Lysine racemization

The primary target of studies on photocatalytic semiconductor suspensions has been water cleavage by visible light. Suspension-based photocatalytic processes are also useful for the removal of inorganic (metal ions) and organic pollutants, the reduction of CO2, the photodestruction of bacteria and viruses, and various organic reactions an example is the use of Pt-loaded CdS for the photocatalytic racemization of L-lysine [210]. 

In some cases enzymes can increase the rate of reaction by up to lO times. Carnell and Roberts (1997) have briefly discussed the scope of biotransformations that are used to make pharmaceuticals like penicillins, cephalosporines, erythromycin, lovastatin, cyclosporin, etc., and for food additives like citric acid, L-glutamate, and L-lysine. A very successful transformation by Zeneca has been that of benzene reduction, with Pseudomonase Putida, to dihydrocatechol and catechol the dihydro derivative is used to produce (+/-) pinitol. Fluorobenzene has been converted to fluorodihydrocatechol, an intermediate for pharmaceuticals. The highly stereo selective Bayer-Villeger reaction has been carried out with genetically engineered S-cerevisvae. Hydrolases have allowed enantioselective, and in some cases regioselective, hydrolysis of racemic esters. 

Toray (1) A large Japanese chemicals manufacturer, perhaps best known for its process for synthesizing /-lysine for use as a dietary supplement. The starting material is cyclohexene which is converted in five steps to racemic lysine. An enzymic process isolates the desired optical isomer, the other is recycled. 

After formation of the aldimine, numerous factors in the enzyme facilitate deprotonation of the a-carbon (Fig. 3, Step II). The lysine liberated by transimi-nation is utilized as a general base and is properly oriented for effective deprotonation [11]. Furthermore, the inductive effects of the ring system are tuned to increase the stabilization of the quinoid intermediate. For example, the aspartate group that interacts with the pyridyl nitrogen of the co enzyme promotes proto-nation to allow the ring to act as a more effective electron sink. In contrast, in alanine racemase, a less basic arginine residue in place of the aspartic acid is believed to favor racemization over transamination [12]. 

Very detailed studies on the inhibition of alanine racemase by fluoroalanines have been conducted. This enzyme catalyzes the racemization of alanine to provide D-alanine, which is required for synthesis of the bacterial wall. This work has demonstrated that a more complex process than that represented in Figure 7.47 could intervene. For instance, in the case of monofluoroalanine, a second path (Figure 7.48, path b) occurs lysine-38 of the active site can also attack the Schiff base PLP-aminoacrylate that comes from the elimination of the fluorine atom. This enamine inactivation process (path b) has been confirmed by isolation and identification of the alkylation compound, after denaturation of the enzyme (Figure 7.48). ... 

Beta-elimination reactions have been observed in a number of proteins. This reaction occurs primarily at alkaline pH conditions. Abstraction of the hydrogen atom from the alpha-carbon of a cysteine, serine, threonine, phenylalanine, or lysine residue leads to racemization or loss of part of the side chain and the formation of dehydroalanine (26). 

Careful inspection of the reported photocatalytic reactions may demonstrate that reaction products can not be classified, in many cases, into the two above categories, oxidation and reduction of starting materials. For example, photoirradiation onto an aqueous suspension of platinum-loaded Ti02 converts primary alkylamines into secondary amines and ammonia, both of which are not redox products.34) ln.a similar manner, cyclic secondary amines, e.g., piperidine, are produced from a,co-diamines.34) Along this line, trials of synthesis of cyclic imino acids such as proline or pipecolinic acid (PCA) from a-amino acids, ornithine or lysine (Lys), have beer. successfuL35) Since optically pure L-isomer of a-amino acids are available in low cost, their conversion into optically active products is one of the most important and practical chemical routes for the synthesis of chiral compounds. It should be noted that l- and racemic PCA s are obtained from L-Lys by Ti02 and CdS photocatalyst, respectively. This will be discussed later in relation to the reaction mechanism. 

The four-step synthesis of a, -diaminocaprolactam shown in Figure 5.29 is part ofa chemoenzymatic route to (S)-lysine, an essential amino add in our diet [135], The racemic caprolactam (azepan-2-one) product is then hydrolyzed selectively to (S)-lysine, using an immobilized (S)-hydrolase enzyme. 

The N,0-acetals are intermediates for the synthesis of enamides 264) and for the production of DL-omithine 265 266) and DL-lysine 267 268>. The amino acid syntheses are not economically competitive with the fermentation methods since they only give the racemates, which then have to be resolved into the antipodes. 

Taking advantage of the ready availability of racemic ibuprofen, the resolution approach for production of (S)-(+)-ibuprofen becomes an attractive alternative. Merck s resolution process involves the formation of a diastereomeric salt of ibuprofen with (S)-lysine, an inexpensive and readily available natural amino acid.45 The racemic ibuprofen is mixed with 1.0 equivalent of (5)-lysine in aqueous ethanol. The slurry is agitated to allow full dissolution. The supernatant, which is a supersaturated solution of ibuprofen-lysine salt, is separated from the solid and seeded with (.S )-ibuprofcn-(.S )-lysine to induce crystallization. The precipitated solid is collected by filtration, and the mother liquor is recycled to the slurry of racemic ibuprofen and (S)-lysine. This process is continued until essentially all (S)-ibuprofen in the original slurry is recovered, resulting in the... 

