c) Substrate-Level Phosphorylation
C is correct. Substrate-level phosphorylation.
In the final step of glycolysis, the phosphate group from phosphoenolpyruvate is transferred to ADP to form ATP and pyruvate. Since the formation of ATP in this reaction does not require oxidation, the electron transport chain, or ATP synthase, this type of reaction is called substrate-level phosphorylation. Answer choice A is incorrect because oxidation states do not change in this reaction. Answer choice B is incorrect, chemiosmosis is the movement of ions across a semipermeable membrane, down their electrochemical gradient. Answer choice D is incorrect because no water molecules are lost during this reaction.
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Recall that the first five steps of glycolysis are known as the preparatory phase of glycolysis. During this phase, two molecules of ATP are consumed to aid the break down of glucose into two molecules of glyceraldehyde-3-phosphate. The next, and final, five steps of the glycolytic pathway are known as the payoff phase (Figure 1). In this phase, two molecules of glyceraldehyde-3-phosphate are processed to form two molecules of pyruvate. Additionally, four molecules of ATP and two molecules of NADH are also formed. Therefore, the net result of glycolysis is the conversion of one glucose molecule into two molecules of pyruvate, two molecules of ATP, and two molecules of NADH.
This MCAT post covers each of the five steps of the payoff phase of glycolysis.
The first step of the payoff phase, or the sixth step of glycolysis, is the oxidation of glyceraldehyde-3-phosphate (G3P) (Figure 2). In this step, G3P is oxidized and an inorganic phosphate molecule is added, forming 1,3-bisphosphoglycerate (1,3-BPG). It is important to note that oxidation and reduction reactions always occur together. This is termed a reduction/oxidation, or redox, reaction. For this specific step, the oxidation of G3P is coupled with the reduction of NAD+ to NADH. In other words, electron transfer occurs from glyceraldehyde-3-phosphate to NAD+. Furthermore, it is always important to note the enzyme catalyzing a given reaction. The sixth step of glycolysis is catalyzed by glyceraldehyde-3-phosphate dehydrogenase. Note that dehydrogenase enzymes catalyze the removal of hydrogens, therefore, catalyzing oxidation reactions.
The next step in the payoff phase, or the seventh step of glycolysis, is the transfer of a phosphate group from 1,3-bisphosphoglycerate (Figure 3) to ADP to form ATP and 3-phosphoglycerate. This method of ATP formation is known as substrate-level phosphorylation, and should not be confused with oxidative phosphorylation, which occurs through the electron transport chain and chemiosmotic coupling. Moreover, since this reaction involves the transfer of a phosphate group, the catalytic enzyme is a kinase. In this case, the kinase is phosphoglycerate kinase.
Step eight of glycolysis is the conversion of 3-phosphoglycerate to 2-phosphoglycerate (Figure 4). In this step, the phosphate group attached to carbon-3 of 3-phosphoglycerate is moved to carbon-2, resulting in the formation of 2-phosphoglycerate. This reaction is catalyzed by phosphoglycerate mutase.
The ninth step of glycolysis is the dehydration of 2-phosphoglycerate (Figure 5). Dehydration indicates the removal of water. In this step, water is removed from 2-phosphoglycerate to form phosphoenolpyruvate. This reaction is catalyzed by the enzyme enolase.
The last step of the payoff phase, and in turn, the last step of glycolysis, involves the transfer of a phosphate group from phosphoenolpyruvate to ADP, forming pyruvate and ATP (Figure 6). This step is another example of substrate-level phosphorylation and is catalyzed by pyruvate kinase. Also note that this step is another irreversible step, meaning that it is an exergonic reaction that releases energy. The importance of irreversible steps in glycolysis will become more clear when we discuss gluconeogenesis, the reverse process of glycolysis, in later lessons. For now, note that the three irreversible steps of glycolysis serve to make sure that glycolytic break down of glucose is not easily undone by gluconeogenic enzymes.
In summary, two molecules of glyceraldehyde-3-phosphate are transformed into two molecules of pyruvate in the payoff phase of glycolysis. For each of these molecules, two ATP and one NADH molecule are produced. However, since there are two molecules of glyceraldehyde-3-phosphate to begin with, the overall energy yield of the payoff phase is four ATP molecules and two NADH molecules.
Depending on physiological conditions, the resulting pyruvate molecules can either be oxidized further by pyruvate dehydrogenase complex and the Krebs cycle or else can be converted back into glucose via gluconeoegenesis.
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