Aspartic reaction

Argininosuccinate synthase links aspartate and citrulline via the amino group of aspartate (reaction 3, Figure 29-9) and provides the second nitrogen of urea. The reaction requires ATP and involves intermediate formation of citrullyl-AMP. Subsequent displacement of AMP by aspartate then forms citrulline. 

FIGURE 17.5 Loading of tRNA molecules with their corresponding amino acid, (a) General equation, (b) Aspartate reaction, (c) The two-step mechanism... 

Enzymes, measured in clinical laboratories, for which kits are available include y-glutamyl transferase (GGT), alanine transferase [9000-86-6] (ALT), aldolase, a-amylase [9000-90-2] aspartate aminotransferase [9000-97-9], creatine kinase and its isoenzymes, galactose-l-phosphate uridyl transferase, Hpase, malate dehydrogenase [9001 -64-3], 5 -nucleotidase, phosphohexose isomerase, and pymvate kinase [9001-59-6]. One example is the measurement of aspartate aminotransferase, where the reaction is followed by monitoring the loss of NADH ... 

Enzymatic Process. Chemically synthesized substrates can be converted to the corresponding amino acids by the catalytic action of an enzyme or the microbial cells as an enzyme source, t - Alanine production from L-aspartic acid, L-aspartic acid production from fumaric acid, L-cysteine production from DL-2-aminothiazoline-4-catboxyhc acid, D-phenylglycine (and D-/> -hydtoxyphenylglycine) production from DL-phenyUiydantoin (and DL-/)-hydroxyphenylhydantoin), and L-tryptophan production from indole and DL-serine have been in operation as commercial processes. Some of the other processes shown in Table 10 are at a technical level high enough to be useful for commercial production (24). Representative chemical reactions used ia the enzymatic process are shown ia Figure 6. 

In principle, aspartame is produced through the coupling of two amino acid moieties. One moiety consists of T.-phenylalanine methyl ester hydrochloride (2) made by treating the amino acid ia methanol and hydrochloric acid the other is aspartic acid anhydride hydrochloride or formic acid salt. The coupling reaction generates two positional isomers, a and p. 

The side chains of the 20 different amino acids listed in Panel 1.1 (pp. 6-7) have very different chemical properties and are utilized for a wide variety of biological functions. However, their chemical versatility is not unlimited, and for some functions metal atoms are more suitable and more efficient. Electron-transfer reactions are an important example. Fortunately the side chains of histidine, cysteine, aspartic acid, and glutamic acid are excellent metal ligands, and a fairly large number of proteins have recruited metal atoms as intrinsic parts of their structures among the frequently used metals are iron, zinc, magnesium, and calcium. Several metallo proteins are discussed in detail in later chapters and it suffices here to mention briefly a few examples of iron and zinc proteins. 

The second electron shuttle system, called the malate-aspartate shuttle, is shown in Figure 21.34. Oxaloacetate is reduced in the cytosol, acquiring the electrons of NADH (which is oxidized to NAD ). Malate is transported across the inner membrane, where it is reoxidized by malate dehydrogenase, converting NAD to NADH in the matrix. This mitochondrial NADH readily enters the electron transport chain. The oxaloacetate produced in this reaction cannot cross the inner membrane and must be transaminated to form aspartate, which can be transported across the membrane to the cytosolic side. Transamination in the cytosol recycles aspartate back to oxaloacetate. In contrast to the glycerol phosphate shuttle, the malate-aspartate cycle is reversible, and it operates as shown in Figure 21.34 only if the NADH/NAD ratio in the cytosol is higher than the ratio in the matrix. Because this shuttle produces NADH in the matrix, the full 2.5 ATPs per NADH are recovered. 

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

Similarly, W-methyl-D-aspartate (NMDA) antagonists 32 with analgesic activity were prepared, again using the Meth-Cohn quinoline synthesis as the key entry reaction, subsequent functional group manipulation giving the desired target compound. 

A solution of 88.5 parts of L-phenylalanine methyl ester hydrochloride in 100 parts of water is neutralized by the addition of dilute aqueous potassium bicarbonate, then is extracted with approximately 900 parts of ethyl acetate. The resulting organic solution is washed with water and dried over anhydrous magnesium sulfate. To that solution is then added 200 parts of N-benzyloxycarbonyl-L-aspartic acid-a-p-nitrophenyl, -benzyl diester, and that reaction mixture is kept at room temperature for about 24 hours, then at approximately 65°C for about 24 hours. The reaction mixture is cooled to room temperature, diluted with approximately 390 parts of cyclohexane, then cooled to approximately -18°C in order to complete crystallization. The resulting crystalline product is isolated by filtration and dried to afford -benzyl N-benzyloxycarbonvI-L-aspartyl-L-phenylalanine methyl ester, melting at about 118.5°-119.5°C. 

A more general method for preparation ofa-amino acids is the amidotnalmatesynthesis, a straightforward extension of the malonic ester synthesis (Section 22.7). The reaction begins with conversion of diethyl acetamidomalonate into an eno-late ion by treatment with base, followed by S 2 alkylation with a primary alkyl halide. Hydrolysis of both the amide protecting group and the esters occurs when the alkylated product is warmed with aqueous acid, and decarboxylation then takes place to vield an a-amino acid. For example aspartic acid can be prepared from, ethyl bromoacetate, BrCh CCHEt ... 

One of the steps in the biosynthesis of uridine monophosphate is the reaction of aspartate with carbamoyl phosphate to give carbamoyl aspartate followed by cyclization to form dihydroorotate. Propose mechanisms for both steps. 

