Biochemistry, Vol. 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
Glucose is formed from CO2 in the Calvin cycle

The reactions illustrated in Figs. 23-17 and 23-19 can account for the net conversion of CO₂ into only one of the six carbon atoms of glucose. How, then, are the remaining five carbon atoms of glucose derived from CO₂? To answer this question, Calvin and his coworkers postulated a complex cyclic mechanism that accomplishes the complete Biosynthesis OF GLUCOSE—that is, The formation of all six of its carbon atoms from carbon dioxide (Fig. 23-20). In The Calvin Cycle, one molecule of ribulose 1,5-bisphosphate is consumed for each molecule of CO₂ fixed, but it is regenerated at the end of the cycle. We see here the same chemical «trick» as in The Citric Acid Cycle or The Urea Cycle: both of these, as we recall, culminate in the regeneration of a compound consumed in the first step of the cycle—namely, oxaloacetate in The Citric Acid cycle (Section 16.3) and Ornithine in the urea cycle (Section 19.17). Figure 23-21 lists as balanced equations the individual reactions that make up the Calvin cycle. Seven of the steps in this cycle [reactions (2) through (8)] are identical to steps of Gluconeogenesis in animal Tissues (Chapter 20). The only difference is that during Photosynthesis, the reductant used in the Formation of glyceraldehyde 3-phosphate is NADPH rather than NADH. The remaining reactions (Fig. 23-21) are catalyzed by six additional Enzymes. To better understand this complex process, the overall equation of the Calvin cycle can be written as follows:

Class="center">6 Ribulose 1,5-bisphosphate + 6 CO₂ +

+ 18 ATP + 12 H₂O + 12 NADPH +

+ 12 H⁺ → 6 Ribulose 1,5-bisphosphate +

+ Glucose + 18 P_i + 18 ADP +

+ 12 NADP⁺.

Fig. 23-20. The Calvin cycle: conversion of CO₂ to D-glucose during photosynthesis. The incoming CO₂ and the end product, glucose, are shown against a red Background. All Other Compounds entering or leaving the cycle are enclosed in boxes. For the balanced reaction equations, see Fig. 23-21. 3-PG, 3-phosphoglycerate; G3P, glyceraldehyde 3-phosphate; DHAP, dihydroxyacetone phosphate; FBP, fructose 1,6-bisphosphate; F6P, fructose 6-phosphate; G6P, glucose 6-phosphate; E4P, erythrose 4-phosphate; Xu5P, xylulose 5-phosphate; SBP, sedoheptulose 1,7-bisphosphate; S7P, sedoheptulose 7-phosphate; R5P, ribose 5-phosphate; Ru5P, ribulose 5-phosphate; RuDP, ribulose 1,5-bisphosphate.

Ribulose 1,5-bisphosphate is written on both sides of the equation to indicate that it is a required component that is regenerated at the end of each turn of the cycle. By canceling it from both sides, we obtain the net equation of the Calvin cycle:

6 CO₂ + 18 ATP + 12 H₂O + 12 NADPH +

+ 12 H⁺ → C₆H₁₂O₆ + 18 P_i + 18 ADP + 12 NADP⁺

Fig. 23-21. Balanced reaction equations for The conversion of CO₂ into glucose (boxed) in the Calvin cycle. Reactions (1) through (8) yield glucose, whereas reactions (9) through (15) regenerate ribulose 1,5-bisphosphate.

The synthesis of each glucose molecule from six molecules of CO₂ requires 18 molecules of ATP and 12 molecules of NADPH, which are subsequently replenished by the light reactions of photosynthesis.

Let us now examine the sequential Reactions of the Calvin cycle (Fig. 23-21). Reactions (1) through (8) lead to the formation of glucose from CO₂ and ribulose 1,5-bisphosphate. Reactions (9) through (15) are concerned with the regeneration of ribulose 1,5-bisphosphate, which is essential for a new turn of the Calvin cycle to begin. Reaction (9) is catalyzed by transketolase, an Mg²⁺-dependent enzyme that employs thiamine pyrophosphate as a prosthetic group. Transketolase catalyzes the reversible transfer of a ketol group (CH₂OH-CO-) from a ketose phosphate—in this case, fructose 6-phosphate—to an aldose phosphate, in this case, glyceraldehyde 3-phosphate (Fig. 23-22). Reaction (10) involves aldolase; it catalyzes the reversible Condensation of an aldehyde, in this case erythrose 4-phosphate, with dihydroxyacetone phosphate to yield the seven-carbon compound sedoheptulose 1,7-bisphosphate (Fig. 23-23). The phosphate group at position 1 is then cleaved from sedoheptulose 1,7-bisphosphate, and the resulting sedoheptulose 7-phosphate enters another transketolase reaction. The products of this reaction are two different pentose phosphates, which are ultimately converted into ribulose 1,5-bisphosphate.

Fig. 23-22. The transketolase reaction. This enzyme, which requires thiamine pyrophosphate and Mg²⁺ ions for activity, catalyzes the reversible transfer of a ketol group (highlighted in red) from a ketose phosphate to an aldose phosphate.

All the reactions listed in Fig. 23-21, with the exception of the first one catalyzed by ribulose bisphosphate carboxylase, also occur in animal tissues. Because they lack ribulose bisphosphate carboxylase, animal organisms are incapable of carrying out the net conversion of CO₂ into glucose. Plants, in which ribulose bisphosphate carboxylase participates in the initial step of CO₂ fixation, are referred to as C₃ plants because CO₂ is incorporated into a three-carbon compound.

Fig. 23-23. Formation of sedoheptulose 1,7-bisphosphate catalyzed by aldolase. This enzyme catalyzes condensation reactions between various aldehydes and dihydroxyacetone phosphate.