Malic Acid Production By Aspergillus Oryzae

Aspergilli are widely used as cell factories for the production of food ingredients, enzymes and antibiotics. The advancement of methods for performing directed genetic modifications has enabled the use of metabolic engineering in order to improve the organisms performance. With the recent availability of genome sequences for several industrially relevant Aspergilli it has become possible to implement systems biology tools for advancing metabolic engineering.

Malic acid belongs to the group of C₄ dicarboxylic acids, which are structurally similar to maleic acid and maleic anhydride, important building blocks of the petrochemical industry. The C₄ dicarboxylic acids are therefore expected to replace petro chemically derived compounds in the future, when increased oil and gas prices favor the production of chemicals from renewable feedstocks. In previous studies A. flavus wildtype strains have shown great malic acid secretion capacities and are among the best C₄ dicarboxylic acid producers. The cultivation conditions for high malic acid production were optimized and nitrogen limitation was identified as main factor for reaching high malic acid yields. Nevertheless, A. flavus is known to produce the mycotoxin aflatoxin and is consequently disqualified for large scale industrial production. Lately it has been shown that the wildtype A. oryzae strain NRRL3488 exhibits similar production capabilities and was therefore chosen for futher optimization of its metabolism towards malic acid production.

In this study, we present the physiological characterization of an engineered A. oryzae strain in which the reductive cytosolic TCA branch from pyruvate via oxaloacetate to malate and a malate transporter have been overexpressed. The use of glucose and xylose as carbon sources in this study bridge over to the use of lignocellulosic biomass in future applications of this organism in a truly sustainable biorefinery.