4- Hydroxy-L-threonine

Similar experiments suggested that 4-hydroxy-L-threonine (43) was an intermediate in synthesis of the three-carbon unit, C-6, C-5, C-5 (after decarboxylation). This was rigorously proved by a chemical synthesis of 4-hydroxy-L-(2,3-13C2)threonine. Incubation of E. coli mutant WG2 with this substrate produced a sample of pyridoxol that was examined by l3C NMR. The presence of doublets in the signals originating from C-5 and C-6 of pyridoxol exclusively, showed that the C-2-C-3 bond of the substrate had been incorporated intact into the predicted site (Scheme 18).42... 

Pyridoxal 5 -phosphate is an essential cofactor for many enzymes responsible for the metabolic conversions of amino acids. The fourth step in its synthetic pathway in Escherichia coli is catalyzed by the divalent metal ion-dependent enzyme 4-hydroxythreonine-4-phosphate dehydrogenase (PdxA), which converts 4-hydroxy-l-threonine phosphate (HTP) to 3-amino-2-oxopropyl phosphate. The... 

The pyridoxal biosynthetic pathway is outlined in Fig. 37. Oxidation of 196 followed by transamination gives 4-hydroxy-L-threonine-4-phosphate 199 [171]. Condensation with l-deoxy-o-xylulose (see also ubiquinone and thiamin sections) and a final oxidation gives the cofactor [172-175]. 

Banks, J. and Cane, D.E. (2004) Biosynthesis of vitamin B6 direct identification of the product of the PdxA-catalyzed oxidation of 4-hydroxy-L-threonine-4-phosphate using electrospray ionization mass spectrometry. Bioorg. Med. Chem. Lett., 14 (7), 1633-1636. 

The entire framework ofpyridoxol (vitamin Bg) (17) has been shown to be derived (in Escherichia colt) from 1-deoxy-D-xylulose (15) and 4-hydroxy-L-threonine (16) (1996JBC30426, 1995NPR555). 

A pathway to pyridoxal itself from l-deoxy-D-xylulose-5-phosphate and 4-hydroxy-L-threonine 4-phosphate will be elaborated upon in the discussion of cofactors. 

It has been shown that phosphorylation changes the local conformation of a protein and thereby affects the activity of the complete protein. 341 Phosphorylation of serine and threonine side chains often occurs (Scheme 2). Phosphoamino acids are readily characterized using 3H and 31P NMR experiments. The H and 31P NMR parameters are distinct for phosphorylated serine, threonine, and tyrosine and have also been used to identify both cis-and trans-O-phospho-4-hydroxy-L-proline. 35 Phosphorylation of Cys is rare, but it can be identified by NMR even in large proteins. 36 ... 

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. 

The occurrence of numerous secondary metabolites and the unusual structural elements l-a-aminobutyric acid, D-alanine, and the C9-amino acid (2S,3/(,4/(,6 )-2-amino-3-hydroxy-4-methyl-6-octenoic acid [(4/ )-4-(( )-2-butenyl)-4-methyl-L-threonine=Bmt] indicate that this peptide is not produced ribosomally but rather post-translationally by a multi-enzyme complex. Additionally offered foreign amino acids are incorporated, e.g., further cyclosporins are obtained in directed biosyntheses by replacement of the a-aminobutyric acid in position 2 by D- or L-allylglycine, of d-alanine in position 8 by D-setine or 3-fluoroalanine, and of butenylmethylthreonine in position 1 by L-/S-cyclohexylalanine... 

L-Threonine-8-L-phenylalanine-9-(/ra 5-4-hydroxy-L-proline)-10-L-alanine-1 6-l-glutamine-17-L-glutamic acid-1 8-l-asparagine-19-L-lysine-39-L-serine-48-L-threonine-49-L-alanineosteocalcin (human), in 0-20047... 

Prokaryotic organisms synthesise a primary form of vitamin pyridoxol 5 -phosphate from 1-deoxy-D-xylulose 5-phosphate (l-deoxy-D-t/treo-pent-2-ulose 5-phosphate) and 2-amino-2-deoxy-D-threo-tetronic (2-amino-2-deoxy-D-threonic) acid, known as 4-(phosphohydroxy)-L-threonine or 4-hydroxy-i-threonine 4-phosphate. Phosphohydroxythreonine arises from D-erythrose 4-phosphate, a product of decomposition of D-fructose 6-phosphate. Non-phosphorylated forms (pyridoxal, pyridoxol and pyridoxamine) are produced by hydrolysis of the corresponding phosphates. Animals do not synthesise vitamin Bg de novo, only convert the non-phosphorylated forms in the Hver, erythrocytes, and other tissues into the corresponding phosphates and the individual forms of each other. Pyridoxal 5 -phosphate arises by oxidation of pyridoxol 5Lphosphate and transamination of pyridoxal 5 -phosphate provides pyridoxamine 5Lphosphate. Both these forms of vitamin Bg are catalytically active. Pyridoxal 5 -phosphate... 

This type of yvn-seleclivc aldol addition has been applied in the synthesis of the unusual L-threonine based amino acid, (2,S, 3/ ,6F)-3-hydroxy-4-methyl-2-methylamino-6-octenoic acid, of cyclosporine104, of the cyclic hexapeptide echinocandin105, and of the antibiotic ionomycin97. 

The metabolism of P-hydroxy-a-amino adds involves pyridoxal phosphate-dependent enzymes, dassified as serine hydroxymethyltransferase (SHMT) (EC 2.1.2.1) or threonine aldolases (ThrA L-threonine selective = EC 4.1.2.5, L-aHo-threonine selective = EC 4.1.2.6). Both enzymes catalyze reversible aldol-type deavage reactions yielding glycine (120) and an aldehyde (Eigure 10.45) [192]. 

Primary cell-walls of dicots contain between 5 and 10% of protein that is rich (20%) in hydroxy-L-proline.4,55,228 The wall protein also contains a relatively high content of L-alanine, L-serine, and L-threonine, and this feature is found in such animal structural-proteins as collagen.228 This characteristic, amino acid composition, together with the fact that it is difficult to extract much of the protein from cell walls under nondegra-dative conditions,229 suggests that it has a structural role in the wall.228-230... 

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