DARK REACTION

In 1945, Melvin Calvin used unicellular algae, ¹⁴CO₂, and brief illumination ® identified a ¹⁴C-containing metabolite as 3-phosphoglycerate with the ¹⁴C in the carboxyl group.

Suggested a 2-C acceptor for CO₂. It turned out to be a 5-C acceptor.

CALVIN CYCLE

(Fig. 18-24)

3 Ribulose-5-P + 3 ATP ® 3 Ribulose-1,5-bisP + 3 ADP + 3 H⁺ (1)

3 CO₂ +3 Ribulose-1,5-bisP + 3 H₂O ® 6 3-Phosphoglycerate + 6 H⁺ (2)

6 3-Phosphoglycerate + 6 ATP ® 6 1,3-Bisphosphoglycerate + 6 ADP (3)

6 1,3-Bisphosphoglycerate + 6 NADPH + 6 H⁺ ®

6 Glyceraldehyde-3-P + 6 NADP⁺ + 6 Pi (4)

2 Glyceraldehyde-3-P ® 2 Dihydroxyacetone P (5)

Glyceraldehyde-3-P + Dihydroxyacetone P ® Fructose-1,6-bisP (6)

Fructose-1,6-bisP + H₂O ® Fructose-6-P + Pi (7)

Fructose-6-P + Glyceraldehyde-3-P ® Xylulose-5-P + Erythrose4-P (8)

Erythrose-4-P + Dihydroxyacetone P ® Sedoheptulose-1,7-bisP (9)

Sedoheptulose-1,7-bisP + H₂O ® Sedoheptulose-7-P + Pi (10)

Sedoheptulose-7-P + Glyceraldehyde-3-P ® Ribose-5-P + Xylulose-5-P (11)

2 Xylulose-5-P ® 2 Ribulose-5-P (12)

Ribose-5-P ® Ribulose-5-P (13)

3 CO₂ + 9 ATP + 6 NADPH + 5 H₂O ®

Glyceraldehyde-3-P + 9 ADP + 8 Pi + 6 NADP⁺ + 3 H⁺

Or:

6 CO₂ + 18 ATP + 12 NADPH + 10 H₂O ®

2 Glyceraldehyde-3-P + 18 ADP + 16 Pi + 12 NADP⁺ + 6 H⁺

Glyceraldehyde-3-P ® Dihydroxyacetone P

Glyceraldehyde-3-P + Dihydroxyacetone P ® Fructose-1,6-bisP

Fructose-1,6-bisP + H₂O ® Fructuse-6-P + Pi

Fructose-6-P ® Glucose-6-P

Glucose-6-P + H₂O ® Glucose + Pi

6 CO₂ + 18 ATP + 12 NADPH + 12 H₂O ®

Glucose + 18 ADP + 18 Pi + 12 NADP⁺ + 6 H⁺

Note that:

· Ribulose-1,5-bisP is the CO₂ acceptor. (Eq. #2)

· 2 3-phosphoglycerate ® ® Fructose-6-P identical to gluconeogenesis except that the glyceraldehyde-3-P dehydrogenase is NADPH- instead of NADH-dependent as in gluconeogenesis.

· Ribulose-5-P can be regenerated.

Stage I: Production Phase (Eqs. 1-4)

3 Ribulose-5-P + 3 CO₂ + 9 ATP + 6 NADPH ® 6 Glyceraldehyde-3-P

Stage II: Recovery Phase (Eqs. 5-13)

5 C₃ ® 3 C₅

3 CO₂ + 9 ATP + 6 NADPH ® Glyceraldehyde-3-P