Metallo-beta-lactamase protein fold
The metallo-β-lactamase (MBL) superfamily constitutes a group of proteins found in all domains of life that share a characteristic αββα fold with the ability to bind transition metal ions. Such metal binding sites may have divalent transition metal ions like Zn(II), Fe(II)/Fe(III) and Mn(II), and are located at the bottom of a wide cleft able to accommodate diverse substrates. The name was adopted after the first members of the superfamily to be studied experimentally: a group of zinc-dependent hydrolytic enzymes conferring bacterial resistance to β-lactam antibiotics. These zinc-β-lactamases (ZBLs) inactivate β-lactam antibiotics through hydrolysis of the β-lactam ring. Early studies on MBLs were conducted on the enzyme βLII isolated from strain 569/H/9 of Bacillus cereus. It was named βLII because it was the second β-lactamase shown to be produced by the bacterium; the first one, βLI, was a non-metallic β-lactamase, i.e., insensitive to inhibition with EDTA (βLII was renamed BcII over time). Low-resolution X-ray crystallographic analyses published in 1995, disclosed the new αββα fold that would become the hallmark of the MBL superfamily, along with a single Zn(II) ion bound to a three-histidine motif, resembling the active site typical of carbonic anhydrases. Thus, BcII and ZBLs in general were thought