β-Sheet capping: Signals that initiate and terminate β-sheet formation

In the present work, we address the question of whether different amino acids have different β-sheet initiating and terminating characteristics. Using a large scale analysis of parallel and antiparallel β-sheets in a non-redundant dataset of proteins, we observed that most of the amino acids show significant under- or over-representation in at least one of the positions at the two ends of β-sheets, which are denoted as N-cap and C-cap. In addition, based on statistical data and structural comparison, we found that certain amino acids, especially Asp, Asn, Gly and Pro have strong tendencies to block β-sheet continuation. Hence, we can consider these residues as β-sheet terminators. It was also proposed that the dipole moments in parallel β-sheets, whose direction is from C-terminal (partially negative) to N-terminal (partially positive), are much stronger than has previously been suggested. In fact, enhancement of dipole moments in parallel β-sheets is a result of the positioning of positively charged residues at N-cap and negatively charged residues at C-cap. This enhancement in dipole moment magnitude leads to strengthened dipolar interactions between parallel β-sheets dipoles and other partners especially α-helices dipoles. The results provide an explanation for the antiparallel alignment of parallel β-sheets with α-helices.