BCH/PPA 503 | Lecture Two Web Notes

Lecture Three

Models integrating electron transport, ATP/ADP, chloroplastic compounds, inhibitors
and the regulation of CO₂ fixation

Outline

1)Molecular approaches to defining rate-limiting steps

Rubisco

1)Structure/ Function Relationships

a)crystallographic structures

b)active site

c)catalytically essential residues

2)Reaction Mechanism

a)rapid quench studies

b)isolation of reaction intermediates

c)dual activity

3)CO₂/O₂ partitioning

a)specificity

b)molecular alterations

c)intractability

4)Constraints to productivity

5)Evolutionary considerations

6)Molecular Engineering and chloroplast transformation

The energetic relationship of the light reactions and CO₂ fixation reactions of photosynthesis can be illustrated as follows:

This image is from M.J. Farabee's web site.

Some plants have evolved strategies for suppressing photorespiration. One such adaptation is C₄ photosynthesis in which CO₂ is 1^st fixed into a 4-C rather than a 3-C acid, as is the case for C₃ photosynthesis. C₄ plants possess 2 types of photosynthetic cells each with a different type of chloroplast and a unique anatomy ('Kranz' anatomy). A layer of cells surrounding the vascular bundle, the bundle-sheath, is common in plants, but they are the only chloroplast containing cells in C₄ plants. These bundle-sheath cells are thick walled and sometimes suberized sealing them off from gas exchange. In C₄ plants phosphoenolpyruvate (PEP) is carboxylated into C₄ acids by PEP carboxylase (PEPC) in mesophyll cells which are transported via plasmodesmata to the bundle-sheath cells where they are decarboxylated. A simplified scheme of the NADP-malic enzyme type of C₄ photosynthesis from Furbank and Taylor [Plant Cell 7,797 (1995)].

In the C₄ photosynthetic pathway CO₂ (in the form of HCO₃^-) is covalently added to PEP by PEPC to form the C₄ acid, oxaloacetate. In the case of the NADP⁺-malate enzyme type C₄ pathway, OAA is reduced by malate dehydrogenase using NADPH from the light reactions to form malate, a C₄ acid that is transported to the bundle-sheath cells. Malate is decarboxylated in the bundle-sheath cells by NADP⁺-malic enzyme where the CO₂ released is fixed by the PCR cycle as in C₃ plants. The 3-C compound pyruvate diffuses back to the mesophyll where it is phosphorylated in an ATP-dependent reaction catalyzed by pyruvate orthophosphate dikinase to generate the carbon acceptor PEP.

The net reaction catalyzed by the C₄ pathway is to transfer CO₂ from the mesophyll to the bundle-sheath at the expense of 2 ATPs per CO₂ transferred. It is in effect an ATP-driven CO₂ pump. It achieves concentrations of inorganic C (CO₂ + HCO₃^-) ~ 150 µM (~ a CO₂ concentration of 70 µM). This is ~ 20 x the CO₂ concentration in mesophyll cells and is sufficient to saturate photosynthesis and essentially completely suppress photorespiration. PEPC has an affinity for inorganic C ~ that of RUBISCO [K_m(HCO₃^-) = 30 µM, ~ 6.4 µM CO₂ at pH 7]. This CO₂ concentrating activity of C₄ plants provides 3 potential advantages over C₃ plants:

Many xerophytic plants have photosynthetic adaptations that can greatly improve water-use efficiency and effectively suppress photorespiration. Water-use efficiencies (g CO₂ fixed per kg H₂O transpired) of CAM plants range from 10 - 40 compared to 1 - 3 for C₃ plants and 2 - 5 for C₄ plants. The main feature of photosynthetic metabolism in CAM plants is assimilation of CO₂ into oxaloacetic acid at night by PEPC that is then reduced to malic acid by malate dehydrogenase, which is stored in vacuoles. During the day the malate is released from the vacuole where it is decarboxylated to provide CO₂ for fixation by the Calvin-Benson cycle behind closed stomata. Thus, CAM plants separate CO₂ assimilation from CO₂ fixation in time rather than spatially as in C₄ plants.

All materials © 1998, 1999, 2000, Dr. David Hildebrand or Dr. Bob Houtz, unless otherwise noted.
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This page was last modified January 17, 2000.