Page 13 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN002H-54 May 17, 2001 20:22

Bioenergetics 109

used in biosynthesis, or anabolic metabolism. The E of phosphate at the 1-position of the bisphosphorylated sugar
0
+
the NADP –NADPH redox pair is −340 mV. Thus, elec- acid is transferred to ADP to form ATP. The conversion of
tron transfer from the reduced iron–sulfur protein of PS I 3-phosphoglycerate to carbohydrates occurs by a pathway
+
to NADP is energetically a very favorable spontaneous that is essentially the reverse of glycolysis. It must be em-
phasized, however, that glycolysis and photosynthetic car-
reaction. It is NADPH that provides the electrons for CO 2
reduction. The ultimate electron donor is water. bon metabolism take place in separate intracellular com-
Two water molecules are oxidized by PS II to yield partments. Glycolysis occurs in the cytoplasm and uses
+
four protons and molecular oxygen. Water is a very weak NAD as the electron acceptor. The photosynthetic re-
reducing agent. Thus, a strong oxidizing agent is needed duction of 3-phosphoglycerate occurs inside chloroplasts
for water oxidation. P680 fits the bill. The midpoint po- in the aqueous space known as the stroma. The enzymes in
+
tential of the P680 –P680 redox pair is on the order of the two compartments are not the same even though they
+
+1 V. Since the water–oxygen redox couple has an E of catalyze similar reactions. For example, the triose phos-

0
+0.815 V, the oxidation of water by P680 is an energet- phate dehydrogenase in the cytoplasm is very specific for
+
+
ically spontaneous reaction. Water oxidation is catalyzed NAD , whereas that in the chloroplast stroma is equally
by a manganese-containing enzyme that is plugged into specific for NADPH.
the energy-converting thylakoid membrane. Therefore, ATP is required for the reduction by
So far, we have seen that the reduced FeS protein of NADPH of 3-phosphoglycerate to the oxidation level of a
PS I is converted to its oxidized form by passing electrons carbohydrate:
+
eventually to NADP . In PS II, P680 is reduced to P680
+
ATP + 3-phosphoglycerate → ADP
with electrons extracted from water. For electron trans-
−
port to continue, the electron acceptor of PS II, Q , and + 1,3-bisphosphoglycerate, (9)
the electron donor of PS I, P700 , must be oxidized and
+
reduced, respectively. The redox potential of the Q–Q − and the bisphosphoglycerate is in turn reduced by
+
couple is about +0.05 V, whereas that of P700 –P700 NADPH:
−
is near +0.450 V. Thus, electron transport from Q to
+
NADPH + H + 1,3-bisphosphoglycerate → NADP +
+
P700 is energetically spontaneous with a free energy of
9.3 kcal/mol for each electron transferred. + P i + glyceraldehyde 3-phosphate. (10)
+
−
Electron transport from Q to P700 is mediated by a
Since two 3-phosphoglycerates are generated for each
quinone, iron–sulfur, and a cytochrome protein complex
CO 2 assimilated, two NADPH and two ATP are required
in the thylakoid membrane. This protein, the cytochrome
for reduction. This reaction is the only one in photo-
b 6 f complex, is remarkably similar to the cytochrome bc 1
complex of the mitochondrial electron transport chain. synthetic carbohydrate metabolism that is an oxidation–
reduction reaction.
Glyceraldehyde 3-phosphate is a sugar phosphate and
B. CO 2 Reduction may be readily converted within chloroplasts to many sug-
ars and the glucose polymer starch. Some of the glyc-
Linear electron transport in oxygenic photosynthesis is the
eraldehyde 3-phosphate is used in a complex series of
reduction of NADP to NADPH by water, which results
+
reactions to regenerate the five-carbon acceptor of CO 2 ,
in the formation of molecular oxygen:
ribulose 1,5-bisphosphate. In the process, one phosphate
+
+
2H 2 O + 2NADP → O 2 + NADPH + 2H . (8) is cleaved from one of the sugar phosphate intermediates.
Thus, ribulose 5-phosphate, the product of the cycle, must
NADPHisincapableofreducingCO 2 byitself;ATPisalso be phosphorylated by using ATP as the phosphoryl donor.
required. The CO 2 acceptor in photosynthesis is the five- As a consequence, three ATP and two NADPH are re-
carbon, phosphorylated sugar ribulose 1,5-bisphosphate. quired for each CO 2 taken up.
CO 2 cleaves this sugar into 2 mol of the three-carbon sugar Photosynthesis must satisfy the energy requirements of
acid 3-phosphoglycerate, a compound that is also an inter- all living tissues in plants, including roots, stems, and de-
mediate in glycolysis. The enzyme that catalyzes this re- veloping fruit. Up to 75% of the triose phosphate formed
action, ribulose 1,5-bisphosphate carboxylase/oxygenase, is exported from the chloroplasts in leaf cells to the cy-
or rubisco, is present in very high concentrations within toplasm where it is converted to sucrose, a major product
chloroplasts, which makes it among the most abundant of photosynthesis. In most plants, sucrose is transported
proteins in the biosphere. to the rest of the plant where it is either stored as starch
Recall that in glycolysis one of the two steps or broken down by glycolysis and the citric acid cycle in
in which ATP is formed is the conversion of 1,3- exactly the same way as it is in animals to produce the
bisphosphoglycerate to 3-phosphoglycerate. The acyl ATP needed to sustain life.