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Glycolytic non-oxidative pathway

Glycolytic non-oxidative pathway


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I am currently digging in some books to understand the three major metabolic pathways involved in physical training. The most difficult one for me is the glycolytic non-oxidative pathway (also more commonly known as the anaerobic lactic pathway) and I would like some help from people versed in this field.

In this pathway, as far as I understand, glycolysis produces pyruvate. In this process, NADH and H+ ions are produced along the way.

Then, if there is still a high energy demand (i.e. glycolysis is still necessary); NADH binds with pyruvate to form lactate and free up NAD+ which is necessary to sustain the glycolysis (otherwhise, pyruvate would be consumed via an oxidative pathway i.e. oxidative glycosis or slow glycolysis). This can theoretically continue until glycogen is depleted or severely diminished as far as I understand.

The problem comes then from the H+ ions produced during the glycolysis. These ions cause acidosis of the muscles if not removed. However, they can be removed if sufficient oxygen is present to form water. And here is my main question :

Why, during high intensity exercise, would oxygen be insufficient to take care of the H+ ions produced by the glycolysis ? Is it because muscles used during high intensity are not the best ones for using/transporting oxygen? Is it also because these H+ ions cannot be transported towards neighbouring muscles able to oxidise H+ ions ?

I understand this is a difficult question and maybe there is no precise answer at the moment. If you could point me toward a good ressource that deals with this question, I would be glad. I currently base myself on McArdle book on exercise physiology.


I am going to try to walk through this problem, in a step-by-step manner in relation to exercise, starting from at rest, and ending at the point in which the body is no longer able to maintain its energy-charge.

At Rest

The body mainly utilises oxidative phosphorylation to maintain its energy-demands. In cells where great amounts of energy need to be produced very quickly, eg. cells that are actively replicating, glycolysis is the preferred mode of energy production because this pathway is able to very quickly produce large amounts of ATP, and lactate is able to quickly diffuse in to the bloodstream. Lactate will travel in the bloodstream, to the liver, to be recycled back to glucose through an anabolic process called gluconeogenesis. This recycling pathway is known as the Cori-cycle. Essentially, what is happening is, parts of the body that are in need of high amounts of energy, will "dump" their wastes to the bloodstream, to be dealt with by other organs.

Science and Skiing VI, 2015, pp 17-30. George Brooks of the University of California summarises lactate recycling quite nicely here.

Beginning Exercise

Muscle cells will begin to quickly utilise ATP-stores, and release glucose from the glycogen-stores, and release oxygen-stores from myoglobin.

As these stores begin to become depleted, the body will begin to go in to overdrive, in an attempt to restore itself to a resting-state. Thus, your heart-rate will increase, breathing will become faster, and glycolytic pathways will be activated through feed-forward mechanisms.

During Exercise

I have touched on this before, in another post that you may find interesting. I believe that if you read through this though, that you will understand why there is low levels of oxygen, and what the body does to attempt to circumvent this.

There will be very low levels of oxygen throughout the active tissues. The body will do everything it can to try to restore this, but it will ultimately fail to do so. Thus, the only real option it has is to rely on glycolysis in the tissues that are causing "problems". The rate of ATP-production is most dependent on the rate at which a cell can take glucose in to the cell.

The electron-transport-chain is only able to synthesise ATP at its maximum velocity - this maximum velocity is proportional to the concentration of oxygen. The rate of glycolysis is, however, essentially, only affected by the rate at which lactate can be removed from the cell and the rate at which glucose can be absorbed by the cell.

Of course, the muscles that are being used will only be concerned with producing energy. Therefore, all of the lactate that they produce will be dumped in to the bloodstream, where it will travel to the liver, in the previously mentioned cori cycle.


Overall, the issue is not in the muscle-cell's ability to regenerate NAD+. This is the easy part, seeing as there is no net-change in the concentration of NAD+ when glucose is converted to lactic acid. And also, things such as malate-aspartate shuttles, citrate-pyruvate shuttles, and glycerol 3-Phosphate shuttles can be used to maintain redox-balance. These are only but a few examples of how to maintain redox-balance.

