Page 167 - Algae Anatomy, Biochemistry, and Biotechnology

150 Algae: Anatomy, Biochemistry, and Biotechnology

FIGURE 3.5 In vivo absorption spectra of photosynthetic compartments of Cyanophyta (a), Prochlorophyta
(b), Glaucophyta (c), and Rhodophyta (d).

As the first product was a C3 acid, Calvin hypothesized that the CO 2 acceptor would be a C2 com-
pound. However, no such C2 substrate was found. Rather, it was realized that the CO 2 acceptor was
a C5 compound, ribulose 1,5-bisphosphate (RuBP), and that the product of carboxylation was two
molecules of 3-PG. This crucial insight allowed the pathway of carbon flow to be determined.
While the CBB cycle involves a total of 13 individual enzymatic reactions, only two enzymes
are unique to this pathway: ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and
phosphorybulokinase (PRK). All other enzymes involved also perform functions in heterotrophic
metabolism. PRK catalyzes the phosphorylation of ribulose-monophosphate to ribulose-1,5-
bisphosphate (RUBP). RUBP in turn is the substrate for RuBisCO, which catalyzes the actual
carbon fixation reaction.

RuBisCO
As a result, the RuBisCO enzyme alone represents the most important pathway by which inorganic
carbon enters the biosphere. It has also been described as the most abundant protein on Earth. It
is thought that as much as 95% of all carbon fixations by C3 organisms (that includes all
phytoplankton) occur through RuBisCO.
RuBisCO is known to catalyze at least two reactions: the reductive carboxylation of ribulose
1,5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate and the oxygenation of
RuBP to form one molecule of 3-PG and one molecule of 2-phosphoglycolate.