Stability/activity tradeoffs in Thermus thermophilus HB27 laccase

Abstract

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 Cu_(T1) 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.

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