You must also remember that carbohydrates are not the only class of molecules involved in these cycles; degraded proteins and fats can also be used to supply the TCA-cycle with alternative means of energy-production. Pyruvate probably would not even be able to be converted in to acetyl-CoA anyway, since pyruvate dehydrogenase requires the presence of oxygen to function.

Problems however do arise once the body is unable to deal with all of the waste that it is producing. There will reach a point, where the liver cells will be physically incapable of accepting any new lactate-molecules. And so, lactate will accumulate in the bloodstream. With an accumulation of lactate in the bloodstream, lactate will be unable to diffuse to the outside of the cell. Thus, cells will be unable to convert pyruvate in to lactate.

The reason why ATP-regeneration eventually fails to keep up with the rate of ATP-usage, is because the rate of conversion to lactate decreases. The rate of conversion to lactate decreases, because the concentration of lactate increases

I tried to make a little illustration of what I mean above, below. Hope this helps to conceptualize it.

In this second picture, there is too much lactate for diffusion to occur.

Supporting information

Taken from Quantifying intracellular rates of glycolytic and oxidative ATP production and consumption using extracellular flux measurements; Mookerjee et al. 2017

"These maximum values of JATPglyc and JATPox define the bioenergetic capacity of the cells. As shown in Fig. 5D, the maximum individual capacities of JATPglyc and JATPox in the bioenergetic space plot intersect at (62.5, 46.5) for a theoretical maximum bioenergetic capacity of 62.5 + 46.5 = 109.0 pmol of ATP/min/μg of protein. At this maximum point, the glycolytic index (GImax capacity) would be 62.5/109 = 57.3%, making C2C12 myoblasts primarily glycolytic when running at their maximum ATP production rate. Compared with the actual value of JATP production in the presence of glucose (55.2), the bioenergetic capacity was 109/55.2 = 197% of the rate with glucose (Fig. 5D). This bioenergetic capacity of 197% of the rate with glucose (alternatively, a reserve capacity of 109.0 − 55.2 = 53.8 pmol of ATP/min/μg of protein) reveals that the C2C12 cells under our experimental assay conditions with added glucose were operating comfortably within their capacity to generate ATP and were well set up to respond to any acute increases in ATP demand by increasing either glycolytic or oxidative ATP production, or both."

In these experiments, the researchers were trying to figure out what proportion of ATP would be created by glycolytic and oxidative pathways in muscle cells (and other cell-types). Their findings, were that, indeed, the majority of ATP production in muscle cells comes from glycolysis.

During exercise, this difference in amounts of ATP production between glycolysis and oxidative phosphorylation would probably be even greater.


Fantastic question! Hopefully this somewhat helps. It's very difficult to simply this process as it's quite complex.

During high intensity exercise, energy demands exceed either:

  1. The O2 supply
  2. The rate of use exceeds the at which it becomes available.

So the respiratory chain cannot process all of the H+ joined to NADH.

Continued release of anaerobic energy during glycolysis depends on NAD+'s availability to oxidize 3-phosphoglyceraldehyde -- otherwise glycolysis grinds to a halt.

During anaerobic glycolysis -- NAD+ "frees up" when excess hydrogen's combine temporarily with pyruvate to form lactate. Lactate formation requires one additional step catalyzed by lactate dehydrogenase.

The storage of H+ with pyruvate represents a temporary "collector" of the end product of anaerobic glycolysis. Once lactate is formed it diffuses away into the interstitial space and blood for buffering and removal.

However, this avenue for energy is temporary. Blood lactate and muscle lactate levels increase and ATP regeneration fails to keep pace with the rate of use. Fatigue sets in and performance diminishes.


EDIT

  • H+ = a free electron

  • Increasing the concentration of free H+ ions = lowers the pH (so a pH of 1 is very acidic and has a very high concentration of free H+ ions, a PH of say 9 has a lower concentration of H+ ions and is less acidic).

