25.4 How Do Organisms Synthesize Amino Acids? 793 Nevertheless, four of the five enzymes necessary for isoleucine synthesis are common to the pathway for biosynthesis of valine, so discussion of isoleucine synthesis is pre- sented under the biosynthesis of the pyruvate family of amino acids. The Pyruvate Family of Amino Acids Includes Ala, Val, and Leu The pyruvate family of amino acids includes alanine (Ala), valine (Val), and leucine (Leu). Transamination of pyruvate, with glutamate as amino donor, gives alanine. Because these transamination reactions are readily reversible, alanine degradation occurs via the reverse route, with ␣-ketoglutarate serving as amino acceptor. Transamination of pyruvate to alanine is a reaction found in virtually all organ- isms, but valine, leucine, and isoleucine are essential amino acids, and as such, they are not synthesized in mammals. The pathways of valine and isoleucine synthesis can be considered together because one set of four enzymes is common to the last four steps of both pathways (Figure 25.29). Both pathways begin with an ␣-keto acid. Isoleucine can be considered a structural analog of valine that has one extra OCH 2 O unit, and its ␣-keto acid precursor, namely, ␣-ketobutyrate, is one carbon longer than the valine precursor, pyruvate. Interestingly, ␣-ketobutyrate is formed from threonine by threonine deaminase (Figure 25.29, reaction 1). This PLP- dependent enzyme (also known as threonine dehydratase or serine dehydratase) is feedback-inhibited by isoleucine, the end product. Note that part of the carbon skeleton for Ile comes from Asp by way of Thr. From here on, the Val and Ile path- ways employ the same set of enzymes. The first reaction involves the generation of hydroxyethyl-thiamine pyrophosphate from pyruvate in a reaction analogous to those catalyzed by transketolase and the pyruvate dehydrogenase complex. The two-carbon hydroxyethyl group is transferred from TPP to the respective keto acid acceptor by acetohydroxy acid synthase (acetolactate synthase) to give ␣-acetolactate or ␣-aceto-␣-hydroxybutyrate (Figure 25.29, reaction 2). NAD(P)H-dependent reduc- tion of these ␣-keto hydroxy acids yields the dihydroxy acids ␣,-dihydroxyisovalerate and ␣,-dihydroxy--methylvalerate (Figure 25.29, reaction 3). Dehydration of each of these dihydroxy acids by dihydroxy acid dehydratase gives the appropriate ␣-keto acid carbon skeletons ␣-ketoisovalerate and ␣-keto--methylvalerate (Figure 25.29, reaction 4). Transamination by the branched-chain amino acid aminotransferase yields Val or Ile, respectively (Figure 25.29, reaction 5). Leucine synthesis depends on these reactions as well, because ␣-ketoisovalerate is a precursor common to both Val and Leu (Figure 25.30). Although Val and Leu dif- fer by only a sin gle OCH 2 O in their respective side chains, the carboxyl group of ␣-ketoisovalerate first picks up two carbons from acetyl-CoA to give ␣-isopropylmalate in a reaction catalyzed by isopropylmalate synthase; the enzyme is sensitive to feed- back inhibition by Leu (Figure 25.30, reaction 1). Isopropylmalate dehydratase (Figure 29.30, reaction 2) converts the ␣-isomer to the -form, which undergoes an NAD ϩ -dependent oxidative decarboxylation by isopropylmalate dehydrogenase (Figure 29.30, reaction 3), so the carboxyl group of ␣-ketoisovalerate is lost as CO 2 . Amination of ␣-ketoisocaproate by leucine aminotransferase (Figure 29.30, reaction 4) gives Leu. The 3-Phosphoglycerate Family of Amino Acids Includes Ser, Gly, and Cys Serine, glycine, and cysteine are derived from the glycolytic intermediate 3-phosphoglycerate. The diversion of 3-PG from glycolysis is achieved via 3-phosphoglycerate dehydrogenase (Figure 25.31, reaction 1). This NAD ϩ - dependent oxidation of 3-PG yields 3-phosphohydroxypyruvate—which, as an ␣-keto acid, is a substrate for transamination by glutamate to give 3-phosphoserine (Figure 25.31, reaction 2). Serine phosphatase then generates serine (Figure 25.31, reaction 3). Serine inhibits the first enzyme, 3-PG dehydrogenase, and thereby feedback- regulates its own synthesis. 