Introduction to Metabolic Engineering in Menthol Production
Menthol, a valuable essential oil component, is biosynthesized through a complex pathway in peppermint plants. Enhancing its yield involves manipulating enzymatic steps via genetic engineering. For a broader understanding of strategies in this field, see Menthol Biosynthesis and Genetic Enhancement in Peppermint Plants.
Key Enzymes Targeted in Metabolic Engineering
- Limonene Synthase (LS): Overexpressed to increase limonene production due to its relatively low catalytic efficiency, considered a potential rate-limiting enzyme. Overexpression resulted only in a moderate increase in menthol precursors.
- Limonene-3-Hydroxylase (L3H): Catalyzes conversion of limonene to trans-isopiperitenol. Due to its low catalytic efficiency, overexpression was attempted but did not enhance essential oil yield.
- Menthofuran Synthase (MFS): Responsible for menthofuran formation, a side pathway. Antisense suppression of MFS reduced menthofuran levels but interestingly also reduced pulegone content, likely due to increased flux toward menthol production.
Combined Genetic Strategies
- Co-expression of 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR), an early enzyme in the MEP pathway, along with antisense suppression of MFS, successfully redirected metabolic flux toward menthol, increasing essential oil yield from 61% to 78% compared to wild-type plants.
Empirical Approaches
- Application of plant growth regulators and environmental manipulation improved both essential oil content and composition in peppermint, demonstrating alternative or complementary methods to genetic engineering.
Current Progress and Challenges
- Various genes (e.g., GPPS, LS, L3H) have been targeted with mixed success.
- RNA interference and antisense approaches proved effective in redirecting pathway flux.
- Overexpression alone often yielded limited improvements due to pathway complexity and regulatory mechanisms.
Future Potential and Strategies
- Employing tissue-specific promoters and promoter stacking for precise gene expression control.
- Using double terminators to enhance transcriptional stability.
- Engineering alternate compound production pathways to develop peppermint varieties with optimized essential oil yield and composition. Related approaches in metabolic reprogramming can be explored in Metabolic Reprogramming in Catharanthus Roseus for Non-Natural Indole Alkaloids.
Conclusion
Advances in metabolic engineering have elucidated critical regulatory steps in menthol biosynthesis, enabling targeted interventions to improve essential oil production. Combining transgenic approaches with empirical methods holds promise for developing high-yielding peppermint cultivars with enhanced menthol content.
Reference: Proceedings of the National Academy of Sciences USA, 2011, Vol.108, pp.16944–16949.
[Music] [Music] welcome to nptl online certification
course on pharmacognosy and metabolic engineering this is lecture number 46 where I will continue with mental story
which I started in lecture number 45 I will continue this that is biosynthesis and pathway
manipulation now we'll see a few metabolic engineering
applications so first of all I just briefly mention the atement first of one such earlier example was they attempted
to overexpress that is overexpression of lonin
synthes so over expression of lorine synthes why they have attempted because this is basically a Rel L low catalytic
efficiency therefore if the extra copies of genes can be put there then that may uh make the process faster and that uh
may lead to the accumulation of more essential o so and and another point is that because this is of low relatively
low catalytic efficiency so this point I just put it here relative L
low catalytic efficiency
that may be considered as a potentially rate limiting
enzyme therefore putting extra copies of this Gene may make impact leading to the
faster flow of the pathway however elevated expression level only led to a moderate increase so
whatever increase happened because of this over expression it was only moderate uh so the result was not that
much exciting another one what they have taken is basically l3h that is OV
expression of of
lonin three hydroxy which basically converts uh lonin to this trans isop
pipol so again this why this this was used this is because okay L3 H because this
step also it was found to be of relatively low catalytic efficiency and then also they thought that this could
be another red limiting enzyme uh which can uh which can be uh made Faster by putting extra copies of
the genes but over expression of lonin 3 hydroxy did not result in increase of
oil yield so in the first one some changes happened but that was not significant
and second one it did not make the Improvement on oil he so then the third attempt what they made is
basically uh they now made an attempt to make
expression of me menthofuran synthes
MFS in anti sense orientation so what is going to happen if you see
the pathway if I draw it in this side that means from
Pon One root goes to L of furan and other root goes
towards menthon and menopur synthes is here
okay and if this is blocked by
antisense what that is that going what is that is going to happen so that straight away
[Music] uh resulted in reduced polygon and menopur
levels this resulted in reduced Pon and menopur level so I just write it here
reduced menthofuran and Pon
levels okay look when antisense suppression menthofuran level will be less this is fine but how it
affects uh this menthon will finally makes Menthol so how it affects
the pogon that is interesting what is expected that Pon content will be more so if MFS or menopur synthesis antisense
is suppressed then what will happen menop Furon will be less that is fine but Pon will be more but what they found
Pon level also suppressed actually what happened because uh maybe Pon level enhance but that polygon immediately got
converted into menthon and subsequently Menthol so if we go to the pathway that is the
uh so this is this menthon reduce menthon reduce is working here and Pon reductors will be working here
so that means antisense separation basically pushing the pathway towards this side so so that is