  • Oxidation is losing electrons - reduction is gaining electrons.

  • During glycolysis NADH is oxidized. The regeneration of NAD+ (this
    is the reduced form of NADH) during the reduction of pyruvate to
    lactate is required for glycolysis to continue under anaerobic
    conditions.

  • Several key steps in the oxidation of glucose to pyruvate involve the reduction of the electron/energy shuttle NAD+ to NADH. It's required to reoxidize the NADH to NAD+ in order to avoid consuming the available pools of NAD+ and to thus avoid stopping glycolysis.

  • Put differently, during glycolysis, cells can generate large amounts of NADH and slowly exhaust their supplies of NAD+. If glycolysis is to continue, the cell must find a way to regenerate NAD+, either by synthesis or by some form of recycling.

    Let me know if that helps!


UPDATE

So these slides will have to do for now (best I have available to post). Put very simply in anaerobic conditions - there comes a point where there is not enough NAD+ available to convert pyruvate to lactate.

Would H+ ions just accumulate and increase local acidosis?


Non-Oxidative Glycolysis For Production Of Acetyl-CoA Derived Compounds

The Liao group at UCLA has constructed a Non-Oxidative Glycolysis pathway for the synthesis of biofuel precursors with a 100% carbon conversion rate.

Background

The use of a petroleum-based fuel infrastructure has been sustained in the last hundred years by the balance of supply and demand. However, with the development and growth of countries and populations, the demand for fossil fuels has begun to exceed the natural supply, threatening worldwide energy security. The production of synthetic fuels from biomass feedstock presents an attractive alternative to lessen dependence on petroleum based fuels as well as reducing greenhouse gas emissions. Alternative fuels such as bioethanol are produced by fermentation, converting simple sugars to alcohol through the glycolysis pathway. In this pathway, simple sugars are broken down into pyruvate, decarboxylated to acetyl-coenzyme A (CoA), and further processed into ethanol. This decarboxylation step represents a major source of carbon loss that greatly impacts the overall economy and efficiency of biorefineries. While carbon fixing methods help mitigate these losses, these added processes incurs additional energetic costs. As such, alternative synthesis pathways are needed to increase yields of biosynthesis to more economically sustainable levels.

Innovation

The Liao group at UCLA has constructed a cyclic non-oxidative glycolysis (NOG) pathway based on enzyme mediated carbon rearrangements. The NOG novel pathway is capable of splitting glucose into three molecules of acetate, achieving 100% yield. Acetate is an important chemical feedstock that can be further processed into ethanol or butanol. Feasibility of NOG in producing acetyl-CoA intermediaries for the efficient production of bioethanol has been demonstrated both in vitro with enzymes in buffer as well as in vivo using genetically engineered E. coli bacteria.

Applications

  • This technology may be used to improve the synthesis of biofuels such as ethanol and butanol.
  • This technology can be used to generate chemical acetyl-CoA derived chemical feed stocks.

Advantages

  • The NOG pathway may increase yields in the production of fuels and chemicals derived from acetyl-CoA by achieving 100% carbon conversion from carbohydrate to alcohols.
  • The NOG pathway can make use of a variety of starting sugar and C1 (methane, methanol, CO2) inputs.

State Of Development

The Liao group has demonstrated the feasibility of acetate production without carbon loss using the NOG pathway in both in vitro and in vivo systems.


Elevated activity of the oxidative and non-oxidative pentose phosphate pathway in (pre)neoplastic lesions in rat liver

