Pyruvate is converted to acetyl-CoA by the pyruvate dehydrogenase complex. Carbon dioxide is released during this reaction, and in addition to this,  is reduced to .

The pyruvate dehydrogenase complex (PDC) is preparing pyruvate for the Krebs cycle by converting it to acetyl-CoA. Because the Krebs cycle functions within the mitochondrial matrix, the PDC is also taking place there.  This ensures quick and easy movement from the PDC into the Krebs cycle.

The Krebs cycle produces the most high energy electron carriers of any process involved in cellular respiration. Per glucose molecule, the Krebs cycle produces  and .

Ubiquinone is a part of the electron transport chain, and has little to do with the Krebs cycle. Glucose is broken down during glycolysis, and does not enter the Krebs cycle directly. Many students make the mistake of thinking that pyruvate enters the Krebs cycle, since it is produced in glycolysis, and the Krebs cycle follows glycolysis. However, pyruvate is first converted to acetyl-CoA by the pyruvate dehydrogenase complex in the mitochondrial matrix, and acetyl-CoA enters the Krebs cycle.

Isocitrate dehydrogenase activation leads to oxidative decarboxylation of isocitrate in a two step process producing alpha-ketoglutarate and . In the mitochondria, the reaction produces also a charged electron carrier molecule, , from . Isocitrate dehydrogenase, inhibited by  and activated by , is a major regulator enzyme of the citric cycle.

The only step of the citric acid cycle listed that results in the production of  as a side product is the conversion of isocitrate to alpha-ketoglutarate. In this step, the enzyme, isocitrate dehydrogenase catalyzes the conversion of isocitrate to alpha-ketoglutarate, while also converting  to  and  as side products, and generating a molecule of  in the process (i.e. reducing the carbon count from 5 in isocitrate to 4 in alpha-ketoglutarate). 

The conversion of alpha-ketoglutarate to succinyl-CoA also produces a molecule of  as a side product. However, this step is not listed as an answer choice.

None of the other answer choices listed produce  as side products. 

We're told in the question stem that phosphofructokinase is an important regulatory enzyme for the glycolysis pathway. In this pathway, glucose is partially oxidized to provide energy for the cell. Therefore, when the cell has plenty of energy available to meet its metabolic needs, the activity of phosphofructokinase will be reduced in order to suppress glycolysis. Alternatively, when the cell is in need of energy, it will turn glycolysis on by increasing the activity of this enzyme. Thus, compounds that indicate the cell has a lot of energy available, such as ATP and NADH, would be expected to be allosteric inhibitors of this enzyme. So, if the cell has a low amount of ATP, the energy carrying molecule, we would expect the activity of this enzyme to be high. High levels of NADH and citric acid cycle intermediates signal that the cell has enough energy, therefore these would serve to reduce the activity of this enzyme. Low levels of ADP would signal that the cell most likely has a high amount of ATP and is thus in a state of energy surplus. Consequently, this scenario would likely reduce the activity of this enzyme.

When there are high levels of ATP in the blood, ATP itself can act as a signal for the inhibition of ATP production. phosphofructokinase-1 (PFK-1) and pyruvate kinase are major sites of glycolytic regulation. ATP can inhibit these enzymes by binding to their allosteric sites. If these allosteric binding sites are lost, ATP can never bind, and glycolysis will continue indefinitely. Conversely, glycolytic intermediates (like fructose-1,6-bisphosphate) and glycolytic activators (like fructose-2,6-bisphosphate) can act on the same enzymes to increase their activity. Loss of allosteric binding sites for fructose-1,6-bisphosphate and fructose-2,6-bisphosphate on pyruvate kinase and PFK-1, respectively, will result in the slowing down or inhibition of glycolysis.

Glycolysis will be stimulated in situations that require the body to make more ATP.  When the pH is low, ATP is depleted, AMP is at high levels, and carbon dioxide is increased, the body is likely going to need more of an energy supply.  All of these are related to exercise - a situation in which more ATP will be required.  However, an increase in levels of pyruvate implies that glycolysis is actually backed up and should not be stimulated.

While phosphofructokinase-2 is responsible for creating fructose-2,6-bisphosphate, this molecule will actually upregulate the enzymatic activity of phosphofructokinase-1 (PFK-1). As a result, fructose-2,6-bisphosphate is responsible for increasing the amount of glycolysis done in cells via activation of the glycolytic enzyme PFK-1.