Interesting questions for critical thinking:
- How the organic acid producers accumulate large amount of the acid in medium against the concentration gradient?
- How the problem of gradient formation (with respect to pH, temp, nutrient/product concentration, etc.) inside a fermentor vessel is linked to stress-regulation of the process organism?
It is due to trans membrane proteins of the organisms, i.e. in case of citric acid production by yeast Candida utilis; two transport systems for citric acid are present, a proton symport and a facilitated diffusion system for the charged and the undissociated forms of the acid, respectively. Here proton symport helps to transfer organic acid outside of the cell. Along with that, the the citric acid (along with other acids), act as inducer for both transport systems. Thus enhancing the transport of acids outside the cell whether the acid concentration outside of the cell is high or low.
ReplyDeletehttp://www.ncbi.nlm.nih.gov/m/pubmed/1664712/
but why would proton symport transfer organic acid outside the cell when the medium is already acidic???
Deleteor you mean to say since it act as an inducer for both the system it will anyways go out of the cell regardless of the environmental transport
there is no Why. it is one way transport in case of symport. cell just send acid outside along with the proton.
Deleteans_2) By gradient formation (with respect to pH, temp, nutrient/product concentration, etc.) inside a fermentor, the fermenter which is continuously moving inside a fermentor (because of impellers) is not getting homogenious or stable environment every time. The organism is not stable respective to its growth phase. The unstable environment inside and outside of the cell leads to production of reactive oxygen species under stress conditions. The solution for this problem is to introduce the robust strain which carries the gene which can produce antioxidants.
ReplyDeletethe concentration gradient also lead to decrease in the yeild production and increase in the by product formation which indirectly affects the ph,temp and concentration.so by introducing the robust strain can this be controlled??
DeleteRobust strain only deals with the 'ROP'. Metabolic engineering may be performed to nullify the effect of byproduct formation by deleting the pathways which leads to their production.
DeleteAn increase in strain robustness (or stress resistance) will be another key field of research because cost reduction is still the main focus if new microbial organic acid production processes are to become industrially viable. The problem is not that we cannot produce a variety of acids yet, but rather that the high costs of these processes prevent large-scale production. By increasing stress resistance of the microorganisms involved the process costs can be reduced appropriately.
Deletereactive oxygen species are produced to unstable conditions of the fermentor which is continuously moving , thus gradient formation occurs and even tough organism is in log phase. antioxidants helps inhibiting the oxygen radical formation under stress condition.
ReplyDeleteA high % of dissolved sucrose in the medium increases the osmotic value in solution that presents a risk for the cells to lose water in order to survive, they accumulate a low molecular wt substances like glycerol. glycerol formed through pentose phosphate pathway and plays a important role in citric acid excretion.glycerol inhibits isocitrate dehydrogenase enzyme that's way help to accumulate citric acid inside the cell. under higher concentration of sucrose more glycerol is synthesized, thus higher sugar concentration favors acid excretion. glycerol is found to function as a osmoregulator in aspergillus niger cells and therefore it seem to be the only inducer of metabolic procedure leading to citric acid excretion.
ReplyDelete1) The Aspargilus niser used in citric acid production is an acid tolerent , which can sustain the low pH to a certain limit(2-3) due to presence of special transporter protein on its plasma membrane. To maintain the internal voltage(nearly -70mV) it exports the internal citrate to outside instead low pH of outside environment. But when pH is too low(<2) the transporter proteins get inactivated and no more growth in citrate titer is seen.
ReplyDelete2) Due to gradient formation inside a vessel, the surrounding environment of an organism changes with respect to pH. oxygen concentration, nutrient supply,and the orhanism also get replaced by the impeller so many genes get activated and suppress to maintain a homeostasis , due to this types of heterogeneity in environment , these express stress response genes producing ROS causing harm to organism
Why a spontaneous rise in intracellular cAMP level was observe in early stages of Aspargilus niser growth under high citric acid yielding condition ?
ReplyDeleteIs it because of initial sucrose concentration in the medium ?
It might be that cAMP synthesis is triggered by intracellular acidification, caused by dissociation of citric acid. Stimulation of cAMP synthesis by acidification was supported by the fact that addition of azide to the fermentation broth caused an imediate rise in cAMP levels. However, no spontaneous rise in cAMP concentration could be detected, if the cells were grown in continuously illuminated cultures, suggesting that A. niger phosphodiesterase might be photoregulated. More evidence for the light activated PDE was obtained by morphological studies under the light and dark conditions in the presence of cAMP or dbcAMP, and it was additionally supported by experiments where specific phosphodiesterase inhibitors were tested.