(Pre)neoplastic lesions in livers of rats induced by diethylnitrosamine are characterized by elevated activity of the first irreversible enzyme of the oxidative branch of the pentose phosphate pathway (PPP), glucose-6-phosphate dehydrogenase (G6PD), for production of NADPH. In the present study, the activity of G6PD, and the other NADPH-producing enzymes, phosphogluconate dehydrogenase (PGD), isocitrate dehydrogenase (ICD) and malate dehydrogenase (MD) was investigated in (pre)neoplastic lesions by metabolic mapping. Transketolase (TKT), the reversible rate-limiting enzyme of the non-oxidative branch of the PPP, mainly responsible for ribose production, was studied as well. Activity of G6PD in (pre)neoplastic lesions was highest, whereas activity of PGD and ICD was only 10% and of MD 5% of G6PD activity, respectively. Glucose-6-phosphate dehydrogenase activity in (pre)neoplastic lesions was increased 25 times compared with extralesional parenchyma, which was also the highest activity increase of the four NADPH-producing dehydrogenases. Transketolase activity was 0.1% of G6PD activity in lesions and was increased 2.5-fold as compared with normal parenchyma. Transketolase activity was localized by electron microscopy exclusively at membranes of granular endoplasmic reticulum in rat hepatoma cells where G6PD activity is localized as well. It is concluded that NADPH in (pre)neoplastic lesions is mainly produced by G6PD, whereas elevated TKT activity in (pre)neoplastic lesions is responsible for ribose formation with concomitant energy supply by glycolysis. The similar localization of G6PD and TKT activity suggests the channelling of substrates at this site to optimize the efficiency of NADPH and ribose synthesis.

Figures

Light micrographs of cryostat sections…

Light micrographs of cryostat sections of liver of rat treated with diethylnitrosamine incubated…

Light micrographs of cryostat sections…

Light micrographs of cryostat sections of liver of rat treated with diethylnitrosamine, incubated…

Electron micrographs of rat hepatoma…

Electron micrographs of rat hepatoma cells incubated for the ultrastructural localization of transketolase…


Fructose is the second most common type of sugar. Its degradation and subsequent production of ATP share several enzymes of the glycolysis pathway.

The main difference in the Fructose pathway is the start of it. Fructose can be catabolized by the Hexokinase enzyme at the muscle level, generating Fructose 6-phosphate. However, it can also be catabolized at the Hepatic level by the enzyme Fructokinase (Also called Ketohexokinase) and converted into Fructose 1-phosphate.

Fructose has a higher affinity for Furctokinase. So most of the Fructose will go on to form Fructose 1-phosphate. This process is achieved through phosphorylation. So a molecule of ATP is spent.

Fructose 1-phosphate is then catabolized by Aldose resulting in 2 molecules: Glyceraldehyde and Dihydroxyacetone phosphate. Glyceraldehyde is converted into Glyceraldehyde-3-phosphate by action of the enzyme Glyceraldehyde Kinase. And as in Glycolysis, Dihydroxyacetone phosphate is isomerized with the help of the enzyme Triphosphate Isomerase, resulting in another molecule of Glyceraldehyde-3-phosphate.


Construction and evolution of an Escherichia coli strain relying on nonoxidative glycolysis for sugar catabolism

20 % of glycolysis flux). Disrupting the EMPP bymore » phosphofructokinase I (pfkA) knockout increased flux through OPPP (

60 % of glycolysis flux) and the native EDP (

14 % of glycolysis flux), while overexpressing edd and eda in this ΔpfkA mutant directed

70 % of glycolytic flux through the EDP. The downregulation of EMPP via the pfkA deletion significantly decreased the growth rate, while EDP overexpression in the ΔpfkA mutant failed to improve its growth rates due to metabolic burden. However, the reorganization of E. coli glycolytic strategies did reduce glucose catabolite repression. The ΔpfkA mutant in glucose medium was able to cometabolize acetate via the citric acid cycle and gluconeogenesis, while EDP overexpression in the ΔpfkA mutant repressed acetate flux toward gluconeogenesis. Moreover, 13 C-pulse experiments in the ΔpfkA mutants showed unsequential labeling dynamics in glycolysis intermediates, possibly suggesting metabolite channeling (metabolites in glycolysis are pass from enzyme to enzyme without fully equilibrating within the cytosol medium). Conclusions: We engineered E. coli to redistribute its native glycolytic flux. The replacement of EMPP by EDP did not improve E. coli glucose utilization or biomass growth, but alleviated catabolite repression. More importantly, our results supported the hypothesis of channeling in the glycolytic pathways, a potentially overlooked mechanism for regulating glucose catabolism and coutilization of other substrates. The presence of channeling in native pathways, if proven true, would affect synthetic biology applications and metabolic modeling. « less


