Threonine chemical structure

It was snbseqnently discovered that the first enzyme in the pathway for isoleucine synthesis, which is threonine deaminase, was inhibited by isoleucine in an extract of E. coli. No other amino acid caused inhibition of the enzyme. Threonine deaminase is, in fact, the rate-limiting enzyme in the pathway for isoleucine synthesis, so that this was interpreted as a feedback control mechanism (Fignre 3.13(a)). Similarly it was shown that the hrst enzyme in the pathway for cytidine triphosphate synthesis, which is aspartate transcarbamoylase, was inhibited by cytidine triphosphate (Fignre 3.13(b)). Since the chemical structures of isoleucine and threonine, or cytidine triphosphate and aspartate, are completely different, the qnestion arose, how does isolencine or cytidine triphosphate inhibit its respective enzyme The answer was provided in 1963, by Monod, Changenx Jacob. 

Table 1.2. CHEMICAL STRUCTURES OF CYCLOSPORINS A-Z Bmt = (2S, 3R, 4R, 6 -2-amino-3-hydroxy-4-methyl-6-octenoic acid (= (4R)-4-(( )-2-butenyl)-4-methyI-L-threonine) Abu = L-a-aminobuty-ric acid Nva = L-norvaline MeVal = V-methyl-L-valine MeLeu = A -methyl-L-leucine. 

Figure 1. Chemical structure of (A) actinomycin D (key Thr, threonine Val, valine Pro, proline Sar, sarcosine and MeFal, methylvaline) (B) adriamycin and daunorubicin (C) DHAQ (D) NQO and (E) HAAF.

Peptide hormones are produced by the endocrine glands (pituitary, thyroid, pineal, adrenal, and pancreas) or by various organs such as the kidney, stomach, intestine, placenta, or liver (Table 3.4). Peptide hormones can have complex, convoluted structures with hundreds of amino acids. Figure 3.2 illustrates the chemical structure of human insulin and its three-dimensional shape. Insulin is made of two amino acid sequences. The A-Chain has 21-amino acids, and the B-Chain has 30-amino acids. The chains are linked together through the sulfur atoms of cysteine (Cys). Peptide hormones are generally different for every species, but they may have similarities [11]. Human insulin is identical to pig insulin, except that the last amino acid of the B-Chain for the pig is alanine (Ala) instead of threonine (Thr) (lUPAC and lUBMB) [9] and [11]. 

Fig. 3. Chemical structure of actinomycin Cj (D). - Abbreviations MeVal, methyl-valine sar, sarcosine Pro, proline Val, valine Thr, threonine.

FIGURE 3.5 Chemical structure of the essential amino adds, (a) Lysine, (b) Tryptophan, (c) Methionine, (d) Threonine, (e) Phenylalanine, (f) Leudne. (g) Isoleudne. (h) Valine, (i) Histidine. 

The (2S, 3R) stereoisomer is the non-superimposable mirror image of the (2R, 3S) stereoisomer and so these structures are also enantiomers having the same chemical and physical properties. Each set of enantiomers is called a diastereomer. Diastereomer are not mirror images of each other and are completely different compounds with different physical and chemical properties. Thus, threonine has two asymmetric centres, i.e. there are two possible diastereomers consisting of two enantiomers each, making a total of four stereoisomers. As the number of asymmetric centres increases, the number of possible stereoisomers and diastereomers increases. For a molecule having n asymmetric centres, the number of possible stereoisomers is 2" and the number of diastereomers is 2n J. 

Schraml et al.(137) found that the Si resonances of a series of silyl ester, alkoxy-silyl, and amino-silyl derivatives appear in different regions of the spectrum. On the basis of this information they suggested that Si NMR can be used for the structure elucidation of silylated hydroxy- or aminoacids. The silicon chemical shifts for DL-serine [55] and DL-threonine [56 ] are shown in ppm. The variation in the shielding for the alkoxy-silyl group demonstrates the previously noted sensitivity of Si NMR to the nature of R in Mc3SiOR. (140)... 

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