Aromatic Side-Chain Interactions In Proteins. Near- And Far-Sequence His-X Pairs

Several studies have analysed aromatic interactions, involving mostly
phenylalanine, tyrosine and tryptophan. Only a few studies have considered
histidine as an interacting aromatic residue. An extensive analysis of aromatic
His-X interactions is performed here on a data set of 593 PDB structures: 68% of
the histidine are involved in aromatic pairs and 1271 non-redundant His-X pairs
were analysed. Thirty percent of these pairs involve an aromatic partner less
than 6 apart in the sequence. These near-sequence pairs correspond to
conformations which stabilise secondary structures, mainly alpha-helices when the
residues are 4 apart and beta-strands when they are 2 apart in the sequence. The
partners of the other His-X pairs (887, 70%) are more than 5 apart in the
sequence. Of these far-sequence pairs, 35% bridge beta strands and only 9%
helices. The near-sequence pairs are sterically constrained as supported by
conformer distribution. The X partners of far-sequence His-X pairs are mainly
"above" the histidine ring with tilted and normal rings, corresponding to a "T
shape; face to edge" orientation. Phenylalanine, the only aromatic residue with
no heteroatom, is a disfavoured partner, whereas histidine is the preferred one.
Heteroatom-heteroatom interactions are favoured in near-sequence as well as in
far-sequence His-His, His-Trp and His-Tyr pairs.