Scope, limitations and classification of lactamases

The hydrolysis of amide bonds is a ubiquitous process in nature and is catalyzed by various enzymes: Whereas N-unsubstituted amides are cleaved by amidases (EC 3.5.1.4), peptidases (EC 3.4.X.X) cleave peptide bonds in proteins and are involved in a number of vital physiological processes. Cyclic amides (lactams) are generally not hydrolyzed by proteases, but require specific lactamases. While the β-lactamase family (EC 3.5.2.6), acting on highly strained β-lactams, is constantly growing, lactamases able to hydrolyze γ- and δ-lactams are largely under-represented, owing to the lack of ring strain of 5- and 6-membered cyclic amides which accounts for their lower reactivity. To date, the only known substrate in which a 5- or 6-membered ring lactam is enzymatically cleaved is (±)-2-azabicyclo[2.2.1]hept-5-en-3-one (rac-Vince lactam), as well as four derivatives thereof. For these industrially relevant substrates, enantiocomplementary biocatalysts have been identified and their stereopreference was found to correlate with their amino acid sequence and protein structure: While (+)-lactamases belong to the amidase signature family, displaying the typical GGSS(S/G)GS motif in the center of the protein sequence and a conserved Ser-Ser-Lys catalytic triad, (-)-lactamase activity has been identified only among serine hydrolases, members of the α/β-hydrolase fold family, possessing a typical Ser-His-Asp catalytic triad. For larger 8- to 13-membered ring lactams, few active proteins have been identified, all are members of the amidase signature family. An enhanced partial CN double bond character in the amide bond explains the lower reactivity of particularly chemically stable lactams.