Figure 1. Problems solved by metabolic engineering. Improving the production of a product in a cell along an existing pathway (a) or by adding a new pathway (b) (given by red arrows). It may also include improvement of existing transport systems in and out of the cell membrane or the addition of new transport systems. Each arrow within the cell is a reaction catalyzed by an enzyme. The horizontal arrows in the cell show the shortest pathway from substrate S to product P. The arrows through the cell membrane represent transport systems for S and P; subscripts o and i denote their extra- and intracellular concentrations, respectively. The substrate S is generally a 6- or 5-C sugar derived from polysaccharides.

Figure 2. Basic molecular structure of biological processes. The information stored in DNA is transcribed into messenger molecules (mRNA), which in turn encode the synthesis of proteins on the ribosomes. Most of the proteins produced act as catalysts for the reactions of the metabolic network. The entirety of the DNA information is termed genome; for microorganisms it contains typically about 5000 genes. The proteome comprises about 4000 proteins, and roughly 2000 metabolites can be identified as comprising the metabolome (Deckwer et al., 2006).

Figure 3. Enzymatic reactions that were implemented in E. coli to direct carbon flow from glucose to glycerol (a) and from glycerol to 1,3-propanediol (b).

Figure 4. Engineered pathways for production of fatty acid alkyl esters (FAAE), fatty alcohols, and wax esters in E. coli. Overexpression of thioesterases (TES), acyl-CoA ligases (ACL), and deletion of β-oxidation (ΔfadE) enhanced free fatty acid (FFA) and fatty alcohol production. FAR: fatty acid reductase AT: acyl transferase; pdc: pyruvate decarboxylase; adhB: alcohol dehydrogenase.

Figure 5. Graphic illustration of a CBP microorganism(van Zyl et al., 2007, reproduced with permission from Springer Berlin/Heidelberg).

Figure 6. Biocatalyst for d-mannitol formation from fructose in a whole-cell biotransformation (GLF: glucose facilitator) (Kaup et al., 2003).

Figure 7. Whole-cell biotransformation of d-glucose to 2,5-diketo-d-gluconic acid and 2-keto-l-gulonic acid (serving as precursor for l-ascorbic acid, vitamin C) by a recombinant strain of Erwinia sp. (in solution, glucose and sugar acids are present in equilibrium as cyclic pyranose and lactones, respectively) (Grindley et al., 1988).

Figure 8. Engineered whole cells of P. pastoris can utilize methanol as substrate for carbon metabolism and for redox equivalents required for recombinant enantioselective redox enzymes from different sources to produce chiral intermediates (Hartner, 2007; Schroer et al., 2010).