Bacteria near-universally contain a cell wall sacculus of murein (peptidoglycan), the synthesis of which has been intensively studied for over 50 years. In striking contrast, archaeal species possess a variety of other cell wall types, none of them closely resembling murein. Interestingly though, one type of archaeal cell wall termed pseudomurein found in the methanogen orders Methanobacteriales and Methanopyrales is a structural analogue of murein in that it contains a glycan backbone that is cross-linked by a L-amino acid peptide. Here, we present taxonomic distribution, gene cluster and phylogenetic analyses that confirm orthologues of 13 bacterial murein biosynthesis enzymes in pseudomurein-containing methanogens, most of which are distantly related to their bacterial counterparts. We also present the first structure of an archaeal pseudomurein peptide ligase from Methanothermus fervidus DSM1088 (Mfer336) to a resolution of 2.5 Å and show that it possesses a similar overall tertiary three domain structure to bacterial MurC and MurD type murein peptide ligases. Taken together the data strongly indicate that murein and pseudomurein biosynthetic pathways share a common evolutionary history.
SummaryFive amino acids (Y105, Y176, Y189, Y189, W207) that constitute the substrate binding site of PHB depolymerase PhaZ7 were identified. All residues are located at a single surface-exposed location of PhaZ7. Exchange of these amino acids by less hydrophobic, hydrophilic or negatively charged residues reduced binding of PhaZ7 to PHB. Modifications of other residues at the PhaZ7 surface (F9, Y66, Y103, Y124, Y169, Y172, Y173, F198, Y203, Y204, F251, W252) had no effect on substrate binding. The PhaZ7 wild-type protein, three muteins with single amino acid exchanges (Y105A, Y105E, Y190E), a PhaZ7 variant with deletion of residues 202-208, and PhaZ7 in which the active-site serine had been replaced by alanine (S136A) were crystallized and their structures were determined at 1.6-2.0 Å resolution. The structures were almost identical but revealed flexibility of some regions. Structural analysis of PhaZ7 (S136A) with bound 3-hydroxybutyrate tetramer showed that the substrate binds in a cleft that is composed of Y105, Y176, Y189 and Y190 and thus confirmed the data obtained by site-directed mutagenesis. To the best of our knowledge this is the first example in which the substrate binding site of a PHB depolymerase is documented at a molecular and structural level.
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