A membrane protein based biosensor: Use of a phosphate – H+ symporter membrane protein (Pho84) in the sensing of phosphate ions.

A label free biosensor for direct detection of inorganic phosphate based on potential-step capacitance
measurements has been developed. The high-affinity Pho84 plasma membrane phosphate/proton symporter
of Saccharomyces cerevisiae was used as a sensing element. Heterologously expressed and purified
Pho84 protein was immobilized on a self-assembled monolayer (SAM) on a capacitance electrode.
Changes in capacitance were recorded upon exposure to phosphate compared to the control substance,
phosphate analogue methylphosphonate. Hence, even without the explicit use of lipid membranes, the
Pho84 membrane protein could retain its capacity of selective substrate binding, with a phosphate detection
limit in the range of the apparent in vivo Km. A linear increase in capacitance was monitored in the
phosphate concentration range of 5–25 �M. The analytical response of the capacitive biosensor is in
agreement with that the transporter undergoes significant conformational changes upon exposure to
inorganic phosphate, while exposure to the analogue only causes minor responses.