Passage
Upon entering muscle cells, glucose is immediately phosphorylated by hexokinase to produce glucose 6-phosphate (G6P) . This phosphorylation event prevents glucose from exiting the cell. G6P may then enter one of three metabolic pathways, depending on the needs of the cell (Figure 1) .
Figure 1 Three potential fates of G6P, determined by the needs of the cellUnder low ATP conditions, G6P enters glycolysis, where it is converted to two pyruvate molecules. The energy released in this process produces a net total of two ATP molecules per glucose molecule consumed. The resulting pyruvate molecules can then enter the mitochondria, where they participate in additional metabolic reactions that produce more ATP.When the cell experiences oxidative stress, G6P enters the pentose phosphate pathway (PPP) and produces NADPH, a reducing agent. During this process, G6P loses one carbon atom in the form of CO₂ and is converted to ribose 5-phosphate (R5P) , which is required for nucleotide synthesis. In addition to low NADPH conditions, G6P may also enter the PPP during replication, when nucleotides are needed for DNA synthesis. If nucleotides are not needed but energy is, R5P can be converted back to glycolytic intermediates and reenter glycolysis. Each glucose molecule that is converted to R5P prior to glycolysis yields an average of 1.67 ATP molecules.When all the cell's metabolic requirements are met, additional G6P can enter glycogenesis and produce glycogen, which stores glucose for later use; this process consumes ATP. When signal molecules bind G protein-coupled receptors on muscle cells, glycogenolysis is activated to release individual monomers from glycogen, allowing them to enter the appropriate pathway.
-Under low ATP conditions, which of the following enzymes would be LEAST beneficial to the cell?

Question

Passage
Upon entering muscle cells, glucose is immediately phosphorylated by hexokinase to produce glucose 6-phosphate (G6P) . This phosphorylation event prevents glucose from exiting the cell. G6P may then enter one of three metabolic pathways, depending on the needs of the cell (Figure 1) .

Figure 1 Three potential fates of G6P, determined by the needs of the cellUnder low ATP conditions, G6P enters glycolysis, where it is converted to two pyruvate molecules. The energy released in this process produces a net total of two ATP molecules per glucose molecule consumed. The resulting pyruvate molecules can then enter the mitochondria, where they participate in additional metabolic reactions that produce more ATP.When the cell experiences oxidative stress, G6P enters the pentose phosphate pathway (PPP) and produces NADPH, a reducing agent. During this process, G6P loses one carbon atom in the form of CO₂ and is converted to ribose 5-phosphate (R5P) , which is required for nucleotide synthesis. In addition to low NADPH conditions, G6P may also enter the PPP during replication, when nucleotides are needed for DNA synthesis. If nucleotides are not needed but energy is, R5P can be converted back to glycolytic intermediates and reenter glycolysis. Each glucose molecule that is converted to R5P prior to glycolysis yields an average of 1.67 ATP molecules.When all the cell's metabolic requirements are met, additional G6P can enter glycogenesis and produce glycogen, which stores glucose for later use; this process consumes ATP. When signal molecules bind G protein-coupled receptors on muscle cells, glycogenolysis is activated to release individual monomers from glycogen, allowing them to enter the appropriate pathway.
-Under low ATP conditions, which of the following enzymes would be LEAST beneficial to the cell?

Quizplus · Accepted Answer

11ecba2d_0e09_a3e7_a3bf_b774d5d39e77_MD0008_00 As stated in the passage, under low ATP conditions the G6P in a cell will enter glycolysis, which will produce ATP. The final ATP molecules of this pathway are produced by the final enzyme in the pathway, pyruvate kinase. Therefore, all glycolysis enzymes upstream of pyruvate kinase are required for maximum ATP synthesis, as their absence would prohibit pyruvate kinase function. These enzymes include the following: Aldolase catalyzes the cleavage of fructose 1,6-bisphosphate into glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (Choice A)_. Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate and is the second to last step in the pathway (Choice B)_. Glyceraldehyde 3-phosphate dehydrogenase, also known as GAPDH, catalyzes the conversion of GAP to 1,3-bisphosphoglycerate in a step that immediately precedes the first ATP-synthesis step of glycolysis (Choice C). 6-Phosphogluconate dehydrogenase (6PGD) is the enzyme in the pentose phosphate pathway that catalyzes the conversion of 6-phosphogluconate to R5P. It is in this step that a carbon atom is converted to CO₂, decreasing the overall potential ATP output. Therefore, 6PGD would be less beneficial to a cell deficient in ATP than would any glycolytic enzyme. Educational objective: Of the metabolic pathways that can be taken by glucose, glycolysis and its downstream pathways produce the most ATP. The enzymes of glycolysis are of the most benefit to cells lacking ATP whereas enzymes of other pathways are beneficial for satisfying other needs.

Passage Upon Entering Muscle Cells, Glucose Is Immediately Phosphorylated by Hexokinase