Which enzyme fixes atmospheric CO2 in the Calvin cycle, and what is the initial five-carbon sugar it carboxylates?

Which enzyme fixes atmospheric CO2 in the Calvin cycle, and what is the initial five-carbon sugar it carboxylates?

• A. Rubisco; ribose-5-phosphate
• B. Ribulose-5-phosphate; ribulose-1,5-bisphosphate
• C. Rubisco; ribulose-1,5-bisphosphate
• D. Carbonic anhydrase; ribulose-1,5-bisphosphate

Answer: (C)

CO2 fixation in the Calvin cycle is carried out by the enzyme Rubisco, which attaches CO2 to the five-carbon sugar ribulose-1,5-bisphosphate (RuBP). This carboxylation forms a six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate. Among the sugar phosphates, RuBP is the correct substrate for this step; ribulose-5-phosphate and ribose-5-phosphate are involved in other pathways, not the CO2 fixation step. Carbonic anhydrase, while important for converting CO2 to bicarbonate in some contexts, does not perform the carboxylation of RuBP.

CO2 fixation in the Calvin cycle is carried out by the enzyme Rubisco, which attaches CO2 to the five-carbon sugar ribulose-1,5-bisphosphate (RuBP). This carboxylation forms a six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate. Among the sugar phosphates, RuBP is the correct substrate for this step; ribulose-5-phosphate and ribose-5-phosphate are involved in other pathways, not the CO2 fixation step. Carbonic anhydrase, while important for converting CO2 to bicarbonate in some contexts, does not perform the carboxylation of RuBP.