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ReplyDeletecitric acid transport over the cell membrane against a very high concentration gradient between intra- and extracellular citrate concentration. The active transport system was influenced by various parameters such as air saturation,
ReplyDeletetemperature, medium composition and growth state of chemostat cells.
However, in case that citrate excretion would be mediated by vacuoles, specificity between excretion of citrate and isocitrate would be attributed to aconitase equilibrium. An argument against this hypothesis is the lower intracellular iron concentration that indicates lower aconitase activity and in the mean time the higher intracellular isocitrate concentration found in present work at pH 3.
A spontaneous rise in intracellular cAMP levels was observed in the early stages of Aspegillus niger growth under high citric acid yielding conditions. The amount of cAMP formed was found to be dependent on initial sucrose concentration in the medium. Under higher sucrose conditions the peaks appeared earlier and were higher, while in lower sucrose media flattened peaks were observed later in fermentation. Since in media with a higher sucrose concentrations intracellular citric acid has started to accumulate earlier and more rapidly, it might be that cAMP synthesis is triggered by intracellular acidification, caused by dissociation of citric acid. Stimulation of cAMP synthesis by acidification was supported by the fact that addition of azide to the fermentation broth caused an imediate rise in cAMP levels. However, no spontaneous rise in cAMP concentration could be detected, if the cells were grown in continuously illuminated cultures, suggesting that A. niger phosphodiesterase might be photoregulated.
ReplyDeleteA spontaneous rise in intracellular cAMP levels was observed in the early stages of Aspegillus niger growth under high citric acid yielding conditions. The amount of cAMP formed was found to be dependent on initial sucrose concentration in the medium. Under higher sucrose conditions the peaks appeared earlier and were higher, while in lower sucrose media flattened peaks were observed later in fermentation. Since in media with a higher sucrose concentrations intracellular citric acid has started to accumulate earlier and more rapidly, it might be that cAMP synthesis is triggered by intracellular acidification, caused by dissociation of citric acid. Stimulation of cAMP synthesis by acidification was supported by the fact that addition of azide to the fermentation broth caused an imediate rise in cAMP levels. However, no spontaneous rise in cAMP concentration could be detected, if the cells were grown in continuously illuminated cultures, suggesting that A. niger phosphodiesterase might be photoregulated.
ReplyDeleteHow does the addition of azide to the fermentation broth causes an immediate rise in cAMP level?
ReplyDeleteAddition of depolarizing agents, such as dinitrophenol azide, under aerobic conditions, is known to cause an increase in the cAMP level within 15 s. it was found that both compounds lowered the intracellular pH drastically within the same time period. Plasma membrane depolarization, however, was much slower: azide had no effect on the membrane potential during, respectively, the first 2 min and the first 10 min after addition. Apparently, the intracellular pH of yeast is much more sensitive to perturbation than the membrane potential. The effect of both compounds on the cAMP level was highly dependent on the extracellular pH: when the latter was raised, the effect disappeared completely between pH 6 and 7. A similar dependence on the extracellular pH was observed for the lowering of intracellular pH. Addition of organic acids, such as acetate and butyrate, at low pH and under aerobic conditions, also caused an immediate increase in the cAMP level and an immediate drop in the intracellular pH. it suggest that agents such as azide do not raise the cAMP level in yeast cells because of their membrane depolarizing properties but because they lower the intracellular pH. Under anaerobic conditions, azide and organic acids were much less effective in increasing the cAMP level. Addition of a small amount of glucose, however, restored their capacity to enhance the cAMP level. This suggests that under anaerobic conditions and in the absence of glucose the ATP level is a limiting factor for cAMP synthesis.
ReplyDeleteWhen the cell has adapted to the new environment, grape juice, fermentation
ReplyDeletebegins. The biological activity of the yeast during fermentation leads to a number of stress conditions, some mild, some potentially severe. The most important factors are nutrient limitation and starvation, ethanol toxicity and temperature variations.
Nutrient limitation and depletion: During wine fermentation, several types of nutrients may become limited or exhausted at different stages of the fermentation, which leads to reductions in growth rate and fermentation efficiency or even a complete arrest of fermentation. Nutrient limitation and starvation are stressinducing phenomena and result in a number of stress-associated responses, including the synthesis of trehalose and the induction of heat shock proteins.
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