Threonine uptake

E. coli An impaired L-threonine uptake system enhanced L-threonine production in E. coli L-threonine producer The inability to take up L-threonine relieved negative effect of remaining feedback inhibition 100.0 Okamoto et al. [53]... 

Okamoto K, Kino K, Ikeda M (1997) Hyperproduction of L-threonine by an Escherichia coli mutant with impaired L-threonine uptake. Biosci Biotech Biochem 61 1877-1882... 

The serine/threonine phosphatase inhibitor okadaic acid (incubation time and concentration vary from 10 nM, 30 minutes to 1 pM, 1 hour at 37°C) leads to selective stimulation of caveolae uptake (18,64). 

Experiments with 14C-labelled fructosyllysine (FL) in rats showed that this Amadori compound is not actively transported out of the intestines. Flowever, experiments with ligated gut segments indicated that uptake by passive diffusion is feasible, at least for e-FL, whereas a-FL and fructosylmethionine are poorly absorbed.342 The presence of FL affects the active transport of other compounds thus the active transport of threonine, proline, and glycine was lowered, but that of lysine, methionine, and galactose remained unaffected.343... 

As mentioned earlier, L-threonine production can be enhanced by engineering the export or uptake system. An efficient L-threonine producer strain of E. coli KY10935, which was derived from the wild-type strain by multiple rounds of random mutation and selection, was able to produce 100 g L-1 L-threonine after 77 h cultivation [53]. In this strain, the two key enzymes in the L-threonine biosynthesis (homoserine dehydrogenase and homoserine kinase) were identified to be still inhibited by much lower intracellular concentrations of L-threonine than... 

Diacetyl, and its reduction products, acetoin and 2,3-butanediol, are also derived from acetaldehyde (Fig 8D.7), providing additional NADH oxidation steps. Diacetyl, which is formed by the decarboxylation of a-acetolactate, is regulated by valine and threonine availability (Dufour 1989). When assimilable nitrogen is low, valine synthesis is activated. This leads to the formation of a-acetolactate, which can be then transformed into diacetyl via spontaneous oxidative decarboxylation. Because valine uptake is suppressed by threonine, sufficient nitrogen availability represses the formation of diacetyl. Moreover, the final concentration of diacetyl is determined by its possible stepwise reduction to acetoin and 2,3-butanediol, both steps being dependent on NADH availability. Branched-chain aldehydes are formed via the Ehrlich pathway (Fig 8D.7) from precursors formed by combination of acetaldehyde with pyruvic acid and a-ketobutyrate (Fig 8D.7). 

The process of amino acid accumulation is catalyzed by at least five different Na, amino acid-symporters transporting (i) asparagine and glutamine, (ii) arginine, lysine and histidine, (iii) alanine, glycine, serine and threonine, (iv) valine, leucine, isoleucine and methionine and (v) phenylalanine, tyrosine and tryptophane. One more symporter seems to be involved in the uptake of glutamate and aspartate. Cysteine is not transported and inhibits the transport of other amino acids (reviewed in [30]). 

Threonine and allothreonine are thermally unstable amino acids. The same is true for the simplest hydroxy amino add, (S)-serine (39). High levels of this amino add have however been reported from certain petroleum brine waters and its preservation may be due to the formation of stable chelates. Ahrens has suggested that metal-amino acid chelates may be important in the sedimentary cycle, and the uptake of certain metals in sediments, particularly in binding of metal-amino acid chelates to clays, has been proposed as a possible mechanism for the incorporation of metal ions in carbonaceous materials. Similar complexes are thought to contribute to the stabilization of amino acids in coals, lignites and peats, but, as has been outlined in a previous section, many other ligands are also present in these media, and their relative importance is difScult to assess. Much of this work is unfortunately of a speculative nature since no well-characterized complex of an amino acid has been isolated from a geological source, as far as the author is aware. 

Grunweller A, Gillen C, ErdmannVA, Rurreck J (2003) Cellular uptake and localization of a Cy3-labeled siRNA specific for the serine/ threonine kinase Pim-1. Oligonucleotides 13(5) 345-352... 

Recombinant DNA techniques were employed to improve the L-threonine producer. A threonine-deficient mutant of E. coli was transformed by the genes of threonine operon obtained from a-amino-/ -hydroxyvaleric acid (AHV)-resistant and feedback-insensitive mutants to amplify the expression of enzymes and to increase the amount of L-threonine. E. coli mutant strain was also constructed to have amplified genes of threonine operon obtained from AHV-resistant and feedback-insensitive mutant by the action of Mu phage on the chromosomal DNA. This strain is used in France in the practical production of L-threonine. The productivity of bacterial strains developed as the L-threonine producer is summarized in Table 2 [14]. L-Threonine hyperproducing E. coli mutant, which can produce 100 g/1 of L-threonine in 77 h, was constructed by Okamoto et al. who suggested that the strain has some impairment in L-threo-nine uptake function [15]. 

Figure 5. Experimental and theoretical kinetics of uptake of amino acids at 14 C. Key A> threonine , valine leucine O, proline , total amino acids.

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