Probing the active site of the sugar isomerase domain from E. coli arabinose-5-phosphate isomerase via X-ray crystallography

Lipopolysaccharide (LPS) biosynthesis represents an underexploited target pathway for novel antimicrobial development to combat the emergence of multidrug-resistant bacteria A key player in LPS synthesis is the enzyme D-arabinose-5-phosphate isomerase (API), which catalyzes the reversible isomerization of D-ribulose-5-phosphate to D-arabinose-5-phosphate, a precursor of 3-deoxy-D-manno-octulosonate that is an essential residue of the LPS inner core API is composed of two main domains an N-terminal sugar isomerase domain (SIS) and a pair of cystathionine-beta-synthase domains of unknown function As the three-dimensional structure of an enzyme is a prerequisite for the rational development of novel inhibitors, we present here the crystal structure of the SIS domain of a catalytic mutant (K59A) of E coli D-arabinose-5-phosphate isomerase at 2 6-angstrom resolution Our structural analyses and comparisons made with other SIS domains highlight several potentially important active site residues In particular, the crystal structure allowed us to identify a previously unpredicted His residue (H88) located at the mouth of the active site cavity as a possible catalytic residue On the basis of such structural data, subsequently supported by biochemical and mutational experiments, we confirm the catalytic role of H88, which appears to be a generally conserved residue among two-domain isomerases.