User:Stephen Mills/Secondary Structure: Helices: Difference between revisions

<Structure load='Tut_alpha_helix.pdb' size='500' frame='true' align='left' caption='' scene='User:Robert_Dutnall/Sandbox_1_alpha_helix/A_helix_mainchain_label/4' name='h-bonds'/>

I'll reset the structure for you. Now, look at the structure carefully and identify the mainchain NH and C=O groups. What is the orientation of these groups with respect to the helix axis? Click <scene name='User:Robert_Dutnall/Sandbox_1_alpha_helix/A_helix_mainchain_label/4' target='h-bonds'>here</scene>  if the image doesn't load properly.

<scene name='User:Robert_Dutnall/Sandbox_1_alpha_helix/A_helix_mainchain_label_hbonds/10' target='h-bonds'>Click here to zoom in a little and add hydrogen bonds for the mainchain (dashed lines).</scene> The hydrogen bonds connect backbone NH groups and C=O groups. The N-H group is the hydrogen bond donor, the oxygen in the C=O group is the acceptor (N-H--->O=C). Each hydrogen bond is approximately 3.0 Angstroms (0.3 nm) in length (measured between the N and the O). These hydrogen bonds connect amino acids that are spaced 4 residues apart in the primary sequence. The NH group is from amino acid X and the O is from amino acid X-4 (where X is the number of the amino acid in the sequence).

<scene name='User:Robert_Dutnall/Sandbox_1_alpha_helix/A_helix_mainchain_label_hbonds/9' target='h-bonds'>Click here to zoom out again.</scene> You should be able to see that all of the mainchain NH and C=O groups are involved in hydrogen bonds. This is possible because 1) all the mainchain NH groups and C=O groups are parallel to the helix axis, 2) all the NH groups point toward the N-terminus of the helix, and 3) all the C=O groups point toward the C-terminus. Only the NH groups in the first (N-terminal) turn, and the C=O groups in the last (C-terminal) turn of the helix do not have hydrogen bonding partners. However, if this helix were part of a larger protein, these groups would participate in hydrogen bonds with other parts of the protein. This is called 'helix capping' and the other groups involved are typically from amino acid side chains (that are called 'helix capping residues').

<scene name='User:Robert_Dutnall/Sandbox_1_alpha_helix/A_helix_sidechains/3' target='h-bonds'>Click here to show the side chains again.</scene> The atoms of each amino side chain have been colored light blue to show them more clearly. <scene name='User:Robert_Dutnall/Sandbox_1_alpha_helix/A_helix_sidechains_axis/1' target='h-bonds'>Click here to change the view to look down the helix axis.</scene>

What is the general orientation of the side chains with respect to the helix axis? You should observe that each side chain points away from the helix axis, but points down toward the N-terminus of the helix (this is clearer if you only show the first bond of each side chain: <scene name='User:Robert_Dutnall/Sandbox_1_alpha_helix/A_helix_sidechains_cropped/1' target='h-bonds'>click here</scene> or <scene name='User:Robert_Dutnall/Sandbox_1_alpha_helix/A_helix_sidechains_cropped/2' target='h-bonds'>click here</scene>).