Review Metabolic Engineering for Expanding the Substrate Range of Yarrowia lipolytica Rodrigo Ledesma-Amaro 1, * and Jean-Marc Nicaud 1 Economically viable biotechnology processes must be characterized by a favorable ratio between the production costs and the product market price. In the bioproduction of bulk chemicals, costs must be minimized so that the process is competitive relative to petroleum-based production. The substrate costs must thus be reduced by employing inexpensive carbon sources, such as industrial wastes. Unfortunately, the most convenient microorganisms for a bioconversion are typically unable to degrade such substrates. Fortunately, the discovery of new enzymes together with advances in synthetic biology has moved metabolic engineering forward, expanding substrate ranges. Here we review the latest advances made using the industrial yeast Yarrowia lipolytica, which can exploit various carbon sources to produce biofuels and chemicals. The Importance of Substrate Identity in Microbial Biotechnology Processes Microorganisms are used in industrial biotechnology processes to convert certain substrates into certain products. The economic viability of a given process strongly depends on the microbial cell performance (high yields, titers, and productivity), robustness, and scalability of the microbial system used, the cost of purifying the product, the cost of the substrate, and the market price of the product (Box 1). Unfortunately, the best microbial producers are often unable to bioconvert the cheapest or most convenient substrates. Moreover, the definition of the most convenient substrate is region and time dependent, since the availability of cheap substrates depends on natural resources and local industries, which in turn depend on the region and time of year. Isolation of novel microbes able to utilize complex and most-convenient substrates, followed by optimization of the bioconversion process, may appear too laborious and expensive. However, exploiting pretreated/-purified substrates, which is frequently the strategy of choice, can render the process economically unfeasible. Given the importance of cost:benefit ratios in microbial biotechnology processes, synthetic biology and, more specifically, metabolic engineering could be used to modify the best-suited microorganisms to use target substrates. An organism's enzymatic machinery could be either expanded or improved to cope with different carbon and nitrogen sources as well as with the toxic compounds often found in industrial wastes. These modifications could target extracellular or intracellular enzymes/receptors as well as the compound's transportation system (Box 2). In this review we focus on Y. lipolytica because it is a widely used biotechnological ‘chassis’ whose ability to degrade substrates has been expanded via metabolic engineering. Y. lipolytica is a widely recognized oleaginous yeast known for its superior characteristics in the production of Trends Even when bioconversion yields are high, biotechnology processes may be economically unviable because of the high and/or unstable cost of com- mon substrates. Raw, inexpensive carbon sources, usually complex sugar polymers, are preferred. Although these substrates can be pretreated using chemical/ enzymatic processes to release their subunits, all-in-one consolidated bio- processes are preferred. Most industrial microorganisms are unable to degrade raw substrates such as lignocellulosic biomass or even cer- tain monosaccharide subunits. They must therefore be engineered to fully degrade target substrates. Metabolic engineering has successfully expanded the range of simple and com- plex substrates that industrial microbes can degrade. For example, the yeast Yarrowia lipolytica has been engineered to break down carbon sources that it cannot degrade naturally. 1 Micalis Institute, INRA, AgroParisTech, Université Paris- Saclay, 78350 Jouy-en-Josas, France *Correspondence: Rodrigo.Ledesma@grignon.inra.fr (R. Ledesma-Amaro) and jean-marc.nicaud@grignon.inra.fr (J.-M. Nicaud). TIBTEC 1382 No. of Pages 12 Trends in Biotechnology, Month Year, Vol. xx, No. yy http://dx.doi.org/10.1016/j.tibtech.2016.04.010 1 © 2016 Elsevier Ltd. All rights reserved.