Working in the Dark
Working in the Dark
Three turns of the Calvin cycle (Figure 11.1), allow the conversion of three (3) equivalents of carbon dioxide (CO2) (i.e., 3 C1 units) along with three (3) equivalents of the five-carbon carbohydrate derivative, ribulose-1,5-bisphosphate (i.e., 3 C5 units) to yield three (3) not yet isolated six-carbon adducts, 2-carboxy-3-ketoribitol-1,5-bisphosphate (3 C1 + 3 C5 = 3 C6) to form. The three (3) C6 species then undergo fragmentation to yield six (6) equivalents of the three (3) carbon dihydroxy monocarboxylate, 3-phosphoglycerate (i.e., 3 C6 = 6 C3). A cartoon representation of this process is shown in Scheme 11.1 for one of the three CO2 units. Of the six (6) three-carbon unit equivalents, five (5) are used to regenerate three (3) equivalents of ribulose-1,5-bisphosphate (i.e., 5 C3 = 3 C5), while the sixth three-carbon fragment is now available to combine with another to make a six (6) carbon sugar (2 C3 = 1 C6) such as glucose (C6H12O6) (Figure 11.2). Additionally, as shown in Figure 11.3, 3-phosphoglycerate can be used to make other small compound building blocks such as glyceric acid, lactic acid, pyruvic acid and even acetic acid (after decarboxylation). Ribulose-1,5-bisphosphate (often abbreviated as RuBP), using the enzyme ribulose-1,5-bisphosphate carboxylase (EC 18.104.22.168, carboxydismutase, rubisco), catalyzes the Mg2+- dependent conversion of the 1,5-bisphosphate ester of the carbohydrate ribulose with carbon dioxide (CO2) to produce two (2) equivalents of 3-phosphoglycerate (PGA). As shown in the Scheme 11.1 and 11.2. A hypothetical the six carbon intermediate, 2-carboxy-3-ketoribitol-1,5-bisphosphate, is often written. It is important to keep in mind that we want the 3-phosphoglycerate for purposes of construction of other important compounds. But, as noted above, three turns of the cycle are necessary to produce six (6) equivalents of 3-phosphoglycerate, and five (5) of them are reused in making the three (3) ribulose-1,5-bisphosphates necessary to turn the cycle three (3) times.
Oxford Scholarship Online requires a subscription or purchase to access the full text of books within the service. Public users can however freely search the site and view the abstracts and keywords for each book and chapter.
If you think you should have access to this title, please contact your librarian.