Is use of the hydrophobic moment a sound basis for predicting the structure-function relationships of membrane interactive alpha-helices?

Amphiphilic alpha-helices play a fundamental role in protein membrane association and show a segregation of polar and apolar amino acid residues. Based on correlations between amphiphilic properties and biological function, a number of theoretical approaches have been developed, which quantify alpha-helix amphiphilicity and then attempt to assign function. The most commonly used measure of amphiphilicity is the hydrophobic moment, < microH >, which, when used in conjunction with an alpha-helix's mean hydrophobicity, < H >, has been used to classify membrane interactive amphiphilic alpha-helices as either surface active or transmembrane. Here, the predictive efficacy of plot methodology is reviewed by examining published data, which compare the function of known membrane interactive amphiphilic alpha-helices to that assigned by this methodology. The results of this review are discussed in relation to the reliability of < microH > as a quantifier of alpha-helical amphiphilicity, and the ability of < microH > and < H > to describe alpha-helical structure / function relationships. It is concluded that hydrophobic moment plot methodology is not a generally reliable predictor of alpha-helical structure / function relationships. It appears that the inefficacy of plot methodology is primarily due to the inability of the plot diagram to accommodate the heterogeneity of the alpha-helical classes it attempts to define. However, the predictive efficacy of the methodology appears to be improved if other alpha-helical parameters are also considered when assigning alpha-helical function. It is suggested that the conventional methodology should be seen only as an indicator for the assignation of structure / function relationships, providing a guide to future experimental investigations.