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【正文】 H...:O (2 kcal/mol) ProteinSecondary Structure 1951,PaulingPZ0?= 57??= 47?p= z0= Helicesrepetitive secondary structureNCHelices are the most abundant form of secondary structure containing approximately 3238% of the residues in globular proteins (Kabsch and Sander, 1983) ahelix310helix?helix ? ? n r P 57 47 310 49 26 ? 57 7 Paral? 119 +113 Antiparal ? 139 +135 Parametersofsecondarystructure[n] is the number of residues per helical turn[r] is the helical rise per residue (nm)[p] is the helical pitch (nm). Hbond Atoms in Hbond loopradius i, i+4 13 310 i, i+3 10 ? i, i+5 16 Parametersofsecondarystructureahelixintroduction3238% of all residues in globular proteins The average length of an alpha helix is 10 residues. Found(64+/7,41+/7) / ideal(,)The structure repeats itself every 197。 along the helix axis, . we say that the ahelix has a pitch of 197。. ahelices have amino acid residues per turn, . a helix 36 amino acids long would form 10 turns. The separation of residues along the helix axis is 197。, . the ahelix has a rise per residue of 197。the phi and psi angles of the alpha helix lie in the center of an allowed, minimum energy region of the Ramachandran (phi, psi) map. Whyalphahelixisabundantinnativeglobularprotein?the dipoles of hydrogen bonding backbone atoms are in near perfect alignment. the radius ( angstrom)of the helix allows for favorable van der Waals interactions across the helical axis side chains are well staggered minimizing steric interference? CO group toward carboxyl terminus? NH group toward amide terminus? Hbond, i(i+4)? Side chain: i(i+3)。 i(i+4)? interactions between i and i+4 stabilize helixDistortions of ahelices Themajorityofahelicesinglobularproteinsarecurvedordistorteds
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