794 Chapter 25 Nitrogen Acquisition and Amino Acid Metabolism Glycine is made from serine via two related enzymatic processes. In the first, serine hydroxymethyltransferase, a PLP-dependent enzyme, catalyzes the transfer of the serine -carbon to tetrahydrofolate (THF), the principal agent of one-carbon metabolism (Figure 25.32a). Glycine and N 5 ,N 10 -methylene-THF are the products. In addition, glycine can be synthesized by a reversal of the glycine oxidase reaction (Figure 25.32b). Here, glycine is formed when N 5 ,N 10 -methylene-THF condenses with NH 4 ϩ and CO 2 . Via this route, the -carbon of serine becomes part of glycine. + 1 NH 3 + P P H 3 C O O – C O C H 3 C O O – C C C OH O H 3 C H 3 C O O – C C OH C OH CH 3 H H 3 C O O – C CC H CH 3 O H 3 C O O – C CC H CH 3 H H 3 C C H OH Thiamine Pyruvate H 3 C O O – C C C OH O H 3 C CH 2 H 3 C O O – C O CCH 2 NH 3 2 NH 3 + H 3 C O O – C CC H OH H H 3 C O O – C C OH CH 2 C CH 3 H OH H 2 O H 2 O CO 2 H 3 C O O – C C H CH 2 C CH 3 O H 3 C O O – C C H CH 2 C H 3 C H NH 3 + 3 4 5 NADP + NADP + NADPH H + ␣-KG ␣-KG Pyruvate ␣-Acetolactate ␣,-Dihydroxyisovalerate ␣-Ketoisovalerate Valine Thiamine pyrophosphate Thiamine pyrophosphate ␣-Ketobutyrate ␣-Aceto-␣-hydroxybutyrate Thiamine pyrophosphate Threonine ␣,-Dihydroxy--methylvalerate ␣-Keto--methylvalerate Isoleucine Glutamate FIGURE 25.29 Biosynthesis of valine and isoleucine. 25.4 How Do Organisms Synthesize Amino Acids? 795 The conversion of serine to glycine is a prominent means of generating one-carbon derivatives of THF, which are so important for the biosynthesis of purines and the C-5 methyl group of thymine (a pyrimidine, see Chapter 26), as well as the amino acid methionine. Glycine itself contributes to both purine and heme synthesis. Cysteine synthesis is accomplished by sulfhydryl transfer to serine (Figure 25.33). In some bacteria, H 2 S condenses directly with serine via a PLP-dependent enzyme- catalyzed reaction (Figure 25.33a), but in most microorganisms and green plants, the sulfhydrylation reaction requires an activated form of serine, O-acetylserine (Figure 25.33b). O-acetylserine is made by serine acetyltransferase, with the transfer of an acetyl group from acetyl-CoA to the OOH of Ser. This enzyme is inhibited by Cys. H 3 C O O – C C H 3 C O O – C C H H 3 C C H H 3 C H 3 C O O – C CC H CH 3 O C CH 3 H O C SCoA O H 3 C 1 H 3 C OH C O – O CH 2 C ++ O O – CC OH 2 3 4 ␣-KG Glutamate H 3 C O O – C CC H CH 3 CH 2 H NH 3 + C H C O – O CH 2 NAD + NADH CO 2 H + CoASH ␣-Ketoisovalerate ␣-Isopropylmalate -Isopropylmalate Leucine ␣-Ketoisocaproate FIGURE 25.30 Biosynthesis of leucine. H 2 C O O – O – P C O 1 + 2 3 C H O OH O – H 2 C O O – O – P C O C O O O – ␣-KG Glutamate H 2 C O O – O – P C O C O O – H NH 3 + H 2 COH O O – C C H NH 3 + NAD + NADH H + H 2 O P i 3-Phosphoglycerate 3-Phosphohydroxypyruvate 3-Phosphoserine Serine FIGURE 25.31 Biosynthesis of serine from 3-phosphoglycerate. 