Summary – Oxidative vs Nonoxidative Pentose Phosphate Pathway

Oxidative and nonoxidative are two distinct phases of the pentose phosphate pathway. Oxidative pathway is the first phase and it is followed by nonoxidative phase. NADPH is produced during the oxidative pentose phosphate pathway and the reactions are not reversible. In contrast, pentoses are produced during the nonoxidative pentose phosphate pathway and the reactions are reversible. Products of the pentose phosphate pathway are useful in different ways. In this regard, ribose-5-phosphate sugar used to make DNA and RNA while the NADPH molecules which help with building other molecules. So, this is the summary of the difference between oxidative and nonoxidative pentose phosphate pathway.

Reference:

1. Berg, Jeremy M. “20.3 The Pentose Phosphate Pathway Generates NADPH and Synthesizes Five-Carbon Sugars.” Biochemistry. 5th Edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here.
2. “Pentose Phosphate Pathway.” Wikipedia, Wikimedia Foundation, 25 Feb. 2020, Available here.

Image Courtesy:

1. “Ox Pentose phosphate pathway” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia
2. “Nichtox Pentosephosphatweg” By Yikrazuul – Own work (CC BY-SA 3.0) via Commons Wikimedia


Glycolysis

The Embden–Meyerhof–Parnas Pathway

Glycolysis can be broadly defined as an energy-yielding pathway that results in the cleavage of a hexose (glucose) to a triose (pyruvate). Although the term is often taken to be synonymous with the Embden–Meyerhof–Parnas (EMP) pathway, other glycolytic pathways exist, among them the Entner–Doudoroff pathway that proceeds via a gluconic acid intermediate and a complex set of rearrangements that proceed via a pentose intermediate ( Figure 1 ).

Figure 1 . The glycolytic pathways of Escherichia coli. The pathway farthest to the left is the Emden-Meyerhof-Parnas pathway the one farthest to the right is the Entner-Doudoroff pathway. The genes that code for the major enzymes of the pathways are shown in italics. Bold arrows indicate the production or consumption of high-energy bonds (in the form of ATP or PEP) or reducing power (as NADH or NADPH). The curved, bold line near the top of the figure represents the cytoplasmic membrane reactions above that curved line occur in the periplasm, those beneath it occur in the cytoplasm.

The EMP pathway is present in organisms from every branch of the bacteria, archaea, and eukarya. Clearly, this is an early evolutionary adaptation, probably present in the ancestor of all current life forms. This suggests that the EMP pathway evolved in an anaerobic, fermentative world. However, the pathway also functions efficiently as the basis for aerobic respiration of glucose. The differences between fermentation and respiration lie largely in the differing fates of the pyruvate produced (see later). For simplicity, this discussion focuses on the EMP pathway in the well-known bacterium Escherichia coli, though the basic features of the pathway are nearly universal.

Before glucose metabolism begins, it must be transported into the cell and phosphorylated. In E. coli, these two processes are intimately coupled such that the glucose is phosphorylated by the phosphotransferase system (PTS) as it passes into the cell. Since glucose-6-phosphate (G-6-P), like most if not all sugar phosphates, is toxic at high cellular concentrations, this transport process is tightly regulated. Transcription of the glucose-specific transporter gene, ptsG, is maximal only when cyclic adenosine monophosphate (cAMP) (signaling energy limitation) accumulates. Moreover, translation of ptsG messenger RNA (mRNA) is inhibited by the small RNA sgrS, which is produced when G-6-P accumulates. Thus, the import and concomitant phosphorylation to G-6-P is reduced whenever the demand for more energy is low or the concentration of G-6-P is dangerously high.