Although these experiments did not provide the desired systems needed to amplify chirality, they were helpful in elucidating the stereochemical mechanism of the role played by additives in the early stages of crystal nucleation. This information was instrumental to the elaboration of appropriate model systems for the amplification of chirality, such as the generation of homochiral lysine via crystals of nickel/caprolactam [131] and the auto catalytic process of the spontaneous segregation of racemic enantiomers of amino acids in aqueous solutions assisted by centrosymmetric glycine crystals grown at interfaces. 

The alanine racemization catalyzed by alanine racemase is considered to be initiated by the transaldimination (Fig. 8.5).26) In this step, PLP bound to the active-site lysine residue forms the external Schiff base with a substrate alanine (Fig. 8.5, 1). The following a-proton abstraction produces the resonance-stabilized carbanion intermediates (Fig. 8.5, 2). If the reprotonation occurs on the opposite face of the substrate-PLP complex on which the proton-abstraction proceeds, the antipodal aldimine is formed (Fig. 8.5,3). The subsequent hydrolysis of the aldimine complex gives the isomerized alanine and PLP-form racemase. The random return of hydrogen to the carbanion intermediate is the distinguishing feature that differentiates racemization from reactions catalyzed by other pyridoxal enzymes such as transaminases. Transaminases catalyze the transfer of amino group between amino acid and keto acid, and the reaction is initiated by the transaldimination, followed by the a-proton abstraction from the substrate-PLP aldimine to form a resonance-stabilized carbanion. This step is common to racemases and transaminases. However, in the transamination the abstracted proton is then tranferred to C4 carbon of PLP in a highly stereospecific manner The re-protonation occurs on the same face of the PLP-substrate aldimine on which the a-proton is abstracted. With only a few exceptions,27,28) each step of pyridoxal enzymes-catalyzed reaction proceeds on only one side of the planar PLP-substrate complex. However, in the amino acid racemase... 

Nutritional and Physiological Effects of Alkali-Treated Proteins. The first effect of the alkaline treatment of food proteins is a reduction in the nutritive value of the protein due to the decrease in (a) the availability of the essential amino acids chemically modified (cystine, lysine, isoleucine) and in (b) the digestibility of the protein because of the presence of cross-links (lysinoalanine, lanthionine, and ornithinoalanine) and of unnatural amino acids (ornithine, alloisoleucine, / -aminoalanine, and D-amino acids). The racemization reaction occurring during alkaline treatments has an effect on the nitrogen digestibility and the use of the amino acids involved. 

These effects were described by de Groot and Slump (99) followed by Provansal et al. (94). Later, it was confirmed that the lysine moiety of lysinoalanine was completely unavailable to the rat and only partly available to the chick, while a certain part of the cysteine moiety of lanthionine (32% to 52% according to the racemic mixture) was available to chicks (100) (see Table I). 

Pyridoxal is the reagent in other reactions of amino acids, all involving the inline as intermediate. The simplest is the racemization of amino acids by loss of a proton and its replacement on the other face of the enamine. The enamine, in the middle of the diagram below, can be reprotonated on either face of the prochiral inline (shown in green). Protonation on the bottom face would take us back to the natural amino acid from which the enamine was made in the first place. Protonation on the top face leads to the unnatural amino acid after hydrolysis of the inline (really transfer of pyridoxal to a lysine residue of the enzyme). 

Considering that sodium glutamate, like other amino acids, is contained in soy sauce, which is a traditional Japanese food, it is not surprising that Japan should have become interested very early in this type of fermentation. Firms like Ajinomoto and Kyowa Hakko dominate the world market for amino acids and particularly for glutamic acid and l-lysine. It is also through enzymes that the resolution of dl-methionine into its optical isomers is achieved since its laboratory synthesis yields the racemic form. 

S Elimination and Racemization. There is some loss of the amino acids cystine, cysteine, serine, threonine, lysine and arginine during the alkaline treatment of proteins (12,22-30). Unlike arginine as shown above, loss of the other amino acids is not due to a hydrolytic reaction but rather to a g-elimination reaction (Equation 6). There is also some racemization of amino... 

More recently, enzymatic, microbiological, and chromatographic techniques have been used to determine extent of racemization. D-Lysine has been found in a sunflower protein... 

The pair of unshared electrons on the carbanion can undergo two reactions (a) it can recombine with a proton from the solvent to regenerate either the original amino acid side chain or its optical antipode, so that it is racemized (b) it can undergo the indicated 6-elimination reaction to form a dehydroalanine derivative, which can then combine with an e-amino group of a lysine side chain to form a lysinoalanine crosslink. 

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