796 Chapter 25 Nitrogen Acquisition and Amino Acid Metabolism O-Acetylserine then undergoes sulfhydrylation by H 2 S with elimination of acetate; the enzyme is O-acetylserine sulfhydrylase. Sulfide Synthesis from Sulfate Involves S-Containing ATP Derivatives Given the prevailing oxidative nature of our environment and the reactivity and toxicity of H 2 S, the source of sulfide for Cys synthesis merits discussion. In microorganisms and plants, sulfide is the product of sulfate assimilation. Sulfate is the common inorganic form of combined sulfur, and its assimilation involves several interesting ATP deriva- tives. ATP sulfurylase (Figure 25.34, reaction 1) catalyzes the formation of adenosine- 5Ј-phosphosulfate (APS). Then, adenosine-5Ј-phosphosulfate-3Ј-phosphokinase cat- alyzes the formation of 3Ј-phosphoadenosine-5Ј-phosphosulfate (PAPS) from APS ϩ ATP (Figure 25.34, reaction 2). Sulfite is then liberated from PAPS through the re- duction by reduced thioredoxin, leaving 3Ј-phosphoadenosine-5Ј-phosphate as a prod- uct (Figure 25.34, reaction 3). Sulfite (SO 3 Ϫ ) is then reduced to sulfide (S 2 Ϫ ) in a mul- tielectron transfer reaction catalyzed by sulfite oxidase (Figure 25.34, reaction 4); + H 2 COH O O – C C H NH 3 + THF N 5 ,N 10 - Methylene THF O O – H 2 C C NH 3 + + NH 4 + ++N 5 ,N 10 - Methylene THF H 3 + N CH 2 C O O – ++THF + CO 2 NAD + NADH H + H 2 O (a) Serine Serine hydroxymethyltransferase Glycine (b) Glycine oxidase acting in reverse Glycine FIGURE 25.32 Biosynthesis of glycine from serine (a) via serine hydroxymethyltransferase and (b) via glycine oxidase. H 2 COH O O – C HC HC HC H 2 O H + NH 3 + O O – C NH 3 + + H 2 S H 2 C SH H 2 COH O O – C NH 3 + Acetyl-SCoA H 2 C O O – C O C O CH 3 H 2 S H 3 C C O O – + O O – C H 2 C SH + CoASH HC NH 3 + HC NH 3 + (a) Serine Pyridoxal phosphate–dependent enzyme Cysteine (b) Serine O-Acetylserine Cysteine FIGURE 25.33 Cysteine biosynthesis. (a) Direct sulfhydrylation of serine by H 2 S. (b) H 2 S-dependent sulfhydryla- tion of O-acetylserine. 25.4 How Do Organisms Synthesize Amino Acids? 797 NADPH is the electron donor. Sulfite reductase, like nitrite reductase, possesses siro- heme as a prosthetic group (see Figure 25.2). 3Ј-Phosphoadenosine-5Ј-phosphosulfate is not only an intermediate in sulfate assimilation; it also serves as the substrate for syn- thesis of sulfate esters, such as the sulfated polysaccharides found in the glycocalyx of animal cells. The Aromatic Amino Acids Are Synthesized from Chorismate The aromatic amino acids, phenylalanine, tyrosine, and tryptophan, are derived from a shared pathway that has chorismic acid (Figure 25.35) as a key intermediate. Indeed, chorismate is common to the synthesis of cellular compounds having ben- zene rings, including these amino acids, the fat-soluble vitamins E and K, folic acid, + 1 P P SO 4 2– O O O – O S P O – O OCH 2 OH OH OH O N N N NH 2 N + APS 2 O O O – O S P O – O OCH 2 O O N N N NH 2 N – O P O – O OH O – O P O – O PAPS + Thioredoxin SH SH 3 + SO 3 2– Thioredoxin S S P O – O OCH 2 O N N N NH 2 N 3 NADPH + 3 H + + SO 3 2– 4 S 2– + 3 NADP + + 3 H 2 O Sulfite – O ATP ATP ADP Adenosine 5-phosphosulfate (APS) 3-Phosphoadenosine 5-phosphosulfate (PAPS) 3-Phosphoadenosine 5-phosphate Sulfate Sulfide FIGURE 25.34 Sulfate assimilation and the generation of sulfide for synthesis of organic S compounds. 798 Chapter 25 Nitrogen Acquisition and Amino Acid Metabolism and coenzyme Q and plastoquinone (the two quinones necessary to electron trans- port during respiration and photosynthesis, respectively). Lignin, a polymer of nine- carbon aromatic units, is also a derivative of chorismate. Lignin and related com- pounds can account for as much as 35% of the dry weight of higher plants; clearly, enormous amounts of carbon pass through the chorismate biosynthetic pathway. Chorismate Is Synthesized from PEP and Erythrose-4-P Chorismate biosynthesis occurs via the shikimate pathway (Figure 25.36). The precursors for this pathway are the common metabolic intermediates phosphoenolpyruvate and erythrose-4- phosphate. These intermediates are linked to form 3-deoxy- D-arabino-heptulosonate-7- phosphate (DAHP) by DAHP synthase (Figure 25.36, reaction 1). Although this reaction is remote from the ultimate aromatic amino