Synthetic Biology Parts, Devices and Applications

14.4 ­ipidnBased Organelles 293

14.4.1 Repurposing Existing Organelles

14.4.1.1 The Mitochondrion
The mitochondrion is the site of oxidative metabolism within the eukaryotic cell,
facilitating both the citric acid cycle and β‐oxidation of fatty acids, and is involved
in numerous critical cell processes, including apoptosis [95]. Its function revolves
around metabolism, and it possesses a singular chemical environment – relatively
high pH (~8), low oxygen concentration, and a reducing redox environment [96].
Besides its commonly associated pathways, the mitochondrion also assists in sev­
eral other biosynthetic pathways including iron–sulfur cluster biogenesis, heme
biosynthesis, and, surprisingly, type II fatty acid synthesis [95, 97]. An understand­
ing of mitochondrial biogenesis is still a work in progress, but its central role in
metabolic diseases has led to a new appreciation of mitochondrial biology.
Proteomics has revealed the parts list of mitochondrial components and putative
pathways and also led to a deeper understanding of relevant synthetic biological
issues such as protein targeting [98].
Recently, the mitochondria’s unique catalytic potential has been leveraged for
metabolic engineering approaches. Farnesyl diphosphate (FDP) is a 15‐carbon
metabolic intermediate in the isoprenoid pathway. It is synthesized from the two
isomers: isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate
(DMAPP). Once synthesized, FDP can be processed by so‐called sesquiterpene
synthases into numerous products including molecules that are potential biofu­
els and pharmaceuticals [99]. Farhi and colleagues hypothesized that since FDP
stands at the intersection of isoprenoid biosynthesis, compartmentalization of
its terminal reactions may enhance production [100]. This hypothesis was cor­
rect, and targeting of a sesquiterpene synthase to the mitochondria using the
known N‐terminal targeting sequence of COX4 [101] led to a 3× increase in the
final product, valencene [100]. Mitochondrial targeting of additional steps to
produce the key intermediate, FDP, led to an additional twofold increase in final
titers. Interestingly, despite these results, it is not known whether targeting is

Mitochondria

Peroxisomes

Vacuole

De novo microdomains

Figure 14.6 Schematic of potential synthetic organelles in the budding yeast.