β‐Lactamase of Bacillus licheniformis 749/C at 2 Å resolution

Abstract

Two crystal (A and B) of the 29,500 Da Class A β‐lactamase (penicillinase) from Bacillus licheniformis 749/C have been examined crystallographically. The structure of B‐form crystals has been solved to 2 Å resolution, the starting model for which was a 3.5 Å structure obtained from A‐form crystals. The β‐lactamase has an α + β structure with 11 helices and 5 β‐strands seen also in a peinicilin target DD‐peptidase of Streptomyces R61.¹ Atomic parameters of the two molecules in the asymmetric unit were refined by simulated annealing at 2.0 Å resolution. The R factor is 0.208 for the 27,330 data greater than 3 (F), with water molecules excluded from the model. The catalytic Ser‐70 is at the N‐terminus of a helix and is within hydrogen bonding distance of conversed Lys‐73. Also interacting with the Lys‐73 are Asn‐132 and the conserved Glu‐166, which is on a potentially flexible helix‐containing loop. The structure suggests the binding of β‐lactam substrates is facilitated by interactions with Lys‐234, Tyhr‐235, and Ala‐237 in a conserved β‐strand peptide, which is antiparallel to the β‐lactam's acylamido linkage; an exposed cavity near Asn‐170 exits for acylamido Substituent. The reactive double bond of clavulanate‐type inhibitors may interact with Arg‐244 on the fourth β‐strand. A very similar binding site architecture is seen in the DD‐peptidase.