acid end products, it is an im- portant point for regulation of aromatic amino acid biosynthesis, as we shall see. In the next step on the way to chorismate, DAHP is cyclized to form a six-membered saturated ring compound, 5-dehydroquinate (Figure 25.36, reaction 2), in a reaction catalyzed by dehydroquinate synthase (NAD ϩ is a coenzyme in this reaction but is not modified by it). A sequence of reactions ensues that introduces unsaturations into the ring through dehydration (Figure 25.36, reaction 3, 5-dehydroquinate de- hydratase) and reduction (reaction 4, shikimate dehydrogenase), yielding shikimate. Phosphorylation of shikimate by shikimate kinase (reaction 5), then addition of PEP by 3-enolpyruvylshikimate-5-phosphate synthase (reaction 6), followed by cho- rismate synthase (reaction 7), gives chorismate. Thus, two equivalents of PEP are needed to form chorismate from erythrose-4-P. Phenylalanine and Tyrosine At chorismate, the pathway separates into three branches, each leading specifically to one of the aromatic amino acids. The branches leading to phenylalanine and tyrosine both pass through prephenate (Figure 25.37). In some organisms, such as E. coli, the branches are truly distinct because prephenate O C – O O HH C O O – HO C C C O C C O C C C C H Chorismate p -Hydroxybenzoate Coenzyme Q Vitamin K p-Aminobenzoate (PABA) Folic acid Prephenate Phenylalanine Vitamin EPlastoquinone Tyrosine Anthranilate Tryptophan Lignin (a complex polymer of C 9 aromatic units) FIGURE 25.35 Some of the aromatic compounds derived from chorismate. 25.4 How Do Organisms Synthesize Amino Acids? 799 does not occur as a free intermediate but rather remains bound to the bifunctional enzyme that catalyzes the first two reactions after chorismate. In any case, chorismate mutase is the first reaction leading to Phe or Tyr (Figure 25.37, reaction 1). In the Phe branch, the OOH group para to the prephenate carboxyl is removed by prephenate dehydratase (Figure 25.37, reaction 2). In the Tyr branch, this OOH is retained; instead, an oxidative decarboxylation of prephenate catalyzed by prephenate dehydrogenase (Figure 25.37, reaction 4) yields 4-hydroxyphenylpyruvate. Glutamate- dependent aminotransferases (phenylalanine aminotransferase [Figure 25.27, reac- tion 3] and tyrosine aminotransferase [reaction 5]) introduce the amino groups into the two ␣-keto acids, phenylpyruvate and 4-hydroxyphenylpyruvate, to give Phe and Tyr, respectively. Some mammals can synthesize Tyr from Phe obtained in the diet via phenylalanine-4-monooxygenase (also known as phenylalanine hydroxylase), using O 2 and tetrahydrobiopterin, an analog of tetrahydrofolic acid, as co-substrates (Figure 25.38). Tryptophan The pathway of tryptophan synthesis is perhaps the most thoroughly studied of any biosynthetic sequence, particularly in terms of its genetic organiza- tion and expression. Synthesis of Trp from chorismate requires six steps (see Fig- ure 25.37). In most microorganisms, the first enzyme, anthranilate synthase (see Figure 25.37, reaction 6), is an ␣ 2  2 -type protein, with the -subunit acting in a ATP ADP NAD + NAD + NADH B H + H 2 O H 2 O E P 1 H 2 C O OPO 3 2– C O H C H CHOH CHOH CH 2 P P CHOH CHOH CH 2 CHO H CH 2 C O COO – – OH 2 P C COO – HO O HO H OH 3 O HO H OH H COO – + + 4 HO H OH H COO – HO H 5 HO H OH H COO – O H P P 6 H 2 C C HO H O H COO – O H P C CH 2 COO – P HO H O H COO – C CH 2 COO – COO – H PEP COO – 7 O 3 2 – P Erythrose-4-P Phosphoenolpyruvate (PEP) 2-Keto-3-deoxyarabino- heptulosonate-7-P (DAHP) 5-Dehydroquinate 5-Dehydroshikimate ShikimateShikimate-5-P3-Enolpyruvylshikimate-5-P Chorismate FIGURE 25.36 The shikimate pathway leading to the synthesis of chorismate. 