The second hybrid system is based on a variant of carbonic
anhydrase (CA), an enzyme that binds Zn2+ with a KD of
approximately 4 pM. In elegant molecular engineering work,
the Fierke laboratory was able to tune the specificity and affinity
of CA to create a series of probes that could respond to
physiological concentrations of Zn2+.159 Early generations of
these probes featured a small-molecule fluorophore covalently
linked to carbonic anhydrase. When Zn2+ binds carbonic
anhydrase, a second cofactor (the fluorophore dapoxyl
sulfonamide) binds to an open site on the Zn2+ ion, allowing
energy transfer from the dapoxyl moiety to the fluorophore on
the enzyme. Recently, Zeng et al. reported a long wavelength,
emission ratiometric modification of the carbonic anhydrase
Zn2+ sensor that could be amenable to imaging in tissues.160
This new system uses Alexa Fluor 594 as a FRET donor and
Chesapeake Blue sulfonamide as the acceptor fluorophore. One
limitation of these versions of the sensor is the requirement for
microinjection or the attachment of cell-penetrating peptides to
introduce it into cells, because covalent attachment of the
fluorophore prevented the probe from being genetically
encodable.40 However, recent iterations have replaced the
small-molecule fluorophore with a FP, thus allowing part of the
probe (CA-FP) to be genetically encoded and transfected into
cells. The membrane-permeable dapoxyl sulfonamide can be
added directly to cells to complete the hybrid system