Structural Studies of the Bacteriophage Lambda Lysozyme Complexed with a Chitohexasaccharide

Abstract

Lysozyme from the bacteriophage lambda, 1aL, complexed with a chitohexasaccharide has been solved to 2.6 Å by molecular replacement using a mutant form of 1aL as a model. The protein packs as a dimer in the crystal with the backbones of both monomers being nearly identical. The inhibitor molecule resides in the deep cleft in the middle of the bilobal enzyme. Subsites A to Dare occupied by one (GlcNAc)₆ molecule, while the remaining sites interact with two rings from the adjacent (GlcNAc)₆ molecule. The binding mode of 1aL is compared to other lysozymes (HEWL, HuL, GEWL, T4) and Slt70. Interesting differences are noted in the stacking interactions in ring D and the extensive interactions in ring E. It is hypothesized in the thesis that one possible role of the peptide moiety is to interact with Tyr-132, preventing it from forming stacking interactions with ring D, allowing the sugar to penetrate deeper into the active site. Ring E is buried deeply in the enzyme and has a low thermal factor. In addition, the active site is much narrower in 1aL than in other lysozyme structures. A possible explanation has been suggested that rings E and F stay in the active site longer than those in lysozyme to prevent water molecules instead of the O6 atom of ring D to participate in the nucleophilic attack at the end of the reaction.