800 Chapter 25 Nitrogen Acquisition and Amino Acid Metabolism OH – + H 2 O H 2 O CO 2 P 1 NH 3 + P P HO H O H COO – C CH 2 COO – HO H O – OOC CH 2 COO – C 2 O CH 2 COO – C H CH 2 COO – C 3 4 Glutamate α-Ketoglutarate 5 + CO 2 O CH 2 COO – C 4-Hydoxyphenylpyruvate OH NH 3 + H CH 2 COO – C OH 6 NH 2 COO – 7 – OOC HN O H HH H OHOH H 2 C OP – O O – O 8 COO – N H C H C C C CH 2 H H 9 CO 2 P N H C C CH 2 H H OHOH 10 N H Serine HC COO – NH 3 + CH 2 OH 11 N H CH 2 C COO – H NH 3 + OH OH HO NAD + NADH Glutamate α-Ketoglutarate Chorismate Prephenate Phenylpyruvate Phenylalanine Tyrosine Glutamine Glutamate + Pyruvate Anthranilate 5-Phosphoribosyl- α-pyrophosphate (PRPP) N-(5'-Phosphoribosyl)-anthranilate Enol-1-o-carboxyphenylamino- 1-deoxyribulose phosphate Indole-3-glycerol phosphate Glyceraldehyde- 3-phosphate Indole Tryptophan FIGURE 25.37 The biosynthesis of phenylalanine, tyrosine, and tryptophan from chorismate. 25.4 How Do Organisms Synthesize Amino Acids? 801 glutamine–amidotransferase role to provide the ONH 2 group of anthranilate. Or, given high levels of NH 4 ϩ , the ␣-subunit can carry out the formation of anthranilate di- rectly by a process in which the activity of the -subunit is unnecessary. Furthermore, in certain enteric bacteria, such as E. coli and Salmonella typhimurium, the second reac- tion of the pathway, the phosphoribosyl-anthranilate transferase reaction (see Figure 25.37, reaction 7), is an activity catalyzed by the ␣-subunit of anthranilate synthase. PRPP (5-phosphoribosyl-1-pyrophosphate), the substrate of this reaction, is also a precursor for purine biosynthesis (see Chapter 26). Phosphoribosyl-anthranilate then undergoes a rearrangement wherein the ribose moiety is isomerized to the ribulosyl form in enol-1- (o-carboxyphenylamino)-1-deoxyribulose-5-phosphate by N-(5Ј-phosphoribosyl)-anthranilate A DEEPER LOOK Amino Acid Biosynthesis Inhibitors as Herbicides Unlike animals, plants can synthesize all 20 of the common amino acids. Inhibitors acting specifically on the plant enzymes that are ca- pable of carrying out the biosynthesis of the “essential” amino acids (that is, enzymes that animals lack) have been developed. These substances appear to be ideal for use as herbicides because they should show no effect on animals. Glyphosate, sold commercially as RoundUp, is a PEP analog that acts as an uncompetitive inhibitor of 3-enolpyruvylshikimate-5-P synthase (Figure 25.36). Sulfmeturon methyl, a sulfonylurea herbicide that inhibits acetohydroxy acid syn- thase, an enzyme common to Val, Leu, and Ile biosynthesis (Figure 25.29), is the active ingredient in Oust. Aminotriazole, sold as Amitrole, blocks His biosynthesis by inhibiting imidazole glycerol-P dehydratase (Figure 25.40). PPT (phosphinothricin) is a potent in- hibitor of glutamine synthetase. Although Gln is a nonessential amino acid and glutamine synthetase is a ubiquitous enzyme, PPT is rela- tively safe for animals because it does not cross the blood–brain bar- rier and is rapidly cleared by the kidneys. Ϫ OP O O Ϫ CH 2 NH CH 2 COO Ϫ C O OCH 3 SO 2 NH C O NH N N CH 3 N H N N NH 2 CH 3 P O O Ϫ CH 2 CH 2 C NH 2 HCOO Ϫ CH 3 Glyphosate Sulfmeturon Aminotriazole -Phosphinothricin methyl DL (PPT) TetrahydrobiopterinO 2 + + C C H NH 3 + O O – DihydrobiopterinH 2 O + C C H NH 3 + O O – HO O HN N H N N H C OH H C OH H CH 3 O HN H 2 N N N N H C OH H C OH H CH 3 CH 2 CH 2 H 2 N NADP + NADPH H + Phenylalanine Tyrosine Phenylalanine-4- monooxygenase FIGURE 25.38 The formation of tyrosine from phenylalanine. 