Molecular modeling and structure-based thermodynamic analysis of the StAR protein

Although much progress has been achieved in the study of the steroidogenic acute regulatory protein (StAR) dependent cholesterol transfer inside mitochondria, not one mechanism can account for all experimental data obtained to date. We have thus investigated the possibility that molecular modeling and structure-based thermodynamic calculations (STC) could enlighten these discrepancies. Starting from the crystallographic data of the human MLN64, a StAR homology model was generated and subjected to STC to verify the importance of StAR structural alterations for proper function. As expected, the model resembled the MLN64 crystal, although no binding site "tunnel" was obtained. Instead, a closed cavity was discovered, approximately the size and shape of cholesterol. This suggests that StAR does indeed require structural alterations to allow cholesterol binding, most evidently by the C-terminal alpha-helix above the U-shaped beta-barrel. Through STC, it is shown that unfolding of this helix is probable and leads to a 2% subpopulation of partially unfolded StAR, supportive of both the intermembrane shuttle and the molten globule hypotheses.