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.
-Which glycolysis intermediate could serve as the sole starting material for the synthesis of R5P in muscle tissue?

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.
-Which glycolysis intermediate could serve as the sole starting material for the synthesis of R5P in muscle tissue?

Quizplus · Accepted Answer

11ecba2d_0e07_cf26_a3bf_031eebf448e7_MD0008_00 Ribose 5-phosphate (R5P) is a phosphorylated five-carbon sugar (pentose phosphate) that is produced in the pentose phosphate pathway (PPP). The PPP consists of two major parts: an irreversible oxidative phase and a reversible nonoxidative phase. Fructose 6-phosphate (F6P) can enter either phase by different means. F6P is readily converted to G6P by the enzyme phosphoglucose isomerase. G6P can then enter the oxidative phase of the PPP, where it is decarboxylated (loss of CO₂) and converted to R5P. The nonoxidative phase recycles R5P by rearranging its carbon atoms to produce F6P and glyceraldehyde 3-phosphate (GAP). Because this phase is reversible, F6P and GAP can also combine and rearrange to become R5P. F6P can be converted to GAP, so F6P alone is sufficient as a starting material to enter this pathway. (Choice B) Fructose 1,6-bisphosphate (F1,6BP) must be converted to F6P before it can enter the PPP. This can be accomplished in liver cells by the gluconeogenic enzyme fructose 1,6-bisphosphatase, but this enzyme is not present in muscle cells. (Choices C and D) GAP can only enter the PPP by combining with F6P. GAP can be converted to dihydroxyacetone phosphate (DHAP), and GAP and DHAP can combine to form F1,6BP. However, muscle cells lack the enzyme necessary to convert F1,6BP to F6P. These molecules are not sufficient for R5P synthesis in muscle. Educational objective: The pentose phosphate pathway can produce ribose 5-phosphate by decarboxylation of glucose 6-phosphate (oxidative phase) or by rearranging the carbon atoms in fructose 6-phosphate (F6P) and glyceraldehyde 3-phosphate (nonoxidative phase). F6P can be converted to all of these intermediates in the muscle.

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