Cell-free systems have great potential for delivering
robust, inexpensive,
and field-deployable biosensors. Many cell-free biosensors rely on
transcription factors responding to small molecules, but their discovery
and implementation still remain challenging. Here we report the engineering
of PeroxiHUB, an optimized H2O2-centered sensing
platform supporting cell-free detection of different metabolites.
H2O2 is a central metabolite and a byproduct
of numerous enzymatic reactions. PeroxiHUB uses enzymatic transducers
to convert metabolites of interest into H2O2, enabling rapid reprogramming of sensor specificity using alternative
transducers. We first screen several transcription factors and optimize
OxyR for the transcriptional response to H2O2 in a cell-free system, highlighting the need for preincubation steps
to obtain suitable signal-to-noise ratios. We then demonstrate modular
detection of metabolites of clinical interestlactate, sarcosine,
and cholineusing different transducers mined via a custom
retrosynthesis workflow publicly available on the SynBioCAD Galaxy
portal. We find that expressing the transducer during the preincubation
step is crucial for optimal sensor operation. We then show that different
reporters can be connected to PeroxiHUB, providing high adaptability
for various applications. Finally, we demonstrate that a peroxiHUB
lactate biosensor can detect endogenous levels of this metabolite
in clinical samples. Given the wide range of enzymatic reactions producing
H2O2, the PeroxiHUB platform will support cell-free
detection of a large number of metabolites in a modular and scalable
fashion.