Threonine discovery

Amino acids are the main components of proteins. Approximately twenty amino acids are common constituents of proteins (1) and are called protein amino acids, or primary protein amino acids because they are found in proteins as they emerge from the ribosome in the translation process of protein synthesis (2), or natural amino acids. In 1820 the simplest amino acid, glycine, was isolated from gelatin (3) the most recendy isolated, of nutritional importance, is L-threonine which was found (4) in 1935 to be a growth factor of rats. The history of the discoveries of the amino acids has been reviewed... 

Single protein kinases such as PKA, PKC, and Ca +-calmodulin (CaM)-kinases, which result in the phosphorylation of serine and threonine residues in target proteins, play a very important role in hormone action. The discovery that the EGF receptor contains an intrinsic tyrosine kinase activity that is activated by the binding of the hgand EGF was an important breakthrough. The insuhn and IGF-I receptors also contain intrinsic... 

The mitogen-activated protein kinase cascade is second-messenger-independent. Although the second-messenger-dependent protein kinases were identified first as playing an important role in neuronal function, we now know that many other types of protein serine-threonine kinase are also essential (Table 23-1). Indeed, one of the most critical discoveries of the 1990s was the delineation of the mitogen-activated protein kinase (MAP kinase or MAPK) cascades. 

Protein phosphorylation is one of the most important mechanisms in the regulation of cellular function. Proteins can be phosphorylated on serine, threonine or tyrosine residues. Most phosphorylation occurs on serine and threonine, with less than 1% on tyrosine (see Ch. 23). This perhaps accounts for the late discovery of tyrosine phosphorylation, which was found first on polyoma virus middle T antigen in 1979 by Hunter and colleagues [1,2]. 

Since the discovery of the CBS catalyst system, many chiral //-amino alcohols have been prepared for the synthesis of new oxazoborolidine catalysts. Compounds 95 and 96 have been prepared93 from L-cysteine. Aziridine carbi-nols 97a and 97b have been prepared94 from L-serine and L-threonine, respectively. When applied in the catalytic borane reduction of prochiral ketones, good to excellent enantioselectivity can be attained (Schemes 6-42 and 6-43). 

Isolation of individual amino acids started about 1820 by 1904 all of the naturally occurring amino acids in proteins had been isolated except methionine (Mueller, 1922) and threonine (Rose, 1937). One of the earliest methods for the separation of amino acids was through the differential volatility of their methyl or ethyl esters (Emil Fischer, 1901). This approach led to the discovery of valine, proline, and hydroxyproline. [In the 1970s Fischer s method was modified for microanalysis of proteins, separating the amino acid esters by gas phase chromatography. Separation is now usually performed by hplc (high pressure liquid chromatography).]... 

In comparison to the level of cellular serine or threonine phosphorylation, protein tyrosine phosphorylation occurs at quite low levels in normal cells but dramatically increases upon oncogenic transformation or stimulation. Since the first discovery in 1978 that the transforming protein from Rous sarcoma virus (pp60vsrc) exhibited intrinsic kinase activity/5 protein kinase activity has also been shown to be inherent to other growth factor receptors such as epidermal growth factor receptor and the insulin receptor,[6 91 and to involve autophosphorylation processes. The diverse biochemical activity exhibited by protein tyrosine phosphorylation has stimulated the development of chemical methods for the preparation of phosphorylated peptides for use as substrates in elucidating the biochemical and physiological activity of phosphorylated site(s). 

Because of their unique biological functions, a significant and growing part of new drug discovery and development is focused on this class of biomolecules. Their biological functions are determined by the exact arrangement, or sequence of different amino acids in their makeup. There are 20 naturally occurring amino acids, 8 of which are essential amino acids, namely, l-isoleucine, L-leucine, L-lysine, L-methionine, L-phenylalanine, L-valine, l-threonine, and L-tryptophan. 

Another important family of kinases for drug discovery is the mitogen-activated protein kinases (MAPKs). These are proline-directed serine/threonine kinases that activate their substrates by dual-phosphorylation. MAPK enzymes are activated by a variety of signals including growth factors and cytokines, discussed in chapter 6. The MAPK family plays a critical role in cell cycle progression. Small molecule inhibitors of MAPK may have utility in the treatment of cancer. 

All of the threonine stereoisomers 19-22 are chiral substances that is, they are not identical with their mirror images. However, it is important to recognize that not all diastereomers are chiral. To illustrate this point, we return to the tartaric acids mentioned previously in connection with Pasteur s discoveries (Section 5-1C). 

McCoy RH, Meyer CE, Rose WC. 1935. Feeding experiments with mixtures of highly purified amino acids. Vlll. Isolation and identification of a new essential amino acid. J Biol Chem 112 283-302. (Reports the discovery of the final lAA, threonine)... 

Davidson and Wiggins (1956) isolated it from ammoniated molasses a few years before its discovery in coffee. It was identified in the pyrolysis products of amino acids by Kato et al. (1973a,b), in the pyrolysis products of trigonelline by Viani and Horman (1974), and in model reactions involving serine and/or threonine with or without sucrose under coffee-roasting conditions by Baltes and Bochmann (1987d). Mottram (1991) explained the possible formation of 2-methylpyridine by reaction of ammonia with 2,4-hexadienal. 

Conformational restriction played an important role in the discovery that water induces the bioactive conformation of CsA. Cyclosporin A (Sandimmune , CsA, (23.37), Fig. 23.7), is a major drug for preventing rejection of transplanted human organs and has been the subject of many synthetic, conformational and mechanism of action studies. To produce immunosuppression, CsA first binds to cyclophilin A (CyP A), a peptidyl prolyl cis-trans isomerase (PPIase), to form the CsA-CyP complex, which then binds to and inhibits calcineuiin (CaN), a calmodulin-dependent serine/threonine protein phosphatase, thereby inhibiting interleukin-2 (IL-2) synthesis. ... 

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