To date there are about 3000 known plant species, making a total of about 200 000 individual chemical compounds. Plant metabolites are divided into two categories depending on the function of the compound in the plant's life cycle. Most known to us are the primary compounds. These are essential in the plant's day to day activities as they contribute to the function and anatomy, and their absence may result in hindrance of important growth processes like photosynthesis and respiration. These include amino acids, nucleic acids, sugars, and fatty acids. Compounds that contribute to colour, aroma and anti-herbivory characteristics to plants are defined as secondary compounds. In plant systems, their does not directly affect the plant growth pattern but their presence facilitate plant-environment interaction and thus it is not surprising that they exhibit a lot of diversity within and between different plants. It follows that production of secondary metabolites is control by the environment and there is no uniformity in accumulation in plants of the same species.
Unfortunately, like primary metabolites, mankind has for ages found uses for secondary compounds food preservatives, and as additives pharmaceutical and cosmeceutical products. Of late the demand has been on the rise putting a lot of natural plant used for industrial production at a risk of extinction. Their structural complexity does not make it any easier to develop synthetic analogues.
Luckily, there is a branch of biotechnology called metabolite engineering. What this field entails is essentially looking at the secondary compound biosynthetic pathways at genomic and protein level. It is basically redirection, restructuring and enhancing of metabolic pathways at a genetic level. When the pathways have been elucidated researchers focus on optimizing production of the metabolites by either manipulating the enzyme system, improving product accumulation or increasing substrate availability. Biochemical processes are largely depended on enzyme catalysis and enzyme kinematics has it that increases reaction efficient and increase product turn-over. In metabolite engineering, the most common approach is to increase the enzyme concentration in the biological system by over-expressing the gene coding production of the enzyme of interest. Consequently, secondary metabolite production will be increased until the substrate becomes the limiting factor (production of the substrate can also be improved using the same technique). Metabolite engineering can also be used to initiate production of metabolites in heterologous organisms that is organisms that do not naturally synthesize the compound. A search through literature will result in the discovery of a multitude of reports on utilization of microbes (especially fungi and E. coli) to produce carotenoids, attempts to produce terpenes and the successful production of polyketides of medicinal products, thus making microorganisms drug factories. Reconstruction of biosynthetic processes in microorganisms increases the efficiency of secondary metabolite production without really compromising the conservation status of pharmaceutical plants.
To conclude, metabolite engineering can be used to improve metabolite production in plants and allow for heterologous synthesis in microbes that can be used in industrial fermentation chambers. The ripple effect of such a system will be a constant supply of high quality metabolites and a reduction in harvest pressure on the natural stands. On this note we can speculate that metabolite engineering is a vital tool in conservation of medicinal plants.
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