A direct spectropolarimetric assay of arabinose 5-phosphate isomerase

Arabinose 5-phosphate isomerase (API) catalyzes the reversible isomerization of Ribulose 5-phosphate (Ru5P) to Arabinose 5-Phosphate (Ar5P) for the production of 3-deoxy-2-octulosonic acid 8-phosphate (KDO), a component of bacterial lipopolysaccharide (LPS) of gram-negative bacteria. API is an attractive target for therapeutic development against gram-negative bacterial pathogens. The current assay method of API activity utilizes a general reaction for keto sugar determination in a secondary, 3-h color development reaction with 25 N sulfuric acid which poses hazard to both personnel and instrumentation. We therefore aimed to develop a more user friendly assay of the enzyme. Since Ru5P absorbs in the UV region and contains at least 2 chiral centers, it can be expected to display circular dichroism (CD). A wavelength scan revealed indeed Ru5P displays a pronounced negative ellipticity of 30,560 mDeg M^-1cm^-1 at 279 nm in Tris buffer pH 9.1 but Ar5P does not have any CD. API enzymatic reactions were monitored directly and continuously in real time by following the disappearance of CD from the Ru5P substrate, or by the appearance of CD from Ar5P substrate. The CD signal at this wavelength was not affected by absorption of the enzyme protein or of small molecules, or turbidity of the solution. Common additives in protein and enzyme reaction mixtures such as detergents, metals, and 5% dimethylsulfoxide did not interfere with the CD signal. Assay reactions of 1-3 min consistently yielded reproducible results. Introduction of accessories in a spectropolarimeter will easily adapt this assay to high throughput format for screening thousands of small molecules as inhibitor candidates of API.