Iron in Heme Coordination Number

[Keerthana Sundar 1K]

Hello! So I was doing a UA worksheet, and the question asked what the coordination number for iron would be in a typical heme complex. The answer to the question in the worksheet was 4, but I was wondering why it wouldn't be 6? Can't the iron also bind to the histidine and oxygen to make a total of 6 ligands? I apologize if I missed the explanation to this in lecture. I would really appreciate any help! Thanks!

[Jeffrey Doeve 2I]

When we talk about the heme complex, we focus on the Fe and the porphyrin ligand(tetradentate). As a result, the coordination number of heme complex is 4 and its shape is square planar.

[Aydin Karatas 1F]

In the week 9, Monday lecture, Dr. Lavelle explained that the porphyrin ring (a chelating ligand structure) fills up 4 of these sites, creating the heme structure. The fifth site is occupied by histidine, an amino acid in the myoglobin. The sixth site is what oxygen would bind to so that oxygen can be transported throughout the body.

So in just the heme group, the coordination number of Fe is 4.

Hope this helps!

[Nicole Attiq 3D]

I would also assume that histidine and oxygen are outside of the coordination sphere, which is why it is not considered when determining the coordination number. Someone correct me if I'm wrong though.

[Samidha Menon 1E]

It isn't included because the O2 and myoglobin are not part of the heme structure, so when talking about the heme structure specifically it would only be 4.