Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 44

in either role offer major advantages in solar efficiency, which, in temperate
climes, could provide significant environmental benefits.
Photorespiration
Returning to synthesis of carbohydrate by the Calvin cycle, as mentioned above,
the first step is the carboxylation of the five-carbon sugar, ribulose diphosphate
catalysed by rubisco. As mentioned in the preceding section, this enzyme may
also function as an oxidase indicated by its full name ribulose diphosphate car-
boxylase oxidase . When this occurs and oxygen replaces carbon dioxide, the
ensuing reaction produces phosphoglycolate in addition to 3-phosphoglycerate.
Since, as a result of illumination, oxygen is consumed and carbon dioxide is
released during the reactions of the glycolate pathway, this process is termed
photorespiration and occurs alongside photosynthesis. The higher the ambient
temperature in which the organism is growing, and the higher the oxygen con-
centration relative to carbon dioxide, the more pronounced the oxidase activity
becomes and consequently the less efficient rubisco is at introducing carbon
dioxide into carbohydrate synthesis. The phosphoglycolate formed as a result of
oxygen acting as substrate for rubisco, is then dephosphorylated to form gly-
colic acid. There follows a series of reactions forming a salvage pathway for
the carbons of glycolic acid involving transfer of the carbon skeleton to the per-
oxisomes, then to the mitochondria, back to the peroxisomes and finally back
to the chloroplast in the form of glycerate which is then phosphorylated at the
expense of ATP to re-enter the Calvin cycle as 3-phosphoglycerate. The result
of this detour, thanks to the oxidase activity of rubisco is to lose a high energy
bond in phosphoglycolate, to consume ATP during phosphorylation to produce
3-phosphoglycerate, consumption of oxygen and release of carbon dioxide. This
pathway resulting from the oxidase activity of rubisco, shown in Figure 2.10
is wasteful because it consumes energy obtained by the light reactions with no
concomitant fixation of carbon dioxide into carbohydrate. The C 3 plants are there-
fore operating photosynthesis under suboptimal conditions especially when the
oxygen tension is high and carbon dioxide tension is low. Why rubisco has not
evolved to lose the oxidase activity is unclear: presumably evolutionary pres-
sures of competition have been insufficient to date. For the reasons indicated in
the preceding section, C 4 plants show little or no photorespiration due to their
ability to channel carbon dioxide to rubisco by a method independent of oxygen
tension. Therefore they are considerably more efficient than their C 3 counterparts
and may operate photosynthesis at much lower concentrations of carbon dioxide
and higher concentrations of oxygen. It is interesting to contemplate the compet-
itive effects of introducing C 4 style efficiency into C 3 plants, but at the moment
this is just speculation.
Balancing the light and the dark reactions in eukaryotes and cyanobacteria
Using the six-carbon sugar, glucose, as an example, synthesis of one molecule
requires six carbon dioxide molecules, 12 molecules of water, 12 protons,