Background

L-threonine is an organic substance, the chemical formula is C4H9NO3, and the molecular formula is NH2-CH(COOH)-CHOH-CH3. L-threonine was discovered in fibrin hydrolyzate in 1935 by W·C·Ro and proved that it is the last essential amino acid to be discovered. Its chemical name is α-amino-β-hydroxybutyric acid, and there are four stereotypes. Heterogeneous, only the L-type has biological activity. L-threonine is generally packaged in a brown wide-mouth glass bottle[1]. 

Picture 1 L-threonine powders

Among the “white biotechnology” industries, production of amino acids is one of the most important in terms of volumes and annual turnover. A couple of recent reviews summarize the status quo in this branch of biotechnology (Leuchtenberger et al. 2005; Hermann 2003). The current total annual worldwide consumption of amino acids is estimated to be significantly[2].

The biggest contributors to this are amino acids like monosodium glutamate, used as flavour enhancer, or l-lysine, d,l-methionine and l-threonine that are mainly used as feed additives. Their market volume is estimated to be significantly higher than one million tons for each group: food and feed applications. The essential amino acid l-threonine, which is the focus of this review, belongs to the aspartate family of amino acids.

A feed additive

Almost exclusively used as a feed additive, l-threonine is primarily added to pig and poultry diets. While, for example, corn germ meal contains similar amounts of l-threonine (0.38%) and l-lysine (0.42%), soybean meal contains almost twice as much l-lysine (2.80%) as l-threonine (1.81%) (Degussa 2006a). The increase of the l-threonine concentration from 0.55 to 0.75% in a corn–sorghum–peanut mealbased diet for young broilers increases the breast meat deposition by more than 15% (Degussa 2006b; Kidd et al. 1999). In 2005 the l-threonine world market had a volume of about 70 000 tons. Thus, l-threonine ranks third in production volume among the biotechnologically produced amino acids behind l-lysine and l-glutamic acid. However, the growth of market size decelerates and the volatility of prices is high with a decreasing tendency (Fig. 1). This underlines the need for process improvements and a strong and sustainable strain development. Major producers of l-threonine are Ajinomoto, Archer Daniels Midland and Degussa.

Even if there are possibilities to manufacture l-threonine by chemical synthesis or separation from other amino acids after hydrolysis of protein sources, the usage of l-threonine-producing bacteria nowadays is the exclusively applied production method. The focus of this review is the description of recent progress in the production of l-threonine through usage of bacteria, especially Escherichia coli and subsequent purification of the product. In the frame of this, we deal with the applied bacterial strains, their physiology and networks of biosynthesis pathways, the production process itself and, finally, the drawbacks of physiology for the production process and vice versa. Although other processes are described and these are even in industrial use, nowadays l-threonine is usually produced by strains of E. coli. Hence, this review will focus on bioprocesses using E. coli strains to produce l-threonine. Due to the limited available information on this topic—only a few recent publications deal with the production of l-threonine–we also took previously unpublished information into consideration and introduced publications without direct connection to threonine production but with an indirect contribution[3].

The synthesis of l-threonine

The synthesis of threonine is normally tightly controlled to produce only the amount of l-threonine required to support cellular activity. Bacteria such as E. coli do not naturally synthesize excess threonine or export l-threonine into the medium. If l-threonine begins to accumulate, the cell uses several mechanisms like feedback regulation at the enzyme level or at the transcription. level to reduce its synthesis (Camajova et al. 2002b; Table 1). The l-threonine pathway is embedded into a large metabolic network, and exchange of information with the rest of the metabolic system becomes crucial for improvement of the performance of microbial strains in fermentation

Market

Bioprocesses for the production of amino acids have been developed since the end of the 1950s. In the beginning, the technologies served only a small market, but over the years the demand grew and new applications were found. With the growing market the size of the plants and the bioreactors increased stepwise. Nowadays, bioreactor sizes from 50 to 500 m3 are standard in amino acid production (depending on the product). Hence, the fermentation industry requires first of all strains that work perfectly under conditions obtained in reactor volumes up to 500 m3.

Reference

1 Xu Qingyang, Feng Zhibin, Sun Yuhua, etc. The effect of dissolved oxygen on L-threonine fermentation. 2007

2 Feng Zhibin, Wang Dongyang, Xu Qingyang, etc. Effects of nitrogen sources on L-threonine fermentation. "Chinese Journal of Bioengineering", 2006

3 Rieping, M., Hermann, T. (2006). L-Threonine. In: Wendisch, V.F. (eds) Amino Acid Biosynthesis ~ Pathways, Regulation and Metabolic Engineering. Microbiology Monographs, vol 5. Springer, Berlin, Heidelberg.