802 Chapter 25 Nitrogen Acquisition and Amino Acid Metabolism isomerase (see Figure 25.37, reaction 8). Decarboxylation and ring closure ensue to yield the indole nucleus as indole-3-glycerol phosphate (indole-3-glycerol phos- phate synthase, reaction 9). The final two reactions (10 and 11 in Figure 25.37) are both catalyzed by tryptophan synthase, an ␣ 2  2 -type protein. The ␣-subunit cleaves indoleglycerol-3-phosphate to form indole and 3-glycerol phosphate. The indole is then passed to the -subunit, which adds serine in a PLP-dependent reaction. X-ray crystallographic analysis of S. typhimurium tryptophan synthase shows that the active sites of the ␣- and -subunits are separated from each other by 2.5 nm but are connected by a hydrophobic tunnel wide enough to accommodate indole (Fig- ure 25.39). Thus, indole, the product of the reaction catalyzed by the ␣-subunit (see Figure 25.37, reaction 10), can be transferred directly to the -subunit, which cat- alyzes condensation with serine to yield Trp (see Figure 25.37, reaction 11). Thus, indole is not lost from the enzyme complex and diluted in the surrounding milieu. This phenomenon of direct transfer of enzyme-bound metabolic intermediates, or tunneling, increases the efficiency of the overall pathway by preventing loss and di- lution of the intermediate. Histidine Biosynthesis and Purine Biosynthesis Are Connected by Common Intermediates Like aromatic amino acid biosynthesis, histidine biosynthesis shares metabolic in- termediates with the pathway of purine nucleotide synthesis. The pathway involves ten separate steps, the first being an unusual reaction that links ATP and PRPP A DEEPER LOOK Intramolecular Tunnels Connect Distant Active Sites in Some Enzymes Molecular tunneling is the transfer of a reaction intermediate pro- duced at one active site to another active site in the same enzyme through an intramolecular tunnel that connects them. Trypto- phan synthase (Figure 25.39) was the first enzyme discovered with this structural feature. For tryptophan synthase, the intermediate is indole, but for most of these enzymes studied thus far, the in- termediate is ammonia (NH 3 ) derived from glutamine. Two such enzymes have been presented earlier in this chapter: asparagine synthetase (see Figure 25.26) and glutamate synthase (see Figure 25.12). Another will soon be considered: imidazole glycerol phos- phate synthase (Figure 25.40). Several enzymes in nucleotide metabolism (see Chapter 26) also have this attribute, including carbamoyl phosphate synthetase II (see Figure 26.14), glutamine 5-phosphoribosyl-␣-pyrophosphate amidotransferase (see Figure 26.3), and CTP synthetase (see Figure 26.16). One advantage of molecular tunnels is that they sequester reactive intermediates from potentially unproductive side reactions in the intracellular environment (some of which might be harmful). Also, by direct- ing the intermediate from one active site to another, these tunnels favor a particular reaction sequence. FIGURE 25.39 Tryptophan synthase ribbon diagram.The ␣-subunit is in blue, and the -subunit is in orange (N-terminal domain) and red (C-terminal domain).The tunnel connecting them is outlined by the yellow dot surface and is shown with several indole molecules (green) packed in head-to-tail fashion.The labels “IPP” and “PLP”point to the active sites of the ␣- and -subunits, respectively, in which a competitive inhibitor (indole propanol phosphate, IPP) and the coenzyme PLP are bound. (Adapted from Hyde, C. C., et al., 1988.Three- dimensional structure of the tryptophan synthase multienzyme complex from Salmonella typhimurium. Journal of Biological Chemistry 263:17857–17871.) . biosynthesis by inhibiting imidazole glycerol-P dehydratase (Figure 25.40). PPT (phosphinothricin) is a potent in- hibitor of glutamine synthetase. Although Gln is a nonessential amino acid and glutamine. Figure 26.16). One advantage of molecular tunnels is that they sequester reactive intermediates from potentially unproductive side reactions in the intracellular environment (some of which might be