1.8.2 N-, P-, and K-Related Carbon Metabolism in Leaves
1.8.2.1 N Metabolism (Fig. 1.20)
Rubisco Rubisco is a key enzyme in carboxylation (the first step of carbon assimilation) and oxidation (the consumption of photosynthesized compounds in the
Calvin cycle, probably the production of the primary amino acids serine and
glycine). As Rubisco makes up almost half of the soluble proteins in ecosystems,
Rubisco is a key protein in carbon metabolism and nitrogen economy (Osaki et al.
1993a, 1995a, b, c, d).
The Photorespiratory Pathway Rubisco has an oxygenation function called the
photorespiratory pathway (Dennis et al. 1997), consuming RuBP in the Calvin cycle.
This pathway is described as a cyclic pathway in the metabolic map, as RuBP is only
consumed through oxidation by Rubisco. However, the photorespiratory pathway is
important for producing serine and glycine (Osaki et al. 1994, 1995d).
Thus, Rubisco is a key enzyme related to carbon and nitrogen metabolism and
economy, suggesting that nitrogen deficiency directly affects this key metabolism.
Photosystem I and Nitrate/Nitrite Reduction The reduction of nitrate to ammonia
is mediated by two enzymes: nitrate reductase (NR), which involves the two-electron
reduction of nitrate to nitrite, and nitrite reductase (NiR), which transforms nitrite to
ammonia in a six-electron reduction (Marschner 1995).
Nitrite reductase is stimulated by light, as is NADH–nitrate oxidoreductase. In
green leaves, the electron donor is reduced ferredoxin, generated in the light by
photosystem I.
Thus, the energy for nitrate/nitrite reduction depends directly on photosystem I.
GS/GOGAT Mediating Amino Acid Formation
The glutamine oxoglutarate aminotransferase (glutamate synthase: GOGAT)/glutamine synthetase (GS) system manufactures glutamate from NH 4
+ and
α-ketoglutarate (2-oxoglutaric acid) (Marschner 1995).
α-ketoglutarate is provided from the TAC cycle. PEPC (related to the TCA cycle)
and SPS (related to sucrose synthesis) are regulated by light and NO
3À , which
activate C3-type PEPC and depress SPS. Thus, triose-P (photosynthate) is more
distributed to the TCA cycle, and α-ketoglutarate increases to produce amino acids
(Marschner 1995).
Carbon/Nitrogen Interactions As mentioned above, carbon and nitrogen metabolisms mutually interact. At the whole-plant level, carbon–nitrogen interactions are
Fig. 1.20 (continued) oxygenase, PEPC PEP carboxylase, SPS sucrose phosphate synthase, PK
pyruvate kinase, GS/GOGAT glutamine synthetase/glutamate synthase, NR/NiR Reductase nitrate/
nitrite reductase
32
M. Osaki et al.
1.8.2.1 N Metabolism (Fig. 1.20)
Rubisco Rubisco is a key enzyme in carboxylation (the first step of carbon assimilation) and oxidation (the consumption of photosynthesized compounds in the
Calvin cycle, probably the production of the primary amino acids serine and
glycine). As Rubisco makes up almost half of the soluble proteins in ecosystems,
Rubisco is a key protein in carbon metabolism and nitrogen economy (Osaki et al.
1993a, 1995a, b, c, d).
The Photorespiratory Pathway Rubisco has an oxygenation function called the
photorespiratory pathway (Dennis et al. 1997), consuming RuBP in the Calvin cycle.
This pathway is described as a cyclic pathway in the metabolic map, as RuBP is only
consumed through oxidation by Rubisco. However, the photorespiratory pathway is
important for producing serine and glycine (Osaki et al. 1994, 1995d).
Thus, Rubisco is a key enzyme related to carbon and nitrogen metabolism and
economy, suggesting that nitrogen deficiency directly affects this key metabolism.
Photosystem I and Nitrate/Nitrite Reduction The reduction of nitrate to ammonia
is mediated by two enzymes: nitrate reductase (NR), which involves the two-electron
reduction of nitrate to nitrite, and nitrite reductase (NiR), which transforms nitrite to
ammonia in a six-electron reduction (Marschner 1995).
Nitrite reductase is stimulated by light, as is NADH–nitrate oxidoreductase. In
green leaves, the electron donor is reduced ferredoxin, generated in the light by
photosystem I.
Thus, the energy for nitrate/nitrite reduction depends directly on photosystem I.
GS/GOGAT Mediating Amino Acid Formation
The glutamine oxoglutarate aminotransferase (glutamate synthase: GOGAT)/glutamine synthetase (GS) system manufactures glutamate from NH 4
+ and
α-ketoglutarate (2-oxoglutaric acid) (Marschner 1995).
α-ketoglutarate is provided from the TAC cycle. PEPC (related to the TCA cycle)
and SPS (related to sucrose synthesis) are regulated by light and NO
3À , which
activate C3-type PEPC and depress SPS. Thus, triose-P (photosynthate) is more
distributed to the TCA cycle, and α-ketoglutarate increases to produce amino acids
(Marschner 1995).
Carbon/Nitrogen Interactions As mentioned above, carbon and nitrogen metabolisms mutually interact. At the whole-plant level, carbon–nitrogen interactions are
Fig. 1.20 (continued) oxygenase, PEPC PEP carboxylase, SPS sucrose phosphate synthase, PK
pyruvate kinase, GS/GOGAT glutamine synthetase/glutamate synthase, NR/NiR Reductase nitrate/
nitrite reductase
32
M. Osaki et al.