▲ FIGURE 3-6 Motifs of protein secondary structure.

(a) Two helices connected by a short loop In a specific conformation constitute a helix-loop-helix motif. This motif exists in many calcium-binding and DNA-binding regulatory proteins. In calcium-binding proteins such as calmodulin, oxygen atoms from five loop residues and one water molecule form ionic bonds with a Ca2+ ion. (b) The zinc-finger motif is present in many DNA-binding proteins that help regulate transcription. A Zn2+ ion is held between a pair of p strands (blue) and a single a helix (red) by a pair of cysteine residues and a pair of histidine residues. The two invariant cysteine residues are usually at positions 3 and 6 and the two invariant histidine residues are combinations of secondary structures have been conserved in evolution. To date, hundreds of motifs have been cataloged and proteins are now classified according to their motifs.

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