strate mimetic that directs the intrinsic synthesis activity of the protease to the side-chain carboxyl moiety of Asp and Glu (Scheme 5.1.4). Similar to linear substrate mimetics, the iso-type counterparts bear a site-specific ester leaving group which is, however, linked to the w-carboxyl moiety of Asp and Glu instead of being at the C-terminus of the peptide donor. This different architecture leads to a shift in the synthetic activity of the enzyme to the side-chain of Asp and Glu; this finally results in the formation of isopeptides.

More efficient than other methods, substrate mimetics enable proteases to react not only with non-specific coded amino acids, but also with non-amino acid-derived acyl donors. By using OGp esters of 4-phenylbutyric acid (Pbu-OGp) and benzoic acid (Bz-OGp) as the donor, a variety of amino acid amides and peptides as acyl acceptors, and clostripain and chymotrypsin as the catalysts the appropriate isosteric peptide products could be obtained in excellent yields [8, 12]. By using the unique specificity of clostripain toward the amino component the approach could even be expanded to the synthesis of a wide variety of N-linked neo-peptidoglycans. The approach enables selective coupling of carboxylate moieties derived from the side chains of Asp, Glu, and the C-terminus of peptides both with simple monomeric and with highly complex carbohydrates, for example D-glucosamine, d-galactosamine, muramic acid, and moenomycin A (Scheme 5.1.5) [13]. Even C-N bonds completely outside peptidic structures have been synthetic targets of the approach. For example, non-amino acid-derived carboxylic components, for example Pbu-OGp and Bz-OGp, and a large number of non-peptidic amino components, for example aliphatic and aromatic amines, amino alcohols, non-a-amino carboxylic acids, and diamines, have been efficiently coupled [12]. To conclude, this remarkable activity opens up new possibilities of easy synthesis of a broad spectrum of linear peptides, isopeptides, all-D peptides, peptide isosteres, peptide-carbohydrate conjugates, and non-amino acid-derived carboxylic acid amides under extraordinary mild conditions unachievable with classical protease approaches.

Scheme 5.1.5. Clostripain-mediated coupling of the Leu-enkephalin sequence Z-Asp-Tyr-Gly-Gly-Phe-Leu-OH with a moenomycin A analog.

396 | 5.1 Protease-catalyzed Formation of C-N Bonds 5.1.5

Good Carb Diet

Good Carb Diet

WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.

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