The production cost of itaconic acid remains high. However, economic feasibility may be improved through optimization of either fermentation or chemical synthesis methods. Some options may include using sugarcane molasses, a sugar- and aconitic acid-rich feedstock, for chemical or enzyme-based decarboxylation or for improved fermentation conditions. Optimized fermentation conditions may include the use of inexpensive feedstocks such as agricultural waste products or through metabolic engineering of microbial strains to eliminate unwanted pathways and directly increase carbon flux toward itaconic acid production.
Some fungi naturally convert aconitic acid to itaconic acid through decarboxylation. For example,
Aspergillus terreus is used in fermentations to produce itaconic acid
[43,44,45,71][5][6][7][8]. In
A. terreus, CAA is first transported by a mitochondrial transporter, At_MttA, from the tricarboxylic acid (TCA) cycle in the mitochondria to the cytosol, where CAA is decarboxylated to itaconic acid by the
cis-aconitic acid decarboxylase Cad-A
[43,46,72][5][9][10]. Subsequently, itaconic acid is transported out of the mycelia by a specific transporter, belonging to the major facilitator superfamily (MFS) type transporter (MfsA)
[73][11]. The genes involved in itaconic acid production in
A. terreus also include a transcription factor, and the four genes are termed the “itaconate gene cluster”
[73][11]. However, in the corn smut basidiomycete,
Ustilago maydis, the mitochondrial transporter Um_Mtt1, also transports CAA from the mitochondria to the cytoplasm, but CAA is first isomerized to TAA by aconitate-Δ-isomerase, Adi1, then decarboxylated by
trans-aconitic acid decarboxylase, Tad1, to itaconic acid
[43,46,47,74][5][9][12][13]. The “itaconate gene cluster” in
U. maydis is induced upon nitrogen limitation
[75][14]. Overall, the metabolic differences in itaconic acid production by these two fungi is notable because it demonstrates the possibility of producing itaconic acid from either the
trans or
cis isomer of aconitic acid in different organisms
[47][12]. Differential microbial conversions of CAA and TAA to itaconic acid may also provide insight into
ex vivo enzymatic conversion options with recombinantly expressed decarboxylases.