Structure of beta-lactam synthetase reveals how to synthesize antibiotics instead of asparagine

The enzyme beta -lactam synthetase (P-LS) catalyzes the formation of the be ta -lactam ring in clavulanic acid, a clinically important beta -lactamase inhibitor. Whereas the penicillin beta -lactam ring is generated by isopeni cillin N synthase (IPNS) in the presence of ferrous ion and dioxygen, beta -LS uses ATP and Mg2+ as cofactors. According to sequence alignments, beta -LS is homologous to class B asparagine synthetases (AS-Bs), ATP/Mg2+-depen dent enzymes that convert aspartic acid to asparagine. Here we report the f irst crystal structure of a beta -LS. The 1.95 Angstrom resolution structur e of Streptomyces clavuligerus P-LS provides a fully resolved view of the a ctive site in which substrate, closely related ATP analog alpha,beta -methy leneadenosine 5 ' -triphosphate (AMP-CPP) and a single Mg2+ ion are present . A high degree of substrate preorganization is observed. Comparison to Esc herichia coli AS-B reveals the evolutionary changes that have taken place i n beta -LS that impede interdomain reaction, which is essential in AS-B, an d that accommodate P-lactam formation. The structural data provide the oppo rtunity to alter the synthetic potential of beta -LS, perhaps leading to th e creation of new beta -lactamase inhibitors and beta -lactam antibiotics.