**9. Enzymes forming amide and ester bonds via carboxylic-phosphate anhydride**

Activation of carboxylic group of amino acid by ATP-phosphorylation yielding mixed car‐ boxylic-phosphate anhydride is quite popular mechanism of synthesis of amide and ester bonds.

*D*-Alanyl-*D*-alanine ligase is one of the key enzymes in peptidoglycan biosynthesis and is an important target for antibacterial drugs. The enzyme catalyzes the condensation of two ala‐ nine molecules using ATP to produce *D*-Ala-*D*-Ala (Fig. 24), which is the terminal peptide of a peptidoglycan monomer. Analogs of *D*-Ala-*D*-Ala, in which phosphonate or phosphi‐ nate moiety replaces peptide bond appear to be potent inhibitors. As determined by kinetic [Ellsworth et al., 1996], X-Ray [Wu et al., 2008], and molecular modeling studies the inhibitor behaves as substrate and reacts with ATP to produce ADP and a tight-binding phosphory‐ lated transition state analogue, which exerts inhibitory action against the enzyme (Fig. 24). Thus, these compounds might be rather considered as suicide substrates.

**Figure 24.** Mechanism of condensation of two molecules of D-Ala catalyzed by *D*-alanine-*D*-alanine ligase.

Similarly acting inhibitors have been found for glutamine synthetase (phosphinothricin and methionine sulfoximine and their analogs) [Berlicki et al., 2005; Berlicki & Kafarski 2006; Berlicki, 2008], γ-glutamylcysteine synthetase [Hibi et al., 2004], or penicillin binding pro‐ teins [Dzekieva et al., 2010; Dzekieva et al., 2012] (Fig. 25).

**Figure 25.** Inhibitors activated by ATP.

**Figure 23.** Transition state analog inhibitor of peptidyl transferase.

**anhydride**

bonds.

346 Drug Discovery

**9. Enzymes forming amide and ester bonds via carboxylic-phosphate**

Activation of carboxylic group of amino acid by ATP-phosphorylation yielding mixed car‐ boxylic-phosphate anhydride is quite popular mechanism of synthesis of amide and ester

*D*-Alanyl-*D*-alanine ligase is one of the key enzymes in peptidoglycan biosynthesis and is an important target for antibacterial drugs. The enzyme catalyzes the condensation of two ala‐ nine molecules using ATP to produce *D*-Ala-*D*-Ala (Fig. 24), which is the terminal peptide of a peptidoglycan monomer. Analogs of *D*-Ala-*D*-Ala, in which phosphonate or phosphi‐ nate moiety replaces peptide bond appear to be potent inhibitors. As determined by kinetic [Ellsworth et al., 1996], X-Ray [Wu et al., 2008], and molecular modeling studies the inhibitor behaves as substrate and reacts with ATP to produce ADP and a tight-binding phosphory‐ lated transition state analogue, which exerts inhibitory action against the enzyme (Fig. 24).

Thus, these compounds might be rather considered as suicide substrates.