The metabolic breakdown of triacylglycerols begins with their hydrolysis to yield glycerol plus fatty acids. The reaction is catalyzed by a lipase, whose mechanism of action is shown in Figure 29.2. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine residues, which act cooperatively to provide the necessary acid and base catalysis for the individual steps. Hydrolysis is accomplished by two sequential nucleophilic acyl substitution reactions, one that covalently binds an acyl group to the side chain -OH of a serine residue on the enzyme and a second that frees the fatty acid from the enzyme. 

Figure 29.2 MECHANISM Mechanism of action of lipase. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine, which react cooperatively to carry out two nucleophilic acyl substitution reactions. Individual steps are explained in the text.

Merck s thienamycin synthesis commences with mono (V-silylation of dibenzyl aspartate (13, Scheme 2), the bis(benzyl) ester of aspartic acid (12). Thus, treatment of a cooled (0°C) solution of 13 in ether with trimethylsilyl chloride and triethylamine, followed by filtration to remove the triethylamine hydrochloride by-product, provides 11. When 11 is exposed to the action of one equivalent of tm-butylmagnesium chloride, the active hydrogen attached to nitrogen is removed, and the resultant anion spontaneously condenses with the electrophilic ester carbonyl four atoms away. After hydrolysis of the reaction mixture with 2 n HC1 saturated with ammonium chloride, enantiomerically pure azetidinone ester 10 is formed in 65-70% yield from 13. Although it is conceivable that... 

Intermediate 10 must now be molded into a form suitable for coupling with the anion derived from dithiane 9. To this end, a che-moselective reduction of the benzyl ester grouping in 10 with excess sodium borohydride in methanol takes place smoothly and provides primary alcohol 14. Treatment of 14 with methanesulfonyl chloride and triethylamine affords a primary mesylate which is subsequently converted into iodide 15 with sodium iodide in acetone. Exposure of 15 to tert-butyldimethylsilyl chloride and triethylamine accomplishes protection of the /Mactam nitrogen and leads to the formation of 8. Starting from L-aspartic acid (12), the overall yield of 8 is approximately 50%, and it is noteworthy that this reaction sequence can be performed on a molar scale. 

A possible explanation for the superiority of the amino donor, L-aspartic add, has come from studies carried out on mutants of E. coli, in which only one of the three transaminases that are found in E. coli are present. It is believed that a branched chain transaminase, an aromatic amino add transaminase and an aspartate phenylalanine aspartase can be present in E. coli. The reaction of each of these mutants with different amino donors gave results which indicated that branched chain transminase and aromatic amino add transminase containing mutants were not able to proceed to high levels of conversion of phenylpyruvic add to L-phenylalanine. However, aspartate phenylalanine transaminase containing mutants were able to yield 98% conversion on 100 mmol l 1 phenylpyruvic acid. The explanation for this is probably that both branched chain transaminase and aromatic amino acid transminase are feedback inhibited by L-phenylalanine, whereas aspartate phenylalanine transaminase is not inhibited by L-phenylalanine. In addition, since oxaloacetate, which is produced when aspartic add is used as the amino donor, is readily converted to pyruvic add, no feedback inhibition involving oxaloacetate occurs. The reason for low conversion yield of some E. coli strains might be that these E. cdi strains are defident in the aspartate phenylalanine transaminase. 

The best results were obtained with L-aspartic add as the amino donor for P. denitrificam and phenylpyruvic add as the amino acceptor. With L-aspartic add, conversion of phenylpyruvic add exceeded 90%. This may be attributed to absence of feedback inhibition of the reaction due to metabolism of file reaction product, oxaloacetic add. When using glutamic acid the conversion of phenylpyruvic add did not exceed 60%. 

Aspartic add may be produced continuously from ammonium fumarate using immobilised E. coli. L-alanine is continuously produced using immobilised Ps. dacuhhae. If both E. coli and P. dacunhae are employed simultaneously, L-alanine could be produced more effidentiy from ammonium fumarate according to the reaction shown in Figure A8.13. 

Antineoplastic agents that cannot be grouped under subheadings 1-9 include miltefosine which is an alkylphosphocholine that is used to treat skin metastasis of breast cancer, and crispantase which breaks down asparagine to aspartic acid and ammonia. It is active against tumor cells that lack the enzyme asparaginase, such as acute lymphoblastic leukemia cells. Side effects include irritation of the skin in the case of miltefosine and anaphylactic reactions in the case of crispantase. Another recent development is the proteasome inhibitor bortezomib which is used to treat multiple myeloma. 

The catalytic cycle of the Na+/K+-ATPase can be described by juxtaposition of distinct reaction sequences that are associated with two different conformational states termed Ei and E2 [1]. In the first step, the Ei conformation is that the enzyme binds Na+ and ATP with very high affinity (KD values of 0.19-0.26 mM and 0.1-0.2 pM, respectively) (Fig. 1A, Step 1). After autophosphorylation by ATP at the aspartic acid within the sequence DKTGS/T the enzyme occludes the 3 Na+ ions (Ei-P(3Na+) Fig. la, Step 2) and releases them into the extracellular space after attaining the E2-P 3Na+ conformation characterized by low affinity for Na+ (Kq5 = 14 mM) (Fig. la, Step 3). The following E2-P conformation binds 2 K+ ions with high affinity (KD approx. 0.1 mM Fig. la, Step 4). The binding of K+ to the enzyme induces a spontaneous dephosphorylation of the E2-P conformation and leads to the occlusion of 2 K+ ions (E2(2K+) Fig. la, Step 5). Intracellular ATP increases the extent of the release of K+ from the E2(2K+) conformation (Fig. la, Step 6) and thereby also the return of the E2(2K+) conformation to the EiATPNa conformation. The affinity ofthe E2(2K+) conformation for ATP, with a K0.5 value of 0.45 mM, is very low. 

E9.3 The deamination of aspartic acid is a reversible reaction catalyzed by the... 

---