In the ecosystems some bacteria like obligate autotrophs, facultative autotrophs,
chemoautotrophs and chemolithoautotrophs which can fix atmospheric CO 2 by the
help of two enzyme Rubisco and Carbonic anhydrase which is present in the
carboxysomes (a polyhedral organelle) in bacterium. Within the carboxysome, the
carbonic anhydrase converts HCO 3
À into CO 2 and the Rubisco fix the captured CO 2
inside the organelle (Yeates et al. 2008, Fig. 11.5). There are several groups of
carbonic anhydrases (e.g. α, β, and γ) and several types of Rubisco found in
prokaryotes (Peña et al. 2010). The affinity of Rubisco with CO 2 is low, but carbonic
anhydrase facilitate CO 2 environment around the Rubisco inside carboxysome.
Therefore rate of CO 2 fixation inside the carboxysomes increases.
11.4.7 Carbonic Anhydrase
Carbonic anhydrase is a zinc-containing enzyme that catalyzes the reversible reaction of HCO 3
À to CO 2 . The reaction is considered to play necessary roles in
numerous biological processes such as respiration, pH homeostasis, ion exchange,
CO 2 acquisition, and photosynthesis (Badger and Price 2003). HCO 3
À is the
Fig. 11.4 Hydroxypropionate–hydroxybutyrate cycle and Dicarboxylate–hydroxybutyrate cycle
(Berg et al. 2010) 2.3.6 Dicarboxylate–hydroxybutyrate cycle
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