Glycolysis: Follow-up

The ultimate fate of pyruvate and NADH produced in glycolysis depends upon the organism and the conditions, most notably the presence or absence of oxygen and other external electron acceptors.

In aerobic organisms, pyruvate typically enters the mitochondria where it is fully oxidized to carbon dioxide and water by pyruvate decarboxylase and the set of enzymes of the citric acid cycle (also known as the TCA or Krebs cycle). The products of pyruvate are sequentially dehydrogenated as they pass through the cycle conserving the hydrogen equivalents via the reduction of NAD+ to NADH. NADH is ultimately oxidized by an electron transport chain using oxygen as final electron acceptor to produce a large amount of ATP via the action of the ATP synthase complex, a process known as oxidative phosphorylation. A small amount of ATP is also produced by substrate-level phosphorylation during the TCA cycle.

Although human metabolism is primarily aerobic, under hypoxic (or partially anaerobic) conditions, for example in overworked muscles that are starved of oxygen or in infarcted heart muscle cells, pyruvate is converted to the waste product lactate. This and similar reactions are known as fermentation, and they are a solution to maintaining the metabolic flux through glycolysis in response to an anaerobic or severely hypoxic environment.

Although fermentation does not produce much energy, it is critical for an anaerobic or hypoxic cell, since it regenerates NAD+ that is required for glycolysis to proceed. This is important for normal cellular function, as glycolysis is the only source of ATP in anaerobic or severely hypoxic conditions.

There are several types of fermentation wherein pyruvate and NADH are anaerobically metabolized to yield any of a variety of products with an organic molecule acting as the final hydrogen acceptor. For example, the bacteria involved in making yogurt simply reduce pyruvate to lactic acid, whereas yeast produces ethanol and carbon dioxide. Anaerobic bacteria are capable of using a wide variety of compounds, other than oxygen, as terminal electron acceptors in respiration: nitrogenous compounds (such as nitrates and nitrites), sulphur compounds (such as sulphates, sulphites, sulphur dioxide, and elemental sulphur), carbon dioxide, iron compounds, manganese compounds, cobalt compounds, and uranium compounds.

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