A recombinant protein is a protein produced by a genetically engineered organism that would not normally produce it. The word 'recombinant' is used because the protein is made from recombinant DNA, which refers to the recombining of different pieces of DNA. By inserting foreign DNA into the host organism's genome, engineers can utilize the protein producing functions of the host cells to make a desired product. For example, the gene encoding proinsulin, the precursor to insulin, was inserted into E. coli which then produced a functional version of insulin nearly identical to the human version and superior to costly extracts of animal insulin, which had been the only alternative at the time. Today, recombinant protein production is a standard procedure in many fields of research.
Any of the thousands of known genes can be inserted into one of the standardized vectors and inserted into a standardized strain of E. coli. While this works for many proteins, it does not always work. Genes with repeated sections have a tendency to not express well. Some genes cause the DNA to fold back over itself, inhibiting transcription. Some genes show little or no product for reasons not yet discovered.
One of the major hurdles of recombinant protein production has been the proper post-translational folding and modifications. Most proteins need to fold in a certain way to function. When produced in a foreign cell, some proteins have a tendency to fold differently, impairing their functionality. One cause of different folding is the presence or lack of chaperone proteins to help guide the polypeptides into their proper conformation. Another post-translational modification is the selective glycosylation of certain proteins. Many human proteins must have sugar side groups attached to function. E. coli, the standard host for recombinant protein expression, do not glycosylate their proteins, and other potential hosts will glycosylate differently. These proteins must be expressed in another host. Yeast are sometimes a viable host for glycosylated proteins; if yeast do not work, human cells, the most difficult and expensive to work with, will most likely be able to express and glycosylate the protein properly.