title: Residues Asp164 and Glu165 at the substrate entryway function potently in substrate orientation of alanine racemase from E. coli: Enzymatic characterization with crystal structure analysis.
abstract: Alanine racemase (Alr) is an important enzyme that catalyzes the interconversion of L-alanine and D-alanine, an essential building block in the peptidoglycan biosynthesis. For the small size of the Alr active site, its conserved substrate entryway has been proposed as a potential choice for drug design. In this work, we fully analyzed the crystal structures of the native, the D-cycloserine-bound, and four mutants (P219A, E221A, E221K, and E221P) of biosynthetic Alr from Escherichia coli (EcAlr) and studied the potential roles in substrate orientation for the key residues involved in the substrate entryway in conjunction with the enzymatic assays. Structurally, it was discovered that EcAlr is similar to the Pseudomonas aeruginosa catabolic Alr in both overall and active site geometries. Mutation of the conserved negatively charged residue aspartate 164 or glutamate 165 at the substrate entryway could obviously reduce the binding affinity of enzyme against the substrate and decrease the turnover numbers in both D- to L-Ala and L- to D-Ala directions, especially when mutated to lysine with the opposite charge. However, mutation of Pro219 or Glu221 had only negligible or a small influence on the enzymatic activity. Together with the enzymatic and structural investigation results, we thus proposed that the negatively charged residues Asp164 and Glu165 around the substrate entryway play an important role in substrate orientation with cooperation of the positively charged Arg280 and Arg300 on the opposite monomer. Our findings are expected to provide some useful structural information for inhibitor design targeting the substrate entryway of Alr.
authors.affiliation: Drug Discovery and Design Center, State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, People's Republic of China.
authors.forename: Dalei
authors.initials: D
authors.lastname: Wu
authors.forename: Tiancen
authors.initials: T
authors.lastname: Hu
authors.forename: Liang
authors.initials: L
authors.lastname: Zhang
authors.forename: Jing
authors.initials: J
authors.lastname: Chen
authors.forename: Jiamu
authors.initials: J
authors.lastname: Du
authors.forename: Jianping
authors.initials: J
authors.lastname: Ding
authors.forename: Hualiang
authors.initials: H
authors.lastname: Jiang
authors.forename: Xu
authors.initials: X
authors.lastname: Shen
mesh_terms: D000411:Alanine Racemase / Q000737:chemistry / Q000378:metabolism*; D000595:Amino Acid Sequence; D001224:Aspartic Acid / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology*; D018698:Glutamic Acid / Q000737:chemistry*; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
chemical_list: D001224:Aspartic Acid; D018698:Glutamic Acid; D000411:Alanine Racemase