In the absence of a PtsG protein, other PTS-linked transporters, especially the mannose-specific transporter, ManXYZ, can also transport and phosphorylate glucose. However, ptsG mutants grow more slowly on glucose than on wild-type strains. Free glucose can also accumulate intracellularly from the degradation of glucose-containing oligosaccharides such as lactose or maltose. Entry of intracellular glucose into the EMP pathway occurs via a hexokinase encoded by the glk gene.

The next two steps in the EMP pathway prepare the G-6-P for cleavage into two triose phosphates. First, a reversible phosphoglucose isomerase (pgi gene) converts G-6-P to fructose-6-phosphate. A pgi mutant can still grow slowly on glucose by using other glycolytic pathways (see later), but the EMP pathway is blocked in a pgi mutant. The resulting fructose-6-phosphate is further phosphorylated at the C1 position to fructose-1,6,-bisphosphate at the expense of adenosine triphosphate (ATP) by a phosphofructokinase encoded by pfkA. A second minor isozyme of phosphofructokinase encoded by pfkB allows slow growth of pfkA mutants. A potentially competing set of phosphatases that remove the C1 phosphate from fructose-1,6,-bisphosphate function during gluconeogenesis but are controlled during glycolysis by a variety of feedback mechanisms to prevent futile cycling.

The next reaction in the pathway is the cleavage of fructose-1,6-bisphosphate to two triose phosphates that gives the pathway its name (glycolysis = sugar breakage). This reversible reaction is carried out by fructose bisphosphate aldolase (fbaA gene) and yields dihydroxyacetone phosphate (DHAP) and glyceraldehyde phosphate (GAP) as products. A second, unrelated aldolase (fbaB gene) is made only during gluconeogenesis and thus plays no role in glycolysis. The two triose phosphates are freely interconvertible via triosephosphate isomerase (tpi gene). DHAP is a key substrate for lipid biosynthesis. GAP is an important node in glycolysis two other common glycolytic pathways (see below) join the EMP pathway at GAP.

Up to this point, the EMP pathway can be regarded as a biosynthetic pathway since it yields three key biosynthetic building blocks (G-6-P, fructose-6-phosphate, and DHAP) at the expense of ATP and without any oxidative steps. The next step is the oxidative phosphorylation of GAP to 1,3-diphosphoglyceric acid, a high-energy compound. The incorporation of inorganic phosphate by GAP dehydrogenase (gapA gene) is coupled to the reduction of NAD + to NADH. Under aerobic conditions, this NADH is reoxidized using the respiratory chain to yield ATP. Under anaerobic conditions, this NADH is reoxidized by coupling to the reduction of products derived from pyruvate or other EMP pathway intermediates. The enzyme phosphoglycerate kinase (pgk gene) then phosphorylates adenosine diphosphate (ADP) to ATP at the expense of the C1 phosphate of 1,3-diphosphoglycerate. This is the first of two substrate-level phosphorylations where phosphate is transferred from a highly reactive substrate directly to ADP without the involvement of the membrane ATP synthase.

The next two steps rearrange the resulting 3-phosphoglycerate to the last high-energy intermediate of the pathway, phosphoenolpyruvate (PEP). First, the phosphate is transferred from the C3 position to the C2 position by a phosphoglycerate mutase. There are two evolutionarily unrelated isozymes, one of which (encoded by the gpmA gene) requires a 2,3-bisphosphoglycerate as a cofactor and the other (gpmM gene) does not. Although E. coli, Bacillus subtilis, and some other bacteria have both isozymes, many organisms have only one or the other. For example, the yeast Saccharomyces cerevisiae, the bacterium Mycobacterium tuberculosis, and all vertebrates have only the cofactor-dependent enzyme, whereas higher plants, the archaea, and the bacterium Pseudomonas syringae have only the cofactor-independent enzyme. A third isozyme (ytjC gene) appears to exist in E. coli, though its role is less clear.

