. 2021 Aug 30;187(2):550–571. doi: 10.1093/plphys/kiab339

Table 3.

Brief summary of the principal definitions used to describe GECIs properties

Biochemical parameter	Description
K _d (µM)	Apparent dissociation constant for Ca²⁺ of the sensor, at this concentration, half of the indicators are bound with Ca²⁺. Given that Hill coefficients are usually higher than 1, the K_d delineates the optimal concentration range at which a GECI should be used.
k _on (s⁻¹)	Indicates the rate of Ca²⁺ association shown by the sensor, and thus the speed by which a sensor responds to an increase in Ca²⁺ levels.
k _off (s⁻¹)	Indicates the rate of Ca²⁺ dissociation shown by the sensor, and thus the speed by which a sensor responds to a decrease in Ca²⁺ levels.
Hill coefficient	Indicates the cooperativity of the sensor in the Ca²⁺ binding process. A value greater than 1 indicates that binding of one Ca²⁺ ion facilitates the binding of another. The CaM-based GECI bind four Ca²⁺ ions. The closer to 1 the more linear the output of the reporter.
Dynamic range	For non-ratiometric indicators indicate the maximal fluorescence intensity (typically in a Ca²⁺-bound state) divided by minimal fluorescence intensity (determined in the presence of EGTA). For ratiometric indicators indicate the maximum fluorescence emissions ratio (typically in a Ca²⁺-bound state) divided by minimal fluorescence emissions ratio (determined in the presence of EGTA).