14.3 roteinnBased Organelles 285of the shell. Depending on the BMC, there are thousands of copies of protomers
(i.e., ~2000 RuBisCO (ribulose 1,5‐bisphosphate carboxylase/oxygenase) mono
mers in a CB) targeted to the lumen [37]. The specific mechanisms of targeting
are discussed in greater detail later.
Before delving into the prospects of reengineering BMCs, it is important to
understand their function in the native context. The CB was the first BMC to be
discovered and characterized, and it remains the paradigm for BMC function
[40]. We give an overview of CB function here to highlight themes of BMC func
tion (Figure 14.3b). Organisms that assimilate carbon using the Calvin–Benson
cycle must compensate for the low affinity of RuBisCO for CO 2 and for its prom
iscuity – RuBisCO can also fix O 2 in the same reaction at a cost to the cell. To
overcome these limitations, cyanobacteria and many chemoautotrophs employ a
carbon‐concentrating mechanism, which consists of inorganic carbon trans
porters to increase intracellular bicarbonate levels, and the CB to facilitate car
bon fixation [41, 42]. After bicarbonate is actively transported into the cell, it
passively crosses the CB shell (details on this later in text) and enters the CB
lumen. The CB encapsulates two enzymes, carbon anhydrase and RuBisCO.
Carbonic anhydrase interconverts bicarbonate into CO 2 and OH−, and RuBisCO
fixes this CO 2 onto ribulose 1,5‐bisphosphate, which must also enter the lumen,
and produces two molecules of 3‐phosphoglycerate. 3‐Phosphoglycerate then
diffuses out of the CB and enters the reductive phase of the Calvin–Benson
cycle. Although modeling indicates the major mechanism benefiting the reac
tion is an increased local concentration of CO 2 to improve the catalytic rate
[23], additional possible mechanisms include excluding the competing substrate
O 2 from the lumen, improving CO 2 channeling from carbonic anhydrase to
RuBisCO via tight clustering of the enzymes [43], and raising the local pH around
RuBisCO to increase its catalytic activity (Figure 14.3c). Most of the experimen
tal evidence for these hypotheses is indirect, for example, catalytically dead
carbonic anhydrase mutants require high CO 2 concentrations to grow [30],
suggesting further physiological experiments will be needed to describe the
actual mechanism(s) used by the CB to facilitate carbon fixation. Finally, it is
important to note that CB comes in two forms, the so‐called α and β type [44].
They are differentiated by sequence in their shell and cargo proteins, particularly
carbonic anhydrase, and by their genomic organization. In general, genes for α‐
CBs occur together in a single operon in the genome, while the β‐CB regulon is
composed of genes spread across the genome. Despite these evolutionary differ
ences, their catalytic activity is the same and their physiological role is assumed
to be similar [45].
14.3.1.1 Targeting
Although the shell is the defining feature of BMCs, it is the targeting of cargo that
endows function. Targeting is mediated via protein–protein interactions and
probably occurs concurrently with assembly of the shell itself. The first direct
evidence for a shell‐interacting motif was found in the PDU [46]. In this case,
bioinformatic analysis of the propionaldehyde dehydrogenase (PduP) reveals an
N‐terminal extension found only in PDU‐containing organisms. Alanine scan
ning, among other biochemical experiments, has shown this putative α‐helical