Threonine formation

W. Stbcklein and H.-C. Schmidt, Evidence for L-threonine cleavage and allo-threonine formation by different enzymes from Ciostridium pasteurianium Threonine aldolase and serine hydroxymethyltransferase, Biodiem. ]., 232 621 (1985). 

Subsequently a variety of evidence appeared that L-asparate was the precursor of both homoserine and threonine. Delluva (97) foimd that the label of aspartate-3-C added to the incubation medium of EschericMa coli appeared to a marked degree in the C-3 of threonine. Hirsch and Cohen (98) observed the accumulation of L-homoserine from L-aspartate in suspensions of a mutant of E. coli blocked in the conversion of homoserine to threonine. Black and Wright (99) found that a cell-free extract of baker s yeast incubated with C -aspartate, formed labeled threonine. The reactions in the pathway of homoserine and threonine formation for aspartic acid shown in Fig. 3, however, were largely established by enzyme studies. 

The increased flavin production by E. ashbyii in the presence of threonine has led Goodwin and Pendlington (6 ) to propose formation of the o-xylene portion of riboflavin by condensation of the carbon skeletons of two threonine molecules. It has been pointed out that such a mechanism is inconastent with the label pattern of ring A obtained with acetate-l-C with A. goaaypii, in relation to known pathways of threonine formation (64). As an alternative, one could postulate threonine being cleaved to acetaldehyde and glycine by threonine aldolase 84, 86). 

Reaction of Bisglycinatocopper(II). Bisglycinatocopper(II) [13479-54-4] condenses with ahphatic aldehydes. Removal of copper from the condensate results in P-hydroxy-a-amino acid. This is a classical synthetic method of DL-threonine, but the formation of i //o-isomer is unavoidable. 

Many kinds of amino acids (eg, L-lysine, L-omithine, t-phenylalanine, L-threonine, L-tyrosine, L-valine) are accumulated by auxotrophic mutant strains (which are altered to require some growth factors such as vitamins and amino acids) (Table 6, Primary mutation) (22). In these mutants, the formation of regulatory effector(s) on the amino acid biosynthesis is genetically blocked and the concentration of the effector(s) is kept low enough to release the regulation and iaduce the overproduction of the corresponding amino acid and its accumulation outside the cells (22). 

The a subunits, for which two isoforms exist in mammals (al, a2), contain conventional protein serine/threonine kinase domains at the N-terminus, with a threonine residue in the activation loop (Thr-172) that must be phosphorylated by upstream kinases (see below) before the kinase is active. The kinase domain is followed by an autoinhibitory domain, whose effect is somehow relieved by interaction with the other subunits. The C-terminal domain of the a subunit is required for the formation of a complex with the C-terminal domain of the (3 subunit, which in turn mediates binding to the y subunit. The al and a2 catalytic subunit isoforms are widely distributed, although a2 is most abundant in muscle and may be absent in cells of the endothelial/hemopoietic lineage. 

Threonine. Threonine is cleaved to acetaldehyde and glycine. Oxidation of acetaldehyde to acetate is followed by formation of acetyl-CoA (Figure 30-10). Catabolism of glycine is discussed above. 

Kinases are enzymes that place a phosphate group on a serine/threonine or a tyrosine residue of a protein or peptide. All kinase reactions use ATP as the phosphate source. Therefore there have been assays developed that monitor the loss or gain of the peptide/protein substrate (LANCE, ULight) [23], the loss of ATP (easylite luminescence kinaseGlo, Perkin Elmer) [20], or the gain of ADP (Tran-screener TR-FRET) [24]. Many of these formats are applicable to cell based assays. 

The synthesis93 of N-(2,4-dinitrophenyl)-3-0-(tetra-0-acetyl-/ -D-glu-copyranosyl)-L-threonine methyl ester (131) involved a two-step procedure. First, formation of the intermediate, L-threonine orthoester 130 was achieved by treatment of tetra-O-acetyl-a-D-glucopyranosyl bromide (128) with the methyl ester of N-(2,4-dinitrophenyl)-L-threonine94 (129) under the conditions of the Koenigs-Knorr reaction (see next paragraph), and this was then converted into the L-threonine glycoside 131. 

The catechol-type ligand appears to be restricted to siderochromes derived from prokaryotic microorganisms. Klebsiella oxytoca, an organism closely related to members of the genus Aerobacter, forms the 2,3-dihy-droxy-N-benzoyl derivates of serine and threonine in three day cultures (72). It is not known if the latter amino acid occurs in trimers but examination of space-filling CPK models does indicate that enterobactin could accomodate a methyl substituent on the carbon of the serine residue. Catechols occur in higher protist organisms but their formation... 

A classical approach to driving the unfavorable equilibrium of an enzymatic process is to couple it to another, irreversible enzymatic process. Griengl and coworkers have applied this concept to asymmetric synthesis of 1,2-amino alcohols with a threonine aldolase [24] (Figure 6.7). While the equilibrium in threonine aldolase reactions typically does not favor the synthetic direction, and the bond formation leads to nearly equal amounts of two diastereomers, coupling the aldolase reaction with a selective tyrosine decarboxylase leads to irreversible formation of aryl amino alcohols in reasonable enantiomeric excess via a dynamic kinetic asymmetric transformation. A one-pot, two-enzyme asymmetric synthesis of amino alcohols, including noradrenaline and octopamine, from readily available starting materials was developed [25]. 

Moser et al. (1968) (one of the co-authors was Clifford Matthews) reported a peptide synthesis using the HCN trimer aminomalonitrile, after pre-treatment in the form of a mild hydrolysis. IR spectra showed the typical nitrile bands (2,200 cm ) and imino-keto bands (1,650 cm ). Acid hydrolysis gave only glycine, while alkaline cleavage of the polymer afforded other amino acids, such as arginine, aspartic acid, threonine etc. The formation of the polymer could have occurred according to the scheme shown in Fig. 4.9. 

III. 3-6>-Glycopeptides of l-Serine or l-Threonine 1. Formation of the 3-O-Glycosidic Linkage... 

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