Predictability of weak binding from X-ray crystallography: Inhaled anesthetics and myoglobin

Xenon and dichloromethane are inhalational anesthetic agents whose binding to myoglobin has been demonstrated by X-ray crystallography. We explore the thermodynamic significance of such binding using differential scanning cal orimetry, circular dichroism spectroscopy, and hydrogen-tritium exchange me asurements to study the effect of these agents on myoglobin folding stabili ty. Though specific binding of these anesthetics might be expected to stabi lize myoglobin against unfolding, dichloromethane actually destabilized myo globin at all examined concentrations of this anesthetic (15, 40, and 200 m M). On the other hand, xenon (1 atm) stabilized myoglobin. Thus, dichlorome thane and xenon have opposite effects on myoglobin stability despite locali zation in comparably folded X-ray crystallographic structures. These result s suggest a need for solution measurements to complement crystallography if the consequences of weak binding to proteins are to be appreciated.