Helix-hairpin-helix
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In molecular biology helix-hairpin-helix (HHH) is a DNA-binding protein structural motif found in proteins that interact with DNA in a non-sequence-specific manner.[2]
The helix-hairpin-helix motif consists of two anti-parallel α-helices connected by a short hairpin loop involved in interactions with DNA which usually contains a consensus glycine-hydrophobic amino acid-glycine sequence pattern (GhG). The two α-helices are packed at an acute angle of ~25–50° that dictates the characteristic pattern of hydrophobicity in the sequences, while other DNA-binding structures like the helix-turn-helix (HTH) motif, which is also formed by a pair of helices, can be easily distinguished by the packing of the helices at an almost right angle.[3]
Function
Many proteins containing the helix-hairpin-helix motif mediate non-sequence-specific DNA binding through hydrogen bonds between protein backbone nitrogens and DNA phosphate groups. The HHH motif differs from other DNA-binding motifs like helix-turn-helix, which typically bind DNA in a sequence-specific manner.[2]