FERMENTATION METABOLIC PATHWAYS

We have always looked at fermentation process or the formation of fermentation products by microorganisms as something beneficial to man. While we are deriving the benefits from the activities of the fermentation microorganisms, in reality the action of these microorganisms are really not to serve the human masters but more to their selfish ends. To these microorganisms, the fermentation activities are their mode of survival. Fermentation is their metabolic and physiological method of deriving energy from the substrates for their growth.
The fermentation products during their metabolism are just their metabolic waste products which have served their duty. If however, the human beings get their pleasure by consuming their waste products……well and good for these pathetic humans. Nothing is stopping these humans from drinking their metabolic urine or alcohol !
In fact the cornerstone of human civilization in history has always depend on these fermentation products. There is a rich diversity of fermentat ion products as reflected in the culture of various countries. In essence, the diversity is quite limited in terms of the microorganisms and metabolism involved. The fermentation products are just given different brand names in different languages and by the use of different substrates found in the specific countries
The fermentative microorganisms metabolize the substrate to extract their enegy. This is executed by various metabolic pathways found in the microorganisms. It should be stressed that there is a diversity of metabolic pathways available, depending on the type of microorganisms, substrates and physiological environment. What is important to note are:
1 There are a number of fermentation pathways available in a microorganism
2 TCA cycle is only available in aerobic microorganism, BUT at the same time these aerobic microorganisms too have anaerobic or fermentation pathways . These microorganisms too are carrying fermentation process in their metabolism
Most students, in their early stages of learning microbial metabolism erroneously tend to think that only one or two pathways are involved. Or they only tend to think that in the study of fermentation metabolism only carbohydrate catabolism is important! Nothing could be further than truth in these assumptions. It is like saying lipids and proteins are not used as substrates in fermentation.
The beauty about having many fermentation pathways interlinking allows the fermentation microorganisms adaptability in the uncertain environment....and their survival!!!
The changing of tracks in metabolic pathways are achieved by the presence of branch end compounds. Branch compounds are compounds which interlinked two or more metabolic pathways. On reaching a branch end compound the microorganism will have the choice to go for one or other pathway depending on the circumstances. A very central branch end compound is pyruvate. From pyruvate the metabolism can go almost any where.
Such branch end compound such as pyruvate allows the effective linking of carbohydrate, lipid and protein metabolism. It also linked the various anabolic, catabolic and intermediary metabolism.


Type rest of the post here.
Read more!

MANIPULATING METABOLISM

The objective in most fermentation industries are:
1 To produce the desired fermentation product using the microbes
2 Produce the products in high concentration and volume

The ability of the microorganisms to produce the fermentation product is ultimately determined by its genetic set up. Within the genetic make up or the genes are all the instructions needed to produce the enzymes which transform the substrate through a series of metabolic pathways to form the products

Trouble is the formation of a particular fermentation product is not exclusively held or controlled by one particular metabolic pathway, Although we often simplify the picture of product formation as:

[A]--> B--> C--> D--> E -->[F]

Where A is the initial substrate to be converted to F the final fermentation product.

From the above simple hypothetical pathway we know for the substrate to be converted to the final fermentation product, it has to pass through a number of enzyme mediated transformations

The problem of the above is seen from two main views:
1 The metabolic pathway is not independent by itself and form part of the intricate metabolic maze with other pathways,substrates, products and intermediates capable of influencing the metabolic pathway forming the fermentation product
2 Each metabolic pathway forming the fermentation product is controlled by the various steps of metabolic reactions which can affect the ultimate fate of the formation of the fermentation product. The activity of each step of enzymatic conversion are affected by the incoming substrate, concentration of the intermediate product formed


We must note that in analyzing the metabolic pathway, in terms of substrate and product formed controlling the metabolic pathway, there are other kinds of substrate and product contributing to the efficiency of the fermentation product formation


A -----------------------> B
SUBSTRATE PRODUCT

H2O, OH- and H+
NADP, NADPH2, ATP
METABOLIC GASES

These 'other products and intermediates' do influence the metabolic fermentation pathway.

Some of the most significant changes that may occur during the fermentation process from these side products are:
1 Affecting the ph of the reaction
2 Affecting the dissociation of NADH2 to NAD+ vice versa
3 High hydrostatic pressure built up in the fermentor will affect the release of gaseous products of the metabolism thus preventing the gas from escaping from the metabolism of the cell. This might cause diversion in the pathway resulting in the formation of unwanted fermentation side products

EFFECT OF PH
-------------
The ph could be explained as the measure of acidity or the concentration of hydrogen ions in the media. If the fermentation broth contains a lot of [H+] then the broth will be acidic. The contribution of the [H*] in the fermentation media could be initially caused by the acid components of the nutrients in the media or by the addition of chemicals that changes the ph of the media or it sould be the resultant changes to the media brought about by the microbial actions such as the formation of strongly acidic fermentation products.

In whatever the case the high acidity produced is the reflection of high [H+] in the fermentation media.
The impact of high ph could result in:

1 Changes in the optimal of of the fermentation reactions making it no longer optimal for the reactions to occur
2 It might affect the growth and behaviour of the microorganisms in the fermentor
3 It might affect the toxicity of certain acids by favouring ionizations or dissociations of certain molecules
4 It might affect precipitations of nutrients

AFFECTING NADH2 DISSOCIATION
-----------------------------
In a normal metabolic reaction in the generation of energy, the redox carrier NADH2 is a very important electron carrier. It functions in the metabolic redox reactions by receiving and donating electrons

The simple equation could be seen as follows:

NADH2---------> NAD+ + H2
<--------- It is a two way reaction in which H+ is liberated During very acidic conditions, if there are too much H+ then the overall reaction might favour the formation of NADH2. This would lead to a condition where there would not be enough NAD+ for the metabolism to function HIGH HYDROSTATIC PRESSURE ------------------------- This is the situation which often occurs in very large fermentors where the hydrostatic pressure could be very significant affecting microorganisms at the bottom of the fermentor. Under very high hydrostatic pressure it could lead to the diversion of the metabolism of the microorganism resulting in the formation of other products. Gases could be one of the products released during the metabolism of the cells. The high hydrostatic pressure on the microorganisms could prevent the release of the gases produced during the metabolism Read more!