Pyruvate is the end product of glycolysis. The sequence of reactions leading to the formation of pyruvate from glucose is the common pathway occurring during anaerobic and aerobic respiration. Depending upon the absence or presence of molecular oxygen and the cellular metabolic need, pyruvate takes up different routes for its metabolism.
In the absence of molecular oxygen, respiratory electron transport chain and oxidative’ respiration phosphorylation can not function in the mitochondrion because molecular oxygen is the CO terminal electron acceptor for these two highly 2 coordinated processes (see later in the chapter).
As a result the oxidation of XADII to form XADT does not take place. There are limited amount of XADT available in the cell. Once all the XADT becomes tied up in the reduced state (XADII), the reaction catalyzed by NAD”-dependent glyceraldehyde 3-phosphate dehydrogenase and, therefore, glycolysis can not continue to operate.
Under the situation of unavailability of molecular oxygen, glycolysis is the main source of chemical energy (ATP) necessary for cell survival. Consequently, XAD’ must be regenerated for glycolysis to proceed. To overcome this problem, different organisms and different cell types metabolize pyruvate by fermentation pathways, which are localized in the cytosoi, to regenerate XAD\ Two fermentation pathways are of prime importance: alcohol fermentation and lactic acid fermentation.
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Under anaerobic conditions, yeast and several other microorganisms produce ethanol from pyruvate by fermentation pathways. The first step is the decarboxylation of pyruvate to produce acetaldehyde.
This reaction is catalyzed by the enzyme pyruvate decarboxylase. In the second step alcohol dehydrogenase catalyzes the reduction of acetaldehyde to ethanol by XADII and regenerates XAD”.
Higher plant tissues or organs are often subjected to anaerobic conditions, for example, plant roots under flooded soil, and produce ethanol from pyruvate. A variety of microorganisms reduce pyruvate to lactate by a process called lactic acid fermentation. This reaction also takes place under anaerobic conditions in some plant tissues and under oxygen limiting conditio:’ animal muscles.
The reduction of pyruvate by XADII to form lactate and regenerate NAD catalyzed by the enzyme lactate dehydrogenate.
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Under aerobic conditions, pyruvate enters the mitochondrial matrix where it is carboxyla oxidized, and the resultant acetyl group is transferred to coenzyme A (CoA) to form acetyl Co. this oxidative decarboxylation reaction, catalyzed by pyruvate dehydrogenase complex, NAD reduced to XADII and CO” is released.
Pyruvate + XAD + CoA——– > Acctyl CoA + C02 + XADII + II
The acctyl group of acctyl CoA then enters the citric acid cycle where it is completely oxidi to release more energy than that is possible under anaerobic respiration. Thus the reaction catal; by pyruvate dehydrogenase complex is the link between glycolysis and citric acid cyclc. X. required for this reaction and also for the oxidation of glyceraldchydes 3-phosphate during glycolj is regenerated from XADII through the electron transport chain of mitochondria.