Before the advent of recombinant protein expression system, a desired protein either for pharmaceutical or industrial purposes has been isolated from the original source in its native tissue or organism. The protein is intracellularly processed to its native conformation. The material is in limited supply and can be expensive to obtain and maintain. So, cost is the limiting factor in the sale of proteins from their source. A number of systems such as bacteria, yeast, fungi insect cells, mammalian cells have been developed and optimized to produce pharmaceutical and industrial proteins on large-scale. Proteins can be synthesized in vitro, which is a fast process, but this approach is only for small protein because larger peptides become less and less accurate in their synthesis as size increases. In addition, no post-translational processing can be performed.
Bacteria (e.g. E.coli) and yeast are two simple systems in which recombinant DNA techniques can be used for the production of foreign proteins. Bacteria efficiently synthesize and secrete proteins but lack the ability for the expression of eukaryotic genes. Additionally, the recombinant protein can be toxic to the bacterial cell or be degraded by bacterial proteases. Yeast or other fungi produce foreign secreted proteins that can be easily purified from fermentation broths. One of the major drawbacks to these systems is the initial investment because both systems require fermentation equipment. The post-translational processing may not be accurate. Purification of material immediately upon harvest is required due to high volumes of liquid and potential instability of the protein in the fermentation broth. Contaminants from the cultures may also co-purify with the products. Animal cell culture and transgenic animal productions of foreign proteins is currently being explored in industrial laboratories. Mammalian and insect cell cultures are nowadays used for producing many pharmaceutical proteins because of their ability to perform glycosylation and to process the recombinant protein similarly to that of in native host. But these systems are slow and expensive. Transgenic animals have recently emerged as promising systems for producing human proteins in milk because of correct post-translational modification. But this system is also less promising due to slow and problematic scale-up of transgenic animal herds to production size even with the potential promise of new animal breeding technology. Additionally, maintenance of large transgenic animal herds is very expensive. Also, issues regarding disposal of transgenic animal carcasses waste and purification of by-products such as milk solids and liquids remain unsolved. Many therapeutic proteins appear to cross the mammary gland/ blood barrier and appear in the bloodstream resulting in poor animal health or death.
Plant production system has several advantages over other systems. Some of the merits include ease of storage of material, ease of purification from plant material, freedom from animal pathogenic contaminants and ease of producing transgenic plants. Plants containing a gene or genes which have been artificially inserted instead of the plant acquiring them through pollination is known as transgenic plants or genetically modified or GM crops. The inserted gene sequence (known as the transgene) may come form another unrelated plant or from a completely different species. This has created a situation where the whole biological world is now being considered as a 'single gene pool'. Hence, genetic engineering is a specific process in which gene from a species are modified or genes from unrelated species can be introduced into the crop species by transformation methods, followed by regeneration, which is the subsequent selection in tissue culture of transformed cell, under conditions where each cell will express its totipotency and finally, form a new viable plant. There are many methods for genetic transformation, such as Agrobacterium-mediated, particle bombardment etc.
Plant seeds are more advantageous than other systems because the seed is well suited for the storage and preservation of recombinant proteins. Thus, recombinants seed can be stored in conventional grain storage facility and transported using the existing grain transport infrastructure without loss of the recombinant protein. So the end user located at great distances from the sources of production can be benefited. Also, processing facilities would not need to be built adjacent to production fields, lowering capitals costs. As compared to other plant systems, the genetics and scale-up of hybrid maize offer a number of advantages such as high yielding hybrids expressing the recombinant protein of interest can be developed by breeding techniques for recombinant maize similar to non-transgenic maize.
The risk associated with mammalian viral or prion contamination in mammalian cell culture or transgenic animals production is virtually eliminated utilizing transgenic plants. There are no known animal pathogens that infect plants and there are no known plant viruses that infect animals. So a safe pharmaceutical product for human use can be produced. There is no need for capital investment in new and larger fermentation facilities because the production simply becomes a function of growing the crop and handling the yield under the existing agricultural infrastructure.
The production and storage of adequate supplies of raw material is important in maintaining a continuity of supply of the end products. Plant seeds are an excellent storage vehicle of recombinant proteins under room temperature conditions without a significant loss of the protein's activity. This reduces the costs associated with storing the raw material. Thus seeds maximize the efficiency of the purification facilities. So, it is concluded that the process of the production of proteins in seed for genetically enhanced plants will revolutionize the industry with products that are easier to produce, safer and cost less to produce. Thus, use of plants as a recombinant protein expression system will hold great promise for the future.
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