Threonine racemase

PLP-dependent enzymes catalyze the following types of reactions (1) loss of the ce-hydrogen as a proton, resulting in racemization (example alanine racemase), cyclization (example aminocyclopropane carboxylate synthase), or j8-elimation/replacement (example serine dehydratase) (2) loss of the a-carboxylate as carbon dioxide (example glutamate decarboxylase) (3) removal/replacement of a group by aldol cleavage (example threonine aldolase and (4) action via ketimine intermediates (example selenocysteine lyase). 

Serine Racemase (EC 5.1.1.16] Serine racemases have been discovered in both bacteria and eukaryotes (for a review see [60, 62). In the latter organisms, serine racemase catalyzing the conversion of L-Ser to D-Ser was at first discovered in the silkworm Bombyx mori it is a PLP-dependent racemase which is also active on L-Ala (-6% of the activity on L-Ser). A serine racemase was also purified from rat brain (and a serine racemase cDNA was cloned from mouse brain). Mammalian serine racemase shows sequence simUarily with L-threonine dehydratase from various sources all the active site residues of the latter enzyme are also conserved in mouse serine racemase. Mammalian serine racemase is a member of the fold-type II group of PLP enzymes (similarly to L-threonine dehydratase, D-serine dehydratase, and so on) and distinct from alanine racemase, which belongs to the fold-type III group. Mouse serine racemase shows a low kinetic efficiency the Km values for L- and D-Ser are -10 and 60 mM, respectively and the V ax values with L- and D-Ser are 0.08 and 0.37 units/mg protein (less than 0.1% of those of alanine racemase on L- and D-Ala, see above). 

L-Amino acids are the naturally occurring form both in proteins and as free amino acids, and the ability of different experimental animals to use different optical isomers of the amino acids has been studied. The chick is only able to use the L-form of threonine, isoleucine and valine but can use D- and L-leucine equally. It can use both D- and L-tryptophan, although the D-form is less effective (see Fisher, 1954). This probably relates to the presence of specific racemases that enable certain D- and L-interconversions to take place. Factors that affect the requirements for specific amino acids which are particularly pronounced in avian species are summarised in Table 2.3. 

Aspartate 4-semialdehyde, seen, for example, in Scheme 12.13, which provided a pathway for the biosynthesis of the essential amino acid methionine (Met, M) and in Scheme 12.14, which holds a representation of the biosynthesis of threonine (Thr, T), is also a place to begin to describe a pathway to lysine (Lys, K). As shown in Scheme 12.19, aspartate 4-semialdehyde undergoes an aldol-type reaction with pyruvate (CHsCOCO ") in the presence of dihydropicoUnate synthase (EC 4.2.1.52) to produce a series of intermediates that, it is presumed, lead to (5)-23-dihydropyridine-2,6-dicarboxylate. Then, dihydrodipicolinate reductase (EC 1.3.1.26) working with NADPH produces the tetrahydropyridine, (S)-2,3,4,5-tetrahydropyridine-2,6-dicarboxylate.This heterocycle, in the presence of glutamate (Glu, E) and water, is capable of transamination directly to 2-oxoglutarate and (2S, 6S)-2,3-diaminopimelate in the presence of LL-diaminopimelate aminotransferase (EC 2.6.1.83), while the latter, in the presence of the pyridoxal dependent racemase... 

D-Amino acids are frequently encountered in these bacterial constituents. The formation of D-a//othreonine can be explained by the action of racemases on the a-carbon atom of L-threonine 69), Similarly, d-fl//oisoleucine can be produced from L-isoleucine (70) (see for example peptidolipin NA or cerexin A). However, in two cases, L-a//othreonine and L-a//oisoleucine have been found no information is available on their biosynthesis. Unusual amino acids have also been observed in some of these peptidolipids, for instance pipecolic acid (amphomycin), L-threo- -hydroxyglutamic and L-t/ireo-P-methylglutamic acids (neopeptines), or D- or L- 2,4-diaminobutyric acid (octapeptines). 

Specific activity for L-2-amino butyric acid amide was 9.5U/mg, which was 2.7% that for l-ACL (350U/mg with a substrate concentration of 100 mM). Activities toward alanine amide, threonine amide, norvaline amide, and norleucine amide were all <2.1% that for ACL. The enzyme did not act on a-amino acid, peptides consisting of alanine, or alanine methyl esters. The values for 2-aminobutyramide and alanine amide were both calculated to be 1.0, which identified them as typical racemase-catalyzed reactions [23] ... 

The final unique stage in the metabolism of L-isoleucine involves the cleavage of 2-methylacetoacetyl-CoA to acetyl-CoA and propionyl-CoA (Section 10.4). The propionyl-CoA is further metabolized to methylmalonyl-CoA by a biotin-dependent carboxylase and subsequently via succinyl-CoA into the tricarboxylic acid cycle. L-Valine is also metabolized ultimately to methylmalonyl-CoA (Section 10.4), and thus these two branched-chain amino acids form the major precursors of propionyl-CoA and methylmalonyl-CoA. Other precursors of propionyl-CoA include methionine, threonine, odd-carbon-number fatty acids and cholesterol. The methyhnalonyl-CoA produced by propionyl-CoA carboxylase occurs as the D(5)-enantiomer and is racemized to the L(/ )-enantiomer by methylmalonyl-CoA racemase. l(/ )-Methylmalonyl-CoA is then metabolized to succinyl-CoA by a vitamin B12-dependent mutase prior to introduction of the modified molecule into the tricarboxylic acid cycle. 

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