Structural and functional consequences of substitutions at the Pro108-Arg14 hydrogen bond in bovine adrenodoxin

Elimination of Pro108 in bovine adrenodoxin is known to result in the formation of a misfolded protein that is not able to incorporate a [2Fe-2S] cluster and rapidly degrades upon expression in E. coli. However, no experimental explanation for this phenomenon has been demonstrated so far. Using the recently obtained 3D structure of the truncated mutant Adx(4-108) we have studied the reasons of the protein stabilization by the proline residue by means of site-directed muta-genesis. Two main results have been obtained (i) the conserved hydrogen bond Pro108-Arg14, that connects different structural domains of Adx, contributes 6 kJ/mol into the protein stability and (ii) the presence of proline at position 108 provides a low conformational entropy of the unfolded state, supporting a gain in the Gibbs energy of 5.4 kJ/mol at 37 degrees C.