Structural studies have shown that electrostatic interactions play a major part in the binding of dihydrolipoyl dehydrogenase (E3) to the peripheral subunit-binding domain (PSBD) of the dihydrolipoyl acyltransferase (E2) in the assembly of the pyruvate dehydrogenase multienzyme complex of Bacillus stearothermophilus. The binding is characterized by a small, unfavorable enthalpy change (ΔH° = +2.2 kcal/mol) and a large, positive entropy change (TΔS° = +14.8 kcal/mol). The contributions of individual surface residues of the PSBD of E2 to its interaction with E3 have been assessed by alanine-scanning mutagenesis, surface plasmon resonance detection, and isothermal titration calorimetry. The mutation R135A in the PSBD gave rise to a significant decrease (120-fold) in the binding affinity; two other mutations (R139A and R156A) were associated with smaller effects. The binding of the R135A mutant to E3 was accompanied by a favorable enthalpy (ΔH° = -2.6 kcal/mol) and a less positive entropy change (TΔS° = +7.2 kcal/mol). The midpoint melting temperature (Tm) of E3-PSBD complexes was determined by differential scanning calorimetry. The R135A mutation caused a significant decrease (5 °C) in the Tm, compared with the wild-type complex. The results reveal the importance of Arg 135 of the PSBD as a key residue in the molecular recognition of E3 by E2, and as a major participant in the overall entropy-driven binding process. Further, the effects of mutagenesis on the ΔCp of subunit association illustrate the difficulties in attributing changes in heat capacity to specific classes of interactions.
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