why polygon content whatever detected was less so that is interesting and then what finally
the uh another attempt what was made that is expression
of uh uh uh
dxr along with anti
sense expression of MFS and that
finally leads to the increase
in essential oil yield in transgenic
plants which can go from 61% to
78% as compared to the wild type that means what you have done that you you put the over Express the dxr that is the
second enzyme of the map pathway so the the pathway flows more and then you put the
antisense in the f in this so that you make the all path clear so pathway can straight away moves more towards the
Menthol not making menthofuran and that leads to the yield of the thing as well
okay and uh expression of lonin synthes actually led to increase of the lonin content because over expression of lonin
synthes will make the lonin but that lonin increase did not make much impact on the final essential oil yeld so this
is in brief about the metabolic engineering attempts made with uh
papermint and this last work is basically published in the Journal called pnas I
have mentioned several times proceedings of the National Academy
CAD of science
USA that is in the year of 2011 uh volume [Music]
108 and uh page number 16944 2
16949 uh apart from this there are other paper published but I have chosen this work because it is really interesting uh
to understand the pathway now before uh I finish this class the last aspect I would like to tell about
the uh current progress and future potential of Mena
okay so first of all is the while type mint while
type mint so this while type mint attempts were
made to over expression also what I
write what expression of dxr even people made attempted for
gpps lonin synthes l3h and so
on okay and also attempts were made to make RNA suppression or antisense suppression there is a different
color suppression of different genes including Ms MFS maybe another a me
transcription Factor was also attempted and so many other genes and the third one is
basically by by the use of which is empirical
approach approach where use of plant growth Regulators along with the manipulation
of environmental factors
that lead to the uh improved essential oil composition of content
so here weate a box here so this is uh
increased e yield and also we can write
improved U composition
okay and uh this leading to the formation of transgenic
mint which also showed the similar properties so that means with
the empirical approach it is Al it was also possible to improve the essential oil composition and and content by
manipulating the environmental factors by application of plant growth Regulators also the other approach is
the fundamental approach where either OV expression or anti sense separation make changes in the transgenic mintt
essential oil content so this is about now the future potential is this what is this that maybe tissue
specific promoters may be explode maybe promoter
stacking or sometimes double Terminators so this can be explode other approach may
be alternate compound
production and that leads to the formation of pot new potential mint so
the potential mint which should show the uh maximum
yeld along with may be optimized or Optimum
op sorry Optimum composition
for so this is basically the present status of Mena current
progress and the future
potential so EO stands for essential oil so this is in brief I have covered the
biosynthesis of Menthol and carbon and the pathway the enzymes and Regulation and few transgenic attempts
lead to Improvement of Mena and what can be done in the future so with this I will end this class thank you
The key enzymes targeted include Limonene Synthase (LS), which catalyzes the initial formation of limonene and is considered rate-limiting; Limonene-3-Hydroxylase (L3H), responsible for converting limonene to trans-isopiperitenol; and Menthofuran Synthase (MFS), involved in a side pathway leading to menthofuran. Manipulating these enzymes through overexpression or suppression helps redirect metabolic flux to increase menthol production.
Antisense suppression of MFS reduces the production of menthofuran, which is a side product, thereby decreasing wasteful metabolic flux. This suppression not only lowers menthofuran levels but also unexpectedly reduces pulegone content, which likely directs more precursors toward menthol biosynthesis, resulting in higher menthol yield.
A combined genetic strategy involving the overexpression of DXR (1-deoxy-D-xylulose 5-phosphate reductoisomerase) from the early MEP pathway together with antisense suppression of MFS has proven effective. This approach redirected the metabolic flux toward menthol biosynthesis, increasing essential oil yield from 61% in wild-type plants to about 78%.
Overexpression of single enzymes like LS or L3H often results in limited enhancement due to the complex regulatory mechanisms and metabolic bottlenecks within the pathway. Enzymes may have low catalytic efficiencies or their activity might be controlled by feedback inhibition, necessitating combined or more precise engineering strategies to achieve significant yield improvements.
Yes, empirical methods such as applying plant growth regulators and manipulating environmental conditions have been shown to enhance both essential oil content and its composition. These approaches can complement genetic engineering by optimizing physiological factors that influence biosynthesis.
Future strategies include using tissue-specific promoters and promoter stacking to finely control gene expression, employing double terminators to increase transcriptional stability, and engineering alternative biosynthetic pathways. These sophisticated genetic interventions aim to optimize essential oil yield and composition more precisely.
For broader context on metabolic reprogramming, especially for producing non-natural compounds, the summary on 'Metabolic Reprogramming in Catharanthus Roseus for Non-Natural Indole Alkaloids' provides valuable insights. This resource highlights similar approaches applied in different medicinal plants to enhance valuable secondary metabolites.
Heads up!
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