Cyanophycin

Cyanophycin is a nitrogen-storage polymer composed of a polyaspartate backbone with arginine side chains attached with their α-amino group to the β-carboxy group of each aspartate (Figure 30.2). It is synthesized by the enzyme cyanophycin synthase and deposited as granules in cyanobacteria as well as several photosynthetic and non-photosynthetic bacteria (Berg et al., 2000; Oppermann-Sanio and Steinbüchel, 2002). While arginine is the most common side chain attached to the polyaspartate backbone, other amino acids have also been found in cyanophycin produced in heterologous hosts, such as lysine, ornithine, or citrulline (Steinle et al., 2009).

Ethylene Diamine Tetraacetate Acid Tetrasodium Salt (EDTA-4Na)

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Figure 30.2. Structure of cyanophycin and potential products derived from cyanophycin

Reproduced with permission from van Beilen and Poirier (2008).

Although cyanophycin is not suitable for material applications, it is a useful source of polyaspartate, which can be used as a super-adsorbent or antiscalant (potential market $450 million per year; Tsao et al., 1999), replacing the chemically synthesized compound (Oppermann-Sanio and Steinbüchel). Cyanophycin could also be used as a valuable source of dipeptide and of amino acids in the nutritional and medical fields; for example, arginine has numerous physiological roles in many cardiovascular, gastrointestinal, and immune disorders (Sallam and Steinbuchel, 2010). Aspartate and other amino acids found in cyanophycin could also serve as a starting point for the synthesis of a range of chemicals that can be obtained by reductions, dehydrations, polymerization, decarboxylation, and deamination reactions (Werpy and Pedersen, 2004; Scott et al., 2007). As an example, reduction of aspartic acid would produce 3-aminotetrahydrofuran and 2-amino-1,4-butanediol, close analogs of high volume chemicals currently used in industry, while arginine could be converted to 1,4-butanediamine used to synthesize nylon-4,6 (Figure 30.2).

Expression of cyanophycin synthase in plants has been pioneered by Broer and co-workers (Neumann et al., 2005). Transgenic tobacco and potato contain cyanophycin up to 1.1% dry weight through expression of the cyanophycin synthase in the cytoplasm of leaf cells (Neumann et al., 2005), with some deleterious effects such as changes in leaf morphology and decreased growth. The adverse effects were mitigated by translocation of cyanophycin synthesis to plastids, which brought an increase in cyanophycin content up to 9.4% of dry weight in tobacco leaves (Hühns et al., 2008). Minimal effects on growth and morphology were also observed when expression of cyanophycin synthase was restricted to potato tubers. Here, up to 7.5% of dry weight was accomplished (Hühns et al., 2009). For comparison, recombinant E. coli, Saccharomyces cerevisiae, and P. pastoris could produce cyanophycin at 39%, 15%, and 23% of the cell dry weight, respectively (Hai et al., 2006; Steinle et al., 2009, 2010). Economically viable levels in plants may require optimization of the pathways involved in supplying aspartic acid and arginine, as well as engineering the cyanophycin synthase for maximal activity in the plant cell environment.