Elucidation and control of low and high active populations of alkaline phosphatase molecules for quantitative digital bioassay

Alkaline phosphatase (ALP), a homo-dimeric enzyme has been widely used in various bioassays as disease markers and enzyme probes. Recent advancements of digital bioassay revolutionized ALP-based diagnostic assays as seen in rapid growth of digital ELISA and the emerging multiplex profiling of single-molecule ALP isomers. However, the intrinsic heterogeneity found among ALP molecules hampers the ALP-based quantitative digital bioassays. This study aims quantitative analysis of single-molecule activities of ALP from Escherichia coli and reveals the static heterogeneity in catalytic activity of ALP with two distinct populations: half-active and fully active portions. Digital assays with serial buffer exchange uncovered single-molecule Michaelis-Menten kinetics of ALP; half-active molecules have halved values of the catalytic turnover rate, kcat, and the rate constant of productive binding, kon, of the fully active molecules. These findings suggest that half-active ALP molecules are heterogenic dimers composed of inactive and active monomer units, while fully active ALP molecules comprise two active units. Static heterogeneity was also observed for ALP with other origins: calf intestine or shrimp, showing how the findings can be generalized across species. Cell-free expression of ALP with disulfide bond enhancer and spiked zinc ion resulted in homogenous population of ALP of full activity, revealing that inactive monomer units of ALP are deficient in disulfide bond formation and zinc ion coordination, and also offering the way to prepare homogenous and active populations of ALP for quantitative digital bioassays of ALP.