The rearranged 2-phosphoglycerate is then dehydrated by an enolase (eno gene) to yield the key intermediate, PEP. Although pyruvate is generally considered to be the end product of the EMP pathway, it can be argued that PEP shares that honor. PEP is the ultimate source of phosphate for the PtsG-mediated transport/phosphorylation of glucose that initiates the pathway. In addition, the enzyme enolase is a required part of the degradasome that functions with the small RNA sgrS (described earlier) to inhibit translation of ptsG mRNA and stimulate degradation of ptsG mRNA. This reduces the generation of the otherwise toxic accumulation of G-6-P.

It is worth noting that PEP is a branch point under both aerobic and anaerobic conditions. The carboxylation of PEP by PEP carboxylase (ppc gene) provides oxaloacetate, which condenses with the acetyl-CoA derived from pyruvate to form citrate for running both the tricarboxylic acid (TCA) cycle and glyoxylate shunt aerobically. During fermentation, this same oxaloacetate is an intermediate in the reductive (NAD regenerating) pathway to succinate. In addition, the PEP-derived oxaloacetate is used (via a portion of the TCA cycle) for the biosynthesis of glutamic acid even under anaerobic conditions.

The last reaction is a substrate-level phosphorylation of ADP to ATP at the expense of PEP to yield pyruvate. The two isozymes of pyruvate kinase (pykA and pykF genes) are activated by sugar phosphates and the product of the pykF gene shows positive cooperativity with respect to the substrate PEP, again tending to prevent accumulation of this phosphorylated intermediate and thus preventing the generation of more G-6-P via the PEP-dependent PtsG transport mechanism.

At the end of the EMP pathway, 1 mol of glucose is converted to 2 mol of pyruvate, which can be used for further catabolism or for biosynthesis. It also yields 2 mol of ATP and 2 mol of NADH (which must be reoxidized for the pathway to continue operating). Since the pathway generates several toxic intermediates, it is not surprising that the flux through the pathway is tightly regulated. The enzymes of the pathway respond rapidly to variations in supply and demand by feedback inhibition and substrate activation of enzyme activities. They also respond (more slowly) by transcriptional regulation of gene expression in response to global regulators that vary from organism to organism.

The EMP pathway functions to generate both biosynthetic intermediates and catabolic energy from glucose. However, it also serves as a central trunk line into which many other catabolic pathways feed. G-6-P, fructose-6-phosphate, DHAP, and GAP are common junction points where catabolic pathways for sugars, alcohols, fats, and organic acids feed into the EMP pathway.


Outcomes of Glycolysis

A couple of things to consider:

One of the clear outcomes of glycolysis is the biosynthesis of compounds that can enter into a variety of metabolic pathways. Likewise compounds coming from other metabolic pathways can feed into glycolysis at various points. So, this pathway can be part of a central exchange for carbon flux within the cell.

If glycolysis is run long enough, the constant oxidation of glucose with NAD + can leave the cell with a problem how to regenerate NAD + from the 2 molecules of NADH produced. If the NAD + is not regenerated all of the cell's NAD will be nearly completely transformed into NADH. So how do cells regenerate NAD + ? This is another design challenge.

Pyruvate is not completely oxidized, there is still some energy to be extracted - how might this happen? Also, what should the cell do with all of that NADH? Is there any energy there to extract?

Note: Strongly suggested discussion/exercise

Can you write an energy story for the overall process of glycolysis. For energy terms, just worry about describing things in terms of whether they are exergonic or endergonic. When I say overall process I mean overall process: glucose should be listed in the reactants and pyruvate listed on the product side of the arrow- you can skip the intermediates. You are of course welcome to do this in a more detailed fashion too.


Trehalose entry

Trehalose breakdown into glucose monomer by the action of trehalase enzyme, now free glucose can enter into the metabolic pathway

Sucrose entry

Sucrose enzymatically breakdown into glucose and fructose with the use of water molecule is the hydrolysis of sucrose, free glucose can into the catalytic pathway. And fructose enters after some other enzymatic reaction.

