Serine dehydratases and

As with fumarase, the reactions catalyzed by serine dehydratase and dihydroxyacid dehydratase do not require release and rebinding of intermediate. The reaction mechanisms should be describable as adapted versions of one-half only of Figure 3. 

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). 

Although the nitrogen atoms of most amino acids are transferred to a-ketoglutarate before removal, the a-amino groups of serine and threonine can be directly converted into NH4 +. These direct deaminations are catalyzed by serine dehydratase and threonine dehydratase, in which PLP is the prosthetic group. 

Serine and threonine dehydrases. Serine and threonine are not substrates in transamination reactions. Their amino groups are removed by the pyridoxal phosphate-requiring hepatic enzymes serine dehydratase and threonine dehydratase. The carbon skeleton products of these reactions are pyruvate and a-keto-butyrate, respectively. 

Enzymes present in the liver cytosol with short half-lives include ornithine decarboxylase, thymidine kinase, tyrosine aminotransferase, tryptophan oxygenase, hydroxymethylglutaryl-CoA reductase, serine dehydratase, and phosphoenolpyruvate carboxykinase. All of these enzymes have degradation rate constants greater than 0.1/h—more than 10 times more rapid than the average ka for liver cytosol proteins (Schimke, 1970). Perhaps a scrutiny of the group can provide information on the enzyme properties as well as the nature of reactions catalyzed by enzymes with rapid turnover rates. 

Pyridoxamine phosphate serves as a coenzyme of transaminases, e.g., lysyl oxidase (collagen biosynthesis), serine hydroxymethyl transferase (Cl-metabolism), S-aminolevulinate synthase (porphyrin biosynthesis), glycogen phosphoiylase (mobilization of glycogen), aspartate aminotransferase (transamination), alanine aminotransferase (transamination), kynureninase (biosynthesis of niacin), glutamate decarboxylase (biosynthesis of GABA), tyrosine decarboxylase (biosynthesis of tyramine), serine dehydratase ((3-elimination), cystathionine 3-synthase (metabolism of methionine), and cystathionine y-lyase (y-elimination). 

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). 

L-Serine dehydratase [EC 4.2.1.13], also known as serine deaminase and L-hydroxyaminoacid dehydratase, catalyzes the pyridoxal-phosphate-dependent hydrolysis of L-serine to produce pyruvate, ammonia, and water. In a number of organisms, this reaction is also catalyzed by threonine dehydratase. 

Serine can be converted to glycine and N5,N10-methylenetetra-hydrofolate (Figure 20.6A). Serine can also be converted to pyru vate by serine dehydratase (Figure 20.6B). [Note The role of tetrahydrofolate in the transfer of one-carbon units is presented on p. 265.]... 

The role of the iron-sulfur clusters in many of the proteins that we have just considered is primarily one of single-electron transfer. The Fe-S cluster is a place for an electron to rest while waiting for a chance to react. There may sometimes be an associated proton pumping action. In a second group of enzymes, exemplified by aconitase (Fig. 13-4), an iron atom of a cluster functions as a Lewis acid in facilitating removal of an -OF group in an a,P dehydration of a carboxylic acid (Chapter 13). A substantial number of other bacterial dehydratases as well as an important plant dihydroxyacid dehydratase also apparently use Fe-S clusters in a catalytic fashion.317 Fumarases A and B from E. coli,317 L-serine dehydratase of a Pepto-streptococcus species,317-319 and the dihydroxyacid... 

The substrates of serine dehydratase [45,46] and dihydroxyacid dehydratase [47] differ from citrate in more than just R. The different substrates are compared in Figure 4. They all have in common the central HO —C—CH COO- fragment, indicating that all enzymes should have the following features ... 

This reaction is readily reversible. Another means of metabolizing serine, which accounts for its glucogenic character, as well as that of glycine, is the conversion of serine to pyruvate, as indicated in Figure 20.12. This reaction is catalyzed by serine dehydratase. A similar enzyme, threonine dehydratase, converts threonine to a-ketobutyrate, and the latter is then converted to propionyl-CoA, as indicated in Figure 20.13. Another similar enzyme, cysteine desulfhydrase, con-... 

A similar stereochemical question as in the /8-replacement reactions can be asked in the a, /8-eliminations where the group X is replaced by a hydrogen, i.e., is the proton added at C-/8 of the PLP-aminoacrylate on the same face from which X departed or on the opposite face This question has been answered for a number of enzymes which generate either a-ketobutyrate or pyruvate as the keto acid product. Crout and coworkers [119,120] determined the steric course of proton addition in the a,/8-elimination of L-threonine by biosynthetic L-threonine dehydratase and of D-threonine by an inducible D-threonine dehydratase, both in Serratia marcescens. Either substrate, deuterated at C-3, was converted in vivo into isoleucine, which was compared by proton NMR to a sample prepared from (3S)-2-amino[3-2H]butyric acid. With both enzymes the hydroxyl group at C-3 was replaced by a proton in a retention mode. Although this has not been established with certainty, it is likely that both enzymes, like other bacterial threonine dehydratases [121], contain PLP as cofactor. Sheep liver L-threonine dehydratase, on the other hand, is not a PLP enzyme but contains an a-ketobutyrate moiety at the active site [122], It replaces the hydroxyl group of L-threonine with H in a retention mode, but that of L-allothreonine in an inversion mode [123]. Snell and coworkers [124] established that the replacement of OH by H in the a, /8-elimination of D-threonine catalyzed by the PLP-containing D-serine dehydratase from E. coli also proceeds in a retention mode. They... 

Elimination pc—R and aC—H Tryptophanase Serine dehydratase Aspartate p-decarboxylase... 

The activities involved in yeast fatty acid biosynthesis are covalently linked as separate domains of two multifunctional polypeptides, a and p, encoded by the fas2 and fasl genes, respectively (Fig. 2) [57,58]. The functionalities associated with the 220 kDa a subunit include -ketoacyl synthase activity, -ketoacyl reductase activity, and an AGP domain which bears a phosphopantetheinylated serine. The 208 kDa -subunit has acetyl and malonyl CoA transacylase, palmi-toyl transferase, -hydroxyacyl-enzyme dehydratase, and enoyl acyl-enzyme reductase activities. The two subunits can be readily dissociated, and the individual activities maybe measured [57]. 

K. Kubota, K. Yakozeki, and H, Ozaki, Effects of L-serine dehydratase activity of L-serine production by Corymbacterium glycinojdiilum and an examination of the properties of the enzyme, /. Ferment. Bioeng, 67 341 (1989). 

Threonine can be broken down by two separate pathways. Serine dehydratase catalyzes the conversion of threonine to 2-ketobut ate plus an ammonium ion 2-ketobutyrate is then converted by branched-chain keto add (BCKA) dehydrogenase to propionyl-CoA plus carbon dioxide. Propionyl-CoA catabolism is described later in this chapter. Threonine can also be broken down by a complex that has been suggested to be composed of threonine dehydrogenase and acetoacetone synthase (Tressel et ah, 1986). Here, threonine catabolism results in the production of acetyl-CoA plus glycine. 

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