Dipole moment in TIM alpha/beta fold proteins

TIM proteins of alpha/beta barrel fold from alpha/beta class as given in SCOP database were taken for dipole moment analysis. In all, 32 structures were analyzed for their dipole moment contributions. Representative structures from 20 super families in the alpha/beta fold, with different enzyme functions and 12 protein domains of TIM family in TIM super family were considered. The active sites of these proteins are located on the C-terminal side of the beta-strands. The molecules of same alpha/beta fold, but differing in their functionality also showed a common electrostatic field pattern along the barrel axis and had the dipole moment along the barrel axis and towards C-terminal end of the beta-strands. However, it is observed from our calculations that the dipole moment direction is possibly a consequence of the structural fold, with distribution of charges playing a modulatory role, and does not contribute to the location of active site. We show here that apart from the commonly held view as proposed by Hol et al [Hol W G L, van Duijnen PT and Berendsen H J C (1978) Nature (London), 273, 443-446] of the role of the alpha helical dipole moment, the beta-sheets in the barrel can also have a considerable dipole moment contribution. Taken together with our dipole moment analysis on integral membrane proteins [Vasanthi G and Krishnaswamy S (2002) Indian J Biochem Biophys 39, 93-100], this suggests the need to examine the role of dipole moment in the case of especially beta sheets forming barrels.