This reaction is clearly favored in the forward direction. In situations of hypoxia (intense muscular effort, for example), or absence of mitochondria, the cell is unable to regenerate NAD+ from NADH through the respiratory chain. It does so using the conversion of pyruvate to lactate, which consumes NADH and releases NAD+ release. It should be noted that the NAD+ is essential for glycolysis to continue to occur, thereby to obtain energy through the catabolism of sugars. Each molecule of pyruvate converted to lactate regenerates a molecule of NAD+. The lactate formed is sent through the bloodstream to the liver where it is converted to glucose in gluconeogenesis. The question that arises is: "So if you can recycle lactate, converting it back into glucose, why is the liver that has to do this and not the muscle, since it is the muscle that produces lactate? If so, the muscle could use directly the product of fermentation to restore the levels of metabolic fuel." In fact, at a first glance it may make sense to think in this way. However, the synthesis of glucose through gluconeogenesis is very expensive, in terms of energy, so that after an intense physical effort, it did not make sense that the muscle has to spend additional energy to synthesize glucose. Thus, the recovery of an intense effort includes not only the restoration of ATP levels in muscle but also an extra consumption of oxygen in the liver, necessary for the synthesis of ATP to be used in gluconeogenesis from lactate. In other words, after muscular efforts, is the liver that has to use lactate, allowing a faster and more efficient muscle recovery. This process is called the Cori cycle.
Dring anaerobic work the concentration of lactate in the muscle fibers can increase about 30 times and it is a commonplace to say that it is this accumulation of lactate ion which causes fatigue. However, the experimental evidence shows that although the concentration of lactate is directly related to the degree of fatigue it does not interfere with the the muscle contractile activity. Fatigue, muscle pain and cramping experienced after an intense physical effort are the result of an acidification caused by lactic acid in muscle (the pH can drop from 7 to 6.5 !!!). The pKa of lactic acid is about 4, which causes that at the cell pH (≈ 7) or plasma (≈ 7.4) occurs the dissociation of lactic acid to lactate + H+. This accumulation of H+ will interfere with the contractile capacity of muscle fibers and will also invade the synaptic cleft. Thus, the inability of the neuromuscular junction in relaying the nerve impulses to muscle fibers is due probably to a lower release of the chemical transmitter acetylcholine by nerve endings, due to acidification of the interstitial fluid and alteration of protein structures (acetylcholine receptors) by the action of H+. This system provides energy for physical activities that result in fatigue after about 60-120 seconds. It is therefore the primary metabolic process associated with activities such as running up to 400-800 m, swimming events of 100-200m, and also provides energy for high intensity moments in football, basketball, volleyball, tennis, among others. The common denominator of these activities is the support of high-intensity efforts lasting 1-2 minutes. Even the best trained athletes are unable to sprint for more than a minute. A highly competitive athlete needs about 30 minutes to recover from a 100m sprint. Some lactobacilli and streptococcus ferment lactose to lactic acid in milk. The ionization of lactic acid lowers the pH and causes denaturation of the casein (main milk protein) and other milk proteins. When this denaturation is controlled, and occurs in the right conditions, you get the yogurt or cheese.
In short, the fermentation is not used to get energy under anaerobic conditions (this misconception is very common ...). It serves to regenerate NAD+ so that glycolysis can continue to occur in the absence of O2, as glycolysis is the process that will produce ATP!
Main bibliographic sources:
- Quintas A, Freire AP, Halpern MJ, Bioquímica - Organização Molecular da Vida, Lidel
- Nelson DL, Cox MM, Lehninger - Principles of Biochemistry, WH Freeman Publishers
