2.4. Other carboxylases
In addition to RuBisCO and PEPc there are other carboxylases in
cyanobacteria. Acetyl-CoA carboxylase catalyzes the first step of fatty
acid synthesis, an important pathway providing lipids for the membrane
structure [61]. However, Davis et al. (2000) [62] showed that not only
the first enzyme contributes to determine the production of fatty acids
but also further steps in the biosynthetic fatty acid pathways are involved
in the regulation. The cyanobacterial acetyl-CoA protein is a multisubunit
protein composed of four different polypeptides. Mechanistically, ATP
and HCO3
− bind to the biotin carboxylase subunit (CT) (encoded by
accC) followed by biotin interacting with the BCCP subunit (encoded
by accB). The CT catalyzes both the dehydration of HCO3
− and the attachment
of a carboxy group to the biotin. Finally, the carboxyltransferase
subunit (a tetramer formed by dimers of the CT-β subunit (encoded by
accD) and dimers of the CT-α subunit (encoded by accA)) uses acetylCoA
and CO2 (attached to the biotin — BCCP subunit) as substrates to
form malonyl — CoA [63].
Carbamoyl phosphate synthetase (CPS) is catalyzing the formation of
carbamoyl phosphate (CP). In prokaryotes, CP is the precursor of
C. Durall, P. Lindblad / Algal Research 11 (2015) 263–270 267
pyrimidine and arginine biosynthesis [64]. In cyanobacteria, the formation
of arginine plays an important role as one of the amino acids of
the nitrogen rich storage compound cyanophycin. Cyanophycin is a copolymer
formed by aspargine and arginine [65]. This co-polymer is an
important compound due to the fact that it stores nitrogen when nonlimiting
nitrogen levels are present [65]. CPS is regulated by metabolic
intermediates of the pyrimidine and arginine biosynthesis. For instance,
CPS is activated by purines but inhibited by pyrimidines [65].
Cyanobacterial CPS studied so far are composed of two subunits [65].
The N-terminus of the large subunit catalyzes the formation of
carboxyphosphate from HCO3
− and ATP. Carboxyphosphate reacts with
ammonia (generated from glutamine) to form carbamate. The Cterminus
of the small subunit catalyzes the reaction to form
carbamoylphosphate using carbamate and ATP as substrates