Dehydroalanine precursors

As reported by Friedman (1982), the major factor controlling the production of lysinoalanine, once the dehydroalanine precursors are formed, could be the location and availability of "partners for crosslink formation. When they are adjacent or close by, lysinoalanine formation could be facilitated. When treatment allows it, more lysinoalanine could be formed, even involving cross-chain links. Thus, the capacity for forming LAL would vary not only with the treatment applied but with the nature of the protein, involving its secondary or tertiary structure as well as its primary one. 

In contrast to the syntheses described above, which all start from cystine derivatives to form lanthionine, the lanthionine syntheses in this section all start from a protected cysteine as the nucleophilic precursor, which is then allowed to react with any of a variety of different substrates. These subsequent reactions are the Michael addition with dehydroalanine, the nucleophilic substitution of halo amino acids, or the ring-opening reaction of serine p-lactones and aziridines, respectively. However, it must be emphasized that the Michael... 

B Schuster, J Retey (1994) Serine-202 is the putative precursor of the active site dehydroalanine of phenylalanine ammonia lyase FEBS Lett 349(2) 252—254... 

The inspection of cinnamycin (4) and duramycin (5)—prompted by their content in lanthionine and /3-methyllanthionine—for the presence of ,/3-unsaturated amino acids was negative. However, cinnamycin as well as duramycin contain lysinoalanine, among other amino acids rarely seen in nature. Did the peptides at one time contain dehydroalanine and did it serve as a precursor for lysinoalanine If the answer to this question were yes, which amino acids, in turn, are potential precursors of dehydroalanine ... 

Let us turn to answering the second question first. Potential precursors of dehydroalanine, by way of -elimination, are the amino acids with functional groups at the /3-carbon atom. Such -elimination reactions are likely to be enzyme-catalyzed in nature. Additionally, the substrates may be suitably substituted at functions, such as the sulfhydryl group (thioether and sulfonium salt formation) or the hydroxyl group (carbohydrate attachment—glycopeptides and proteins phosphorylation). 

The ,/ -unsaturated amino acid dehydroalanine is a necessary precursor for the formation of lysinoalanine. The chemical events covered in the preceding discussion took place in the in vitro environment. Do a S-unsaturated amino acids play a role in any in vivo environment other than that of microorganisms From the latter domain have been isolated the peptides richest in ,/ -unsaturated amino acids, nisin (I) and subtilin (10). No direct evidence is available at this time to document convincingly the possible physiological role of ,/ -unsaturated amino acids in higher organisms. That dehydroalanine is the constituent amino acid of a plant protein has been reported for phenylalanine ammonia lyase from potato tubers (22). There is one case on record for the presence of... 

Additional common differences include demethylation of the amino acid residues 3 and/or 7 (41,78) whereby the dehydroalanine -variant was found to be 5-fold less toxic than the related methylated microcystin (39). Replacement of the Mdha residue with the precursor amino acid residue N-methyl-L-Ser is again minimally affecting the toxicity (34), but the variant with lanthionine in this position shows very weak toxicity probably due to the bulky modification (79). This indicates that the dehydro-amino acid residue itself is unimportant, a fact which is further supported by the observation that dihydro-microcystins, obtained by reduction of Mdha with sodium boron hydride, are hilly active both as the stereoisomeric mixture or as isolated isomers, i.e. microcystin-L-Ala and microcystin-D-Ala (80). 

Interesting work found in early patent literature deals with the use of dehydroalanine as an electrophile in the Michael addition of aminophosphinic acids, and diversification of the amino group appeared in the Pi position by coupling reactions [105]. This concept was extended by Vassifiou et al. by allowing a similar precursor (43) to participate efficiently in benzylation diversification reactions (Scheme 22) [106]. 

Fifure 7 Hypothetical chiosttepcon peptide synthetase complex. Shown in the arrows represent ing the peptide synthetases are the amino acids activated. In the growing polypeptide, the amino acids are shown as their proposed modihed counteiparts. Synthetase I catalyres the assembly of the first four amino acids. Synthetase II assembles and modifies the next five amino acids, and syn thetase IV releases the modified peptide precursor Abbreviations dA, dehydroalanine dT, dehy-drothreonine tr. thiaiole dtr, dihydrochiazole pyr, tetrahydropyridine. 

As already mentioned, dehydroalanine is the postulated reactive precursor for lysinoalanine. Direct evidence for dehydroalanine reactivity was obtained by Friedman et al. (1977). They showed that dehydroalanine derivatives convert lysine side chains in casein, bovine serum albumin, lysozyme, wool, or polylysine to lysinoalanine residues at pH 9 to 10. Related studies showed that protein SH groups generated by reduction of disulfide bonds are completely alkylated at pH 7.6 to lanthionine side chains. These studies demonstrate that lysinoalanine and lanthionine residues can be introduced into a protein under relatively mild conditions, without strong alkaline treatment. They also imply that it should be possible to explore nutritional and toxicological consequences of lysinoalanine and lanthionine consumption in the absence of racemiza-tion (see below). 

Stoichiometric quantities of Pd(OAc)2 have been used to effect the incorporation of the elements of dehydroalanine at the 3-position of an IV-protected form of 4-bromoindole and so providing a useful precursor to clavicipitic acid (eq 93). The reaction is carried out under an oxygen atmosphere. ... 

Amino acids with appropriate 3-substituents have frequently been employed as a,p-dehydroamino acid precursors, particularly in the form of peptide chain subunits. Elimination reactions from derivatives of P-hydroxy- and P-mercapto-a-amino acids are the examples most often encountered. Of less importance are amino acid derivatives with P-halogen substituents (38, 132, 337, 386, 394, 395), as these require more drastic elimination conditions. Mannich bases of monoalkyl acylaminomalonates have also found use as precursors of dehydroalanine derivatives (177, 438). 

---