When hemoglobin is in the T-state, what is the state of the iron in the heme group?

In the T-state, or tense state, of hemoglobin, the iron ion (Fe²⁺) in the heme group is indeed displaced from the plane of the heme. This displacement occurs because the heme group adopts a conformation that is less favorable for oxygen binding, contributing to hemoglobin’s lower affinity for oxygen in this state.

In the T-state, the heme iron is not in a planar position with respect to the surrounding porphyrin ring. The presence of deoxyhemoglobin, where not all heme groups are bound to oxygen, results in a situation where the iron is pulled slightly out of the plane because of the configuration of the surrounding amino acid residues and the overall protein structure. This arrangement stabilizes the T-state and inhibits oxygen binding.

When hemoglobin shifts to the R-state (relaxed state) upon oxygen binding, the iron moves downward into the plane of the heme group, allowing for a more effective interaction with the oxygen molecule. Therefore, the condition of the iron in the T-state, being displaced from the plane of the heme, is a crucial factor in understanding how hemoglobin regulates oxygen transport and release.