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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.

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  • $\begingroup$ This is a great question, but not really on topic for fitness. I am migrating it to biology, where it may get a higher level of attention. $\endgroup$ – JohnP Aug 28 '17 at 14:30
  • $\begingroup$ There are a couple of points that need attention, I think. Firstly, glycolysis 'produces' ATP as the major source of free energy, not NADH. There is no net oxidation or reduction in lactic acid glycolysis (of NAD or anything else), but paradoxically glycolysis requires a stoichiometric supply of NAD+ in order to continue. The LDH reaction replenishes the NAD+ used in the GAPdh reaction by reducing pyruvate to lactate. $\endgroup$ – user1136 Aug 28 '17 at 20:10
  • $\begingroup$ As I said here, one of the key steps to understanding glycolysis is to ask the question: how is the NADH produced in the GAPdh reaction oxidized back to NAD+?. (You might also be interested in this answer) $\endgroup$ – user1136 Aug 28 '17 at 20:13
  • $\begingroup$ In addition, I think you are confusing ionization with oxidation/reduction. O2 is the terminal electron acceptor, and it is the transport of electrons from glucose to an acceptor such as pyuvate or O2 that conserves free energy. O2 can never be 'insufficient to take care of H+ ions produced during glycolysis' but it might be insufficient to 'get rid of' reducing equivalents fast enough to generate enough ATP during strenuous exercise, and a muscle cell then begins 'dumping' electrons from NADH onto pyruvate (to form lactate), thus allowing more ATP to be produced via glycolysis $\endgroup$ – user1136 Aug 29 '17 at 10:07
  • $\begingroup$ And the paradox is that by 'dumping' electrons onto pyruvate, a more efficient pathway of ATP generation (oxidative phosphorylation) is deprived of a valuable 'fuel' (pyruvate). The key point IMO is that the GAPdh reaction requires a continuous supply of NAD(+) in order for glycolysis continue, and the cell is 'willing' to be very wasteful in order to achieve this during strenuous exercise. $\endgroup$ – user1136 Aug 29 '17 at 10:21
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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.

enter image description here

In this second picture, there is too much lactate for diffusion to occur. enter image description here

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.

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    $\begingroup$ True. However, it is not really about the amount of ATP that can be produced. It is about how quickly that ATP can be produced. By using glycolysis, the cell will be able to produce a greater amount of ATP than it would be able to if it were trying to cycle through the entire TCA-cycle & ETC. Also, the TCA/ETC does not shut down.. Not at all; it will however utilize alternative substrates, such as amino acids or fatty acids. The amino acids and fatty acids will be present in sufficient amounts to fuel the TCA relative to the amounts of oxygen that is present. And, as I said, when a cell is $\endgroup$ – Bob Aug 29 '17 at 0:13
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    $\begingroup$ From What I understand (I am an engineer, not a chemist), I'll tag along with Bob. What I understand is, during intense exercise, your body is doing whatever it can to maintain its balance. So oxidative processes (using fat mainly I guess) are running at 100% (VO2max) but are not sufficient to produce ATP quickly enough (the ATP production rate is not enough to compensate ATP use rate). So non-oxidative processes such as ATP-PC (quickly depleted) and non-oxidative glycolysis enter the game. Oxidative glycolysis is not able to proceed because $\endgroup$ – FenryrMKIII Aug 29 '17 at 5:22
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    $\begingroup$ Let's forget the word efficient. Rather, non-oxidative glycolysis is the pathway that is able to provide ATP at a sufficient rate so that ATP stores are replenished sufficiently fast compared to their use. Now one thing I don't understand from your answer is how can the body use glucose with oxidative processes while under intense activity ? Since in such situation, according to my understanding, glucose is needed for the non-oxidative glycolysis and oxidative glycolysis being the only other pathway to use glucose, $\endgroup$ – FenryrMKIII Aug 29 '17 at 5:53
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    $\begingroup$ @FenryrMKIII "Non-oxidative glycolysis ... provides ATP at a sufficient rate", correct! And, I do not think that the body would use very much glucose for oxidative phosphorylation - since the enzyme (PDH) that converts glucose in to AcCoa will not be active due to the low levels of oxygen. Instead, I think the TCA cycle would more so be powered by amino acids and fat (although minimal amounts of glucose might still get theough). $\endgroup$ – Bob Aug 29 '17 at 6:02
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    $\begingroup$ pyruvate dehydrogenase requires oxygen as a cofactor to function No, it does not. In biochemistry a cofactor is a non-protein component of an enzyme's active site. The linked paper explores the way in which hypoxia influences the regulation of PDH through phosphorylation. $\endgroup$ – Alan Boyd Aug 29 '17 at 7:32
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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.

enter image description here

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.

enter image description here

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?

Update

enter image description here

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    $\begingroup$ I edited my post. Hopefully that helps $\endgroup$ – Mike-DHSc Aug 27 '17 at 21:00
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    $\begingroup$ Two errors in the edit: H+ is not an electron; NAD+ is the oxidised form; there is only one step in glycolysis where NADH is generated. $\endgroup$ – Alan Boyd Aug 28 '17 at 16:54
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    $\begingroup$ (i) I think it is the electrons from NADH that are 'processed' by the electron transport chain, not H+ (and, of course NADH cannot cross the inner mitochondrial membrane). (ii) I think you over-emphasise the importance of H+. Pyruvate, for example, is completely ionized at physiological pH, and cannot, in any sense, be thought of as a repository of H+ (which is not an electron)?. $\endgroup$ – user1136 Aug 28 '17 at 20:22
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    $\begingroup$ @Bob Well, obviously, but that isn't what the sentence Several key steps in the oxidation of glucose to pyruvate involve the reduction of the electron/energy shuttle NAD+ to NADH. conveys, at least to me. $\endgroup$ – Alan Boyd Aug 28 '17 at 21:55
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    $\begingroup$ @Bob (i) NAD (red/ox) does NOT cross the inner mito membrane, but shuttle systems (eg aspartate/malate shuttle) transfer reducing equivalents across. This is fundamental. (ii) There is no net oxidation or reduction in lactic acid fermentation. If you consider that glycolysis produces 2 GAP, it also produces two pyruvate. (iii) The carboxylate group of both pyruvate and lactate are completely ionized at physiological pH. (iv) IMO, the sentence 'During anaerobic glycolysis -- NAD+ "frees up" when excess hydrogen's combine temporarily with pyruvate to form lactate' is complete nonsense $\endgroup$ – user1136 Aug 28 '17 at 23:33

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