Thermodynamic and kinetic characterization of two methyl-accepting chemotaxis heme sensors from Geobacter sulfurreducens reveals the structural origin of their functional difference

The periplasmic sensor domains GSU582 and GSU935 are part of methyl-accepting chemotaxis proteins of the bacterium Geobacter sulfurreducens containing one c-type heme and a PAS-like fold. Their spectroscopic properties were shown previously to share similar spectral features. In both sensors, the heme group is in the high-spin form in the oxidized state and low-spin after reduction and binding of a methionine residue. Therefore, it was proposed that this redox-linked ligand switch might be related to the signal transduction mechanism. We now report the thermodynamic and kinetic characterization of the sensors GSU582 and GSU935 by visible spectroscopy and stopped-flow techniques, at several pH and ionic strength values. Despite their similar spectroscopic features, the midpoint reduction potentials and the rate constants for reduction by dithionite are considerably different in the two sensors. The reduction potentials of both sensors are negative and well framed within the typical anoxic subsurface environments in which Geobacter species predominate. The midpoint reduction potentials of sensor GSU935 are lower than those of GSU582 at all pH and ionic strength values and the same was observed for the reduction rate constants. The origin of the different functional properties of these closely related sensors is rationalized in the terms of the structures. The results suggest that the sensors are designed to function in different working potential ranges, allowing the bacteria to trigger an adequate cellular response in different anoxic subsurface environments. These findings provide an explanation for the co-existence of two similar methyl-accepting chemotaxis proteins in G. sulfurreducens.