Structural Motifs in Proteins
In proteins, structure motifs usually consist of just a few elements; e.g., the 'helix-turn-helix' has just three. Note that, while the spatial sequence of elements is the same in all instances of a motif, they may be encoded in any order within the underlying gene. Protein structural motifs often include loops of variable length and unspecified structure, which in effect create the "slack" necessary to bring together in space two elements that are not encoded by immediately adjacent DNA sequences in a gene. Note also that, even when two genes encode secondary structural elements of a motif in the same order, they may specify somewhat different sequences of amino acids. This is true not only because of the complicated relationship between tertiary and primary structure but also because the size of the elements varies from one protein and the next.
- Beta hairpin:
Extremely common. Two antiparallel beta strands connected by a tight turn of a few amino acids between them.
- Greek key:
4 beta strands folded over into a sandwich shape.
- Omega loop:
a loop in which the residues that make up the beginning and end of the loop are very close together.
- Helix-loop-helix:
Consists of alpha helices bound by a looping stretch of amino acids. This motif is seen in transcription factors.
- Zinc finger:
Two beta strands with an alpha helix end folded over to bind a zinc ion. Important in DNA binding proteins.
See also: structural domain- Helix-turn-helix:
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