Ketosteroid Isomerase: Difference between revisions
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====Alpha-Helix Capping Motifs==== | ====Alpha-Helix Capping Motifs==== | ||
α-helix capping motifs are defined by specific patterns of hydrophobic interactions and hydrogen bonding that occur at both the initiation and termination of these secondary structural elements. <ref name="Aurora">PMID:9514257 </ref> KSI contains examples several of these motifs involved in both C-terminal and N-teminal capping, with two illustrations of C-terminal capping motifs as illustrated below (nomenclature adapted from that of Aurora and Rose <ref name="Aurora" />): | |||
[[Image: helix_capping_motifs.png|thumb|center|1000px|'''Examples of C-terminal helix capping motifs present in KSI.''' (A) α-L capping motif. (B) ]] | |||
=====Alpha-L C-terminal capping motifs==== | |||
An α-L C-terminal motif (type VIa) located at the terminus of helix A (Ala17(C3) to Leu23(C’’’) and is defined by the following amino acid sequence ALNA-GDLD where A20 is in the C-cap position. The α-L motif is stabilized by a hydrogen bond between the backbone carbonyl of Ala17 (C3) and the backbone amide hydrogen of Gly21 (C’) and hydrophobic interactions between Leu18 (C2) and Leu23 (C’’’). The unusual use of the C2 side chain stabilizing this hydrophobic interaction is most likely due the constrains put on the system by Gly21 being the only residue between helices 2 and 3 restricting Leu23 (C’’’) from participating in the expected hydrophopic interaction with Ala17 (C3). Dihedral angles are in approximate agreement with those observed in other α-L motifs within approximately 30o. ¬ | An α-L C-terminal motif (type VIa) located at the terminus of helix A (Ala17(C3) to Leu23(C’’’) and is defined by the following amino acid sequence ALNA-GDLD where A20 is in the C-cap position. The α-L motif is stabilized by a hydrogen bond between the backbone carbonyl of Ala17 (C3) and the backbone amide hydrogen of Gly21 (C’) and hydrophobic interactions between Leu18 (C2) and Leu23 (C’’’). The unusual use of the C2 side chain stabilizing this hydrophobic interaction is most likely due the constrains put on the system by Gly21 being the only residue between helices 2 and 3 restricting Leu23 (C’’’) from participating in the expected hydrophopic interaction with Ala17 (C3). Dihedral angles are in approximate agreement with those observed in other α-L motifs within approximately 30o. ¬ | ||
[[Image: helix_capping_motifs.png|thumb|center|1000px|'''Examples of helix capping motifs present in KSI''']] | [[Image: helix_capping_motifs.png|thumb|center|1000px|'''Examples of helix capping motifs present in KSI.''' (A) α-L capping motif. (B) ]] | ||
