(1-4). The enzyme is an ␣ 2 ␤ 2 complex, consisting of two ␣-subunits bound to a central ␤ 2 -dimer. Each ␣-subunit independently catalyzes the reversible aldol cleavage of indole-3-glycerol phosphate to indole and D-glyceraldehyde 3-phosphate (␣-reaction, Reaction 1). The ␤-subunit catalyzes the conversion of indole and Lserine to L-Trp, with PLP as the cofactor (␤-reaction, Reaction 2). The overall reaction is the combination of the ␣-reaction and the ␤-reaction (␣␤-reaction, Reaction 3). Indole is not observed as a free intermediate in the ␣␤-reaction (5-9), suggesting that indole is not released from the ␣-subunit of Trp synthase into solution during turnover; hence, Reaction 3 is the physiological reaction of Trp synthase. X-ray crystallographic investigations of Trp synthase from Salmonella typhimurium have revealed a 25-30-Å long interenzyme tunnel linking the ␣-and ␤-active sites, through which it has been proposed that indole travels to the ␤-active site directly from the ␣-active site after formation (10 -14). Moreover, intermediate complexes of the ␣-and ␤-reactions reciprocally regulate catalysis at the other site in an allosteric manner (11,(15)(16)(17). These allosteric interactions coordinate the forward progress of the ␣␤-reaction.In the absence of substrate, PLP is bound to ␤Lys87 in the form of the internal aldimine (E A ) (Scheme 1) (18). The reaction of L-Ser at the ␤-site, promoted by the presence of indole-3-glycerol phosphate bound at the ␣-active site, forms an external aldimine (E EA-Ser ), which is converted to an ␣-aminoacrylate Schiff's base (E AA ) with elimination of water, by the action of ␤Lys87 as a general base (19). In return, formation of the ␣-aminoacrylate Schiff's base with PLP at the ␤-site activates the ␣-reaction (20). D-Glyceraldehyde 3-phosphate bound to the ␣-site keeps the ␣-site closed, blocking the release of indole into solution and facilitating transfer of indole from the ␣-and ␤-active sites through the interenzyme tunnel. Nucleophilic addition of indole to the ␣-aminoacrylate intermediate at the ␤-site then affords the quinonoid intermediate of L-Trp (E Q ). ␤Glu109 activates the addition of indole, a weak nucleophile, probably by acting as a general base or H-bond acceptor to the N-H of the reacting indole (21). At this point, ␤Lys87 acts as a general acid, providing the proton for the conversion of E Q to the L-Trp external aldimine (E EA-Trp ) (19). Release of D-glyceraldehyde 3-phosphate from the ␣-site results in an open conformation, which then allows release of the L-Trp product from the ␤-active site. Nucleophilic attack of ␤Lys87 on E EA-Trp

BACKGROUND: Pharmaceutical companies continue to evaluate β-amino acids and β-lactams in a range of drug candidates. The development of a highly efficient and selective bioresolution of cyclic β-lactam substrates could yield enantiopure lactams and β-amino acids with medicinal potential. The aim of this work was to discover and develop a biocatalyst capable of selectively hydrolysing β-lactam substrates.

Nitration of Nα,N1-bis(trifluoroacetyl)-L-tryptophan methyl ester with HNO3 in acetic anhydride at 0° C provides Nα-trifluoroacetyl-2-nitro-L-tryptophan methyl ester in 67% yield, whereas nitration in trifluoroacetic acid at 0° C gives Nα-trifluoroacetyl-6-nitro-L-tryptophan methyl ester in 69% yield.

Methods for the regioselective nitration of N α ,N 1 -bis(trifluoroacetyl)-L-tryptophan methyl ester (I) in the 2-or 6-position are studied. -(OSBORNE, A. S.; SOM, P.; METCALF, J. L.; PHILLIPS*, R. S.; Bioorg. Med. Chem. Lett. 18 (2008) 21, 5750-5752; Dep. Chem., Univ. Ga., Athens, GA 30602, USA; Eng.) -M. Bohle 10-188