Main chain H-bonds.

<p>Upon ligand binding, the main chain around heme anchor gains a total of four H-bonds. However, these additions do not occur simultaneously, and they take place differently in α and β. a. HIM of α. b. HIM of β. An HIM is a specialized distance matrix that contains only helix-forming H-bond lengths in a main chain, and has only several rows as labeled +1, γ, 3/10, α, π, and +6. The <i>i</i>-th column contains distances from a main chain carbonyl O of residue <i>i</i> to the amide Ns of residues <i>i</i> + 1 to <i>i</i> + 6. A distance value in a row within the proper range for H-bonding is colored in dark blue to show strong H-bond and to indicate a specific type of helix or turn. Large distances fade to white and short distances turns into yellow. PDB entries are labeled on the left. c. Main chain consensus around heme anchor in all states and subunits. T state structure is in light blue. A ligated state in R or R2 is in pink. The C-terminus of F that anchors the heme group is shown on the right. All side chains except the proximal His are removed. The constant H-bonds in all states are presented by dark green rods. The newly formed H-bonds in ligated states are in light green. These H-bonds not yet formed in T state are in brown. The strong H-bond gained at proximal His87αO-Val93αN is marked +6. The equivalent H-bond in β proximal His92βO-Val98βN already exists in T state. A weaker H-bond Ala88αO-Arg92αN continues the α conformation as labeled α. In R2 state, an additional strong H-bond Asp85αO-Lys90αN extends the π helix further upstream. In β, the equivalent new H-bond Glu90βO-Lys95βN is the only addition that extends the π helix upstream in both R and R2 states as labeled π. A weaker addition of H-bond in G on the left is Pro95αO-Lys99αN, which puts the beginning of Gα in a transition between 3/10 and α conformation. This transitional conformation is constant in β.