We report the temperature dependence of the formal potential of type 1 copper (Cu_(T1)) in Thermusthermophilus HB27 laccase. Employing [Ru(NH₃)₄ (bpy)](PF₆)₂ (0.505 vs. NHE) as the redox titrant, we found that the Cu_(T1)^(2+/+) potential decreased from approximately 480 to 420 mV (vs. NHE) as the temperature was raised from 20 to 65 °C. Of importance is that the ΔS_(rc)° of − 120 J mol⁻¹ K⁻¹ is substantially more negative than those for other blue copper proteins. We suggest that the highly unfavorable reduction entropy is attributable to CuT1 inaccessibility to the aqueous medium. Although the active site residues are buried, which is critical for maintaining thermostability, the flexibility around Cu_(T1) is maintained, allowing enzyme activity at ambient temperature
