research or industrial application.
So how to modify proteins in living cells? One subtle way is to make the cell biosynthesized it differently.
Biosynthesis of proteins relies on the translation of nucleic acids. Simply put, Sequences of nucleotides are translated in sequences of amino acids (the chemical unit of proteins), using the genetic code. Each amino acid is encoded by a triplet of nucleotide, called codon. Therefore, modifications of a coding nucleotide sequence will translate into modifications of amino-acid sequence, and consequently to modification of protein properties.
Since genetic engineering grants possibility to manipulate accurately artificial nucleotide sequences, it is a perfect technology to achieve protein engineering.