Recombinant Proteins

Recombinant proteins are a new combination of genes that forms DNA. Recombinant DNA technology allows for the production of wild type and modified human and mammalian proteins at bulk quantities. Recombinant proteins are made from cloned DNA sequences which usually encode an enzyme or protein with known function

Recombinant proteins are made through genetic engineering, also called gene splicing or recombinant DNA technology. By putting human, animal or plant genes into the genetic material of bacteria, mammalian or yeast cells, these microorganisms can be used as factories or producers to make proteins for medical, academic and research uses.

A vector is simply a tool for manipulating DNA and can be viewed as a "transport vehicle" for the production of proteins from specific DNA sequences cloned into them. Purification and expression of a protein can sometimes be quite complicated & time-consuming, therefore an additional tag is used in addition to the specific DNA sequence which will facilitate the purification & expresion of the recombinant protein.

Recombinant Proteins are proteins that their DNA that has been created artificially. DNA from 2 or more sources which is incorporated into a single recombinant molecule. The DNA is first treated with restriction endonuclease enzyme which the ends of the cut have an overhanging piece of single-stranded DNA. These are called "sticky ends" because they are able to base pair with any DNA molecule containing the complementary sticky end. DNA ligase covalently links the two strands into 1 recombinant DNA molecule.

Recombinant DNA molecule must be replicated many times to provide material for analysis & sequencing. Producing many identical copies of the same recombinant DNA molecule is called cloning. Cloning is done in vitro, by a process called the polymerase chain reaction (PCR). Cloning in vivo can be done in unicellular microbessuch as E. coli, unicellular eukaryotes like yeast and in mammalian cells grown in tissue culture.

Recombinant DNA must be taken up by the cell in a form in which it can be replicated and expressed. This is achieved by incorporating the DNA in a vector. A number of viruses (both bacterial and of mammalian cells) can serve as vectors.

Recombinant DNA is also sometimes referred to as chimera. When combining two or more different strands of DNA.There are 3 different methods by which Recombinant DNA is made. 1. Transformation, 2. Phage-Transfection 3.Yeast, Plant & Mammalian Transformation. When using the method of transformation one needs to select a piece of DNA to be inserted into a vector, cut a piece of DNA with a restriction enzyme and ligate the DNA insert into the vector with DNA Ligase. The insert contains a selectable marker which allows for identification of recombinant molecules. An antibiotic marker is used in order to cause death for a host cell which does not contain the vector when exposed to a certain antibiotic.

Trasnformation is the insertion of the vector into the host cell. The host cells are prepared to take up the foreign DNA. Selectable markers are used for antibiotic resistance, color changes, or any other characteristic which can distinguish transformed hosts from untransformed hosts. Yeast, Plant & Mammalian Transformation is done by micro-injecting the DNA into the nucleus of the cell being transformed. Phage-Transfection process, is equivalent to transformation except for the fact that phage lambda or MI3 is used instead of bacteria.

These phages produce plaques which contain recombinant proteins which can be easily distinguished from the non-recombinant proteins by various selection methods.

Significant amounts of recombinant protein are produced by the host only when expression genes are added. The Protein’s expression depends on the genes which surround the DNA of interest, this collection of genes act as signals which provide instructions for the transcription and translation of the DNA of interest by the cell. These signals include the promoter, ribosome binding site, and terminator.

The recombinant DNA is inserted into expression vectors which contain the promoter, ribosome binding site, and terminator.

In prokaryotic systems, the promoter, ribosome binding site, and terminator have to be from the same host since the bacteria is unlikely to understand the signals of human promoters and terminators. The designated gene must not contain human introns since the bacteria does not recognize it and this results in premature termination, and the recombinant protein may not be processed correctly, be folded correctly, or may even be degraded.

The peptide sequence can be added as an extension at the N-terminal. Researchers can select the specific purification system which they would like to use. The unique vectors available contain several features needed for the production of bulk quantities of the target protein. The peptide sequence is usually placed in the vector so that it is designed to be a point of attack for a specific protease. Thus, after the recombinant protein is expressed and extracted from bacteria, specific peptide extension can be used to purify the protein and subsequently removed from the target protein to generate a nearly natural sequence on the final product.

6 or more consistent Histidine residues act as a metal binding site for recombinant protein purification and expression. The hexa-His sequence is called a His-Tag sequence which can be placed on the N-terminal of a target protein by using vectors from various commercial molecular biology companies. The His-Tag contains a cleavage site for a specific protease. His-Tag recombinant proteins are purified by Metal Chelate Affinity Chromatography such as nickel ion columns that are used as the heavy metal ion and the His-Tag protein is eluted from the metal-chelate column with Histidine or imidazole. Then the purified His-Tag protein is treated with the specific protease to cleave off the His-Tag or not if the tag doesn’t affect the active site of the protein.

Proteins have metal binding sites which can be used for the purification of recombinant and natural proteins. This type of purification is rather simple when using a gel bead which is covalently modified so that it displays a chelator group for binding a heavy metal ion like Ni2+ or Zn2+. The chelating group on the gel bead contains a small amount of the ligands needed to hold the metal ion. So when the protein’s metal binding site finds the heavy metal, it will bind by providing the ligands from its metal binding site to attach to the metal ion displayed on the chelator location of the gel bead. This purification method is quite identical to affinity chromatography when purifying metal-binding class of proteins.