Lactose entry

With the help of lactase enzyme lactose breakdown into glucose and galactose, now galactose undergoes some other enzymatic reaction before entry.


Fermentation Pathways

Fermentation pathway is characterized by its essential trait, where an organic compound (e.g., pyruvic acid) is the final electron acceptor for the electrons removed from substrates of this pathway. Furthermore, this final acceptor is directly produced in the pathway thus, no external acceptor is required.

As the result, by contrast to aerobic respiration, fermentation doesn’t need molecular oxygen as common electron acceptor. When accomplished, it produces a variety of low-energy compounds, termed end products the nature of these products strongly depends on the species of bacteria.

Different species of bacteria can utilize an extremely broad number of organic substances to obtain energy and reducing power by means of various metabolic pathways.

Glycolysis (Gr. glycos – “sugar” and Gr. lysis – “dissolution”) is the most common pathway for degrading sugars. It is often termed as Embden-Meyerhoff pathway, named after the two scientists who identified its major steps in the 1930s.

Many bacteria and yeasts use this pathway to degrade glucose and other sugars. Here one glucose molecule is converted into two molecules of pyruvic acid, as well as into reducing power equivalents such as hydrogen atoms.

Pyruvic acid, generated from a primary set of reactions, is used further for both fermentation or respiration.

The differences in molecular energy between glucose and pyruvic acid are accumulated in high-energy bonds in ATP. ATP synthesis results from substrate phosphorylation, where energy-rich phosphate anhydride bond is directly transferred from organic donor molecules to ADP. The final ATP yield in fermentation is equal to 2 ATP molecules.

In the absence of respiration or photosynthesis, mirobial cells are completely dependent on substrate phosphorylation to gain energy. Therefore, in this case ATP synthesis ensues from chemical transformations of primary organic substances. A great variety of substrates are metabolized within diverse fermentation pathways.

For instance, a lot of bacteria may perform the lactic acid fermentation. Many of them produce only a single product (lactic acid) being called as homofermenters. Nonetheless, some other lactic acid-producing bacteria are heterofermenters, releasing CO2 and ethanol, as well as lactic acid end products.

Closely related species of bacteria can be reliably discriminated by their products of fermentation.

Due to the fermentation activity of acetic acid bacteria (Acetobacter spp.) acetic acid is formed (acetic acid fermentation).

Ethanol or alcoholic fermentation takes place under the influence of the enzymes of yeasts Saccharomyces cerevisiae (yeasts), mucor moulds, Zymomonas mobilis bacteria, etc.

Lactic acid fermentation is caused by the fermentative activity of Lactobacillus casei, Lactococcus lactis, etc. The enzymes of lactic acid bacteria break down glucose with the production of lactic acid. The representatives from family Enterobacteriaceae are heterofermenters and produce lactic, succinic, and acetic acids, ethanol, carbon dioxide and hydrogen.

Butyric acid fermentation is performed with the anaerobes Clostridium perfringens or Clostridium butyricum resulting in the production of butyric acid

Propionic acid fermentation is demonstrated by anaerobes from genus Propionibacterium spp.

Other Pathways of Glucose Degradation

The major scheme for the degradation of glucose harnesses the glycolytic pathway. However, not all bacteria are able to use this process. They derive energy through other pathways.

The alternate route is termed the Entner-Doudoroff pathway or 2-keto-3-deoxyphosphogluconate pathway. It gives pyruvic acid from glucose oxidation via 2-keto-3-deoxyphosphogluconic acid intermediate. This pathway generates only 1 molecule of ATP after transformation of 1 molecule of glucose as substrate.

Another sugar metabolizing pathway that is present in most bacteria is termed the pentose phosphate pathway. The pathway is not of great significance for the production of energy, but it is valuable as the products of the pathway are 5-carbon and 4-carbon molecules that serve as precursor metabolites for nucleic acid and amino acid synthesis. Further, it provides reducing power required in the biosynthesis of cell components. This pathway is important in the organisms that carry out fermentations where reducing power (NADH) is not available for biosynthetic reactions.