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) equiv¬alents 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 220.127.116.11, 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 Schemes 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.
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