Serine hydroxymethyltransferase reactions

Figure28-7. The serine hydroxymethyltransferase reaction. The reaction is freely reversible. (H4 folate, tetrahyd rofolate.)...

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]. 

FIGURE 40-3 Glycine cleavage system and some related reactions. Glycine and serine are readily interchangeable. Enzymes (1) Glycine cleavage system (2) and (4) Serine hydroxymethyltransferase (3) N5 10-methylenetetrahydrolate reductase. N5 I0-CH2-FH4, N5>10-methylenetetrahydrolate FH4, tetrahydrofolic acid. 

The enzyme serine hydroxymethyltransferase requires pyridoxal phosphate as cofactor. Propose a mechanism for the reaction catalyzed by this enzyme, in the direction of serine degradation (glycine production). (Hint See Figs 18-19 and 18-20b.)... 

Side chain cleavage (Group c). In a third type of reaction the side chain of the Schiff base of Fig. 14-5 undergoes aldol cleavage. Conversely, a side chain can be added by (3 condensation. The best known enzyme of this group is serine hydroxymethyltransferase, which converts serine to glycine and formaldehyde.211-21313 The latter is not released in a free form but is transferred by the same enzyme specifically to tetrahydrofolic acid (Eq. 14-30), with which it forms a cyclic adduct. 

A5, A1 °-Methylcnctctrahydrofolate is a hydroxymethyl-group donor substrate for several enzymes and a methyl-group donor substrate for thymidylate synthase (fig. 10.15). It arises in living cells from the reduction of A5,A10-methenyltetrahydrofolate by NADPH and also by the serine hydroxymethyltransferase-catalyzed reaction of serine with tetrahydrofolate. 

The pyridoxal-5 -phosphate dependent serine hydroxymethyltransferase (SHMT EC 2.1.2.1) in vivo catalyzes the interconversion of L-serine 158 and glycine 149 by transfer of the /1-carbon of L-serine to tetrahydrofolate (THF) by which the activated formaldehyde is physiologically made available as a C,-pool. The reaction is fully reversible and provides a means for the stereoselective synthesis of 158 in vitro from donor 149 and formaldehyde. Economical yields (88-94%) of L-serine have thus been obtained on a multimolar scale using raw cell extracts of recombinant Klebsiella aerogenes or E. coll in a controlled bioreactor at final product concentrations > 450 gl 1 [461,462], Several SHMTs have been purified and characterized from various organisms including animal tissues [463,464], eucaryotic [465] and procaryotic... 

Serine hydroxymethyltransferase is a PLP-dependent aldolase. It catalyzes interconversion between glycine and various P-hydroxy-a-amino acids, such as serine and threonine, via formation of a quinoid intermediate derived from PLP with the amino acid substrate (Scheme 2.9). This aldolase-type reaction is of interest as an asymmetric synthesis of a-amino acids via C-C bond formation. 

Serine is formed from 3-phosphoglycerate (Fig. 15-3). Serine is also synthesized from glycine in a reaction catalyzed by serine hydroxymethyltransferase ... 

Cells making DNA must also be able to make deoxythymidine triphosphate (dTTP). The key step in the synthesis of dTTP is the conversion of dUMP to dTMP via thymidylate synthase. The reaction requires a source of N5,Nw-methylene tetrahydrofolate (see Sec. 15.7, Fig. 15-19) to provide the methyl group. In this reaction, the tetrahydrofolate is oxidized to dihydrofolate. Dihydrofolate must be reduced to tetrahydrofolate via the enzyme dihydrofolate reductase so that more Af5,A,l0-methylene tetrahydrofolate can be made from serine in a reaction catalyzed by serine hydroxymethyltransferase. These three reactions, which are essential for the formation of dTMP, are shown below. 

With the exception of N5-methyl-THF, the THF derivatives are directly synthesized from a C unit in the appropriate oxidation state, and THF. The major anaplerotic reaction is that catalyzed by serine hydroxymethyltransferase (Sec. 15.1). 

The glycine-dependent aldolases are pyridoxal 5-phosphate dependent enzymes that catalyze the reversible aldol reaction, where glycine and an acceptor aldehyde form a (i-hydroxy-a-amino acid (Scheme 5.47).74 Serine hydroxymethyltransferases, SHMT (EC 2.1.2.1), and threonine aldolases, two types of glycine dependent aldolases, have been isolated. In... 

Transamination Reactions of Other Pyridoxal Phosphate Enzymes Inaddition to theirmainreactions, anumberofpyridoxalphosphate-dependent enzymes also catalyze the half-reaction of transamination. Such enzymes include serine hydroxymethyltransferase (Section 10.3.1.1), several decarboxylases, and kynureninase (Section 8.3.3.2). 

Disposal of Surplus One-Carbon Fragments With the exception of serine hydroxymethyltransferase (Secdon 10.3.1.1), aU of the reactions shown in Figure 10.4 as sources of one-carbon subsdtuted folates are essentially catabolic reactions. When there is a greater entry of single carbon units into the folate pool than is required for biosynthetic reactions, the surplus can be oxidized to carbon dioxide byway of 10-formyl-tetrahydrofolate, thus ensuring the avaUabUity of tetrahydrofolate for catabolic reactions. 

Serine Hydroxymethyltransferase Serinehydroxymethyltrans-ferase is a pyridoxed phosphate-dependent aldolase that catalyzes the cleavage of serine to glycine and methylene-tetrahydrofolate (as shown in Figure 10.5). Serine is the major source of one-carbon substituted folates for biosynthetic reactions. At times of increeised cell proliferation, the activities of serine hydroxymethyltransferase emd the enzymes of the serine biosynthetic pathway cue increased. The other product of the reaction, glycine, is also required in increased cimounts under these conditions (for de novo synthesis of purines). 

Figure 10.5. Reactions of serine hydroxymethyltransferase (EC 2.1.2.1) and the glycine cleavage system (EC 2.1.2.10). THE, tetrahydrofolate.

H. Y. Hsiao and T. T, Wei, Stabilization of tetrahydrofoUc acid and serine hydroxymethyltransferase in reaction mixtures for the synthesis of L-serine, European latent Application 84309137,2, 1984,... 

A transferase that also has aldolase activity and has been used to prepare a number of chiral compounds is the enzyme serine hydroxymethyltransferase (SHMT) (EC 2.1.2.1). This enzyme, also known as threonine aldolase, catalyzes the physiological reaction of the interconversion of serine and glycine with pyridoxal phosphate (PLP) and tetrahydrofolate (FH4) as the shuttling cofactor of the C-1 unit. It also catalyzes a number of other reactions, some of which are independent of PLP and FH4 [72]. The SHMT-catalyzed aldolase reaction generates two stereocenters, which it does stereospecifically at the (/.-carbon, whereas it is less strict at the [l-carbon (Scheme 13). Nevertheless, this enzyme from porcine liver, Escherichia coU and Candida humicola (threonine aldolase) has been used to prepare a number of P-hydroxy-a-amino acids [73-76],... 

Tetrahydrofolate (Fig. le) and derivatives (Fig. If, g) are the biologically active forms of folate. This cofactor is involved in many, distinct enzymatic reactions, ranging from the amino acid metabolism, such as serine hydroxymethyltransferase (SHMT) (Fig. 5a), to nucleotide biosynthesis, such as thymidylate synthase (TS) (Fig. 5b) and dihydrofolate reductase DHFR (Fig. 5c). These enzymes are targets for anticancer drugs because they participate in the formation of thymidylate, the only nucleotide that cannot be obtained via the salvage reactions (30). Whereas the search for inhibitors of SHTM has only recently... 

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