Biology Reference
In-Depth Information
major reactions: carboxylation, reduction, and regeneration. The carboxylation part
can be summarized as:
3 Ribulose-1,5-bisphosphate (RuBP)
þ
3CO
2
þ
H
2
O
þ
2
3 Phosphoglycer-
ate (PGA)
The chloroplast enzyme ribulose1,5 bis-phosphate carboxylase/oxygenase
(RUBISCO) is central in the carboxylation phase. The enzyme has a complex
structure and is formed by eight large catalytic subunits of approximately 53 kDa
encoded in the chloroplast genome and eight small 15 kDa mass peptides encoded in
the nucleus. Different types of RUBISCO (I, II, III, and IV), which vary in different
groups of photosynthetic organisms according to their kinetic characteristics, have
been recognized (Tabita et al.
2008
). Only variants of the type I have been found in
seaweeds (Raven
1997
). Although the enzyme is dispersed in the chloroplast stroma,
a portion of the RUBISCO is located in proteinaceous bodies denominated pyrenoids,
which are present in many seaweed groups (McKay et al.
1991
). The role of
pyrenoids in carbon metabolism is still not well understood; however, the evidence
that other enzymes (e.g., RUBISCO activases) are also located in these bodies
suggests its functional involvement in carbon fixation in the chloroplast.
The Calvin-Benson cycle is autocatalytic, i.e., its reactions generate the biochemi-
cal intermediates, which enhance the rate of carbon fixation in the case of increase in
its concentration. It has been demonstrated that thallus regions of seaweeds with
higher relative abundances in RUBISCO exhibit the highest rates of carbon fixation
(Cabello-Pasini andAlberte
2001b
). On the other hand, activity of RUBISCO is highly
dependent on the environmental factors (Raven
1997
; Bischof et al.
2002
). During the
photosynthetic carboxylation, CO
2
is incorporated into the carboxyl group of the
RuBP to form an unstable intermediate (enediol) and finally 3-phosphoglycerate
(PGA), which is the first and most important compound of the cycle that is labeled
in presence of
14
C(Calvin
1956
; Beer and Israel
1986
). In the reduction phase, the
main reaction is the conversion of PGA in glyceraldehyde 3-phosphate (GAP) through
the use of ATP and NADPH formed in the photochemical reactions by GAP-
dehydrogenase (GAP-DH). Due to this dependence, a feedback between both pro-
cesses has been examined in some seaweeds, mainly in relation with environmental
factors that affect thylakoid membranes (Bischof et al.
2002
). The third stage of
the carbon fixation process is the regeneration that allows maintaining the operation
of the cycle through the constant supply of RuBP frommolecules of triose phosphate.
In the ATP-consuming process, which includes isomerization, condensation,
hydrolization, and phosphorylation reactions, three molecules of RuBP are formed
from five molecules of triose phosphate (Nelson and Cox
2004
).
2.3.2 Rates of Carbon Fixation
Rates of RUBISCO-catalyzed carbon fixation measured using radioactive carbon
isotopes (H
14
CO
3
) vary in seaweeds depending on different environmental and
endogenous factors. Values compiled in Table
2.1
indicate that, irrespective of taxa
