Terpenoid Metabolic Engineering in Photosynthetic Microorganisms

Abstract

Terpenoids are a group of natural products that have a variety of roles, both essential and non-essential, in metabolism and in biotic and abiotic interactions, as well as commercial applications such as pharmaceuticals, food additives, and chemical feedstocks. Economic viability for commercial applications is commonly not achievable by using natural source organisms or chemical synthesis. Engineered bio-production in suitable heterologous hosts is often required to achieve commercial viability. However, our poor understanding of regulatory mechanisms and other biochemical processes makes obtaining efficient conversion yields from feedstocks challenging. Moreover, production from carbon dioxide via photosynthesis would significantly increase the environmental and potentially the economic credentials of these processes by disintermediating biomass feedstocks. In this paper, we briefly review terpenoid metabolism, outline some recent advances in terpenoid metabolic engineering, and discuss why photosynthetic unicellular organisms-such as algae and cyanobacteria-might be preferred production platforms for the expression of some of the more challenging terpenoid pathways.

Keywords: cyanobacteria; diatoms; metabolic engineering; photosynthetic microorganisms; terpenoids.

Figure 1

Biosynthesis of terpenoids. The pathways have been conceptually separated into four modules, which is representative of the modularized method many metabolic engineers use to approach isoprenoid pathway engineering. Mevalonate (MVA) and methyl-D-erythritol phosphate (MEP) pathways lead to IPP (isopentenyl pyrophosphate) and DMAPP (dimethylallyl pyrophosphate) (module I). Additions of IPP produce higher-order prenyl phosphates (module II), dephosphorylation (often coincident with or followed by bond rearrangement and/or cyclisation) to form specialized terpenoid backbones (module III), chemical decorations, and other modifications to yield end products. Note that not all end products undergo decorations of the carbon skeleton.

Figure 2

Schematic representation of photosynthetic organisms as heterologous terpenoid biofactories. (A) In cyanobacteria and in algal chloroplasts, photosynthesis provides ATP and NADPH, which are, in turn, used to fix carbon dioxide. Glyceraldehyde 3-phosphate (G3P) links carbon fixation with the rest of metabolism. Pyruvate (PYR) is produced from G3P via glycolysis and these two metabolites are the precursors for the MEP pathway. (B) In eukaryotic algae, mitochondria produce acetyl-CoA (Ac-CoA) from which the MVA pathway is initiated and IPP/DMAPP are produced. Chloroplasts also contribute to the terpenoid precursor pool. Metabolite exchange between cellular compartments, which is not yet fully elucidated, makes terpenoid metabolism more complex.