R-Structure: Definition & Explanation

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn about the R-structure of hemoglobin. We will learn how the R-structure allows oxygen to be bound to oxygen in the lungs. We will also learn about how carbon monoxide can affect the R-structure of hemoglobin.


As you take a deep breath in, think about what the breath is doing. We know that the oxygen in the air is needed to keep us alive. If we don't have oxygen for even 1 minute then brain cells begin to die, and within 15 minutes the brain is usually completely dead. But how does the oxygen get from the breath we take to our cells? It is through a compound called hemoglobin.

Hemoglobin is the body's oxygen transport system. Hemoglobin is structured so that at times it has more affinity to oxygen or it can slightly change its structure and have less affinity to oxygen. When it has a greater affinity to oxygen this is referred to as the R-Structure(or the relaxed state) and when it has less affinity to oxygen this is referred to as the T-structure (or the tense state). In this lesson we will discuss the R-structure and why this structure is important.

Hemoglobin is a part of the blood and transports oxygen from the lungs to the tissue through the blood. In this image of hemoglobin notice the iron (abbreviated as 'Fe') in the center.

A hemoglobin unit has iron in the center that binds to oxygen to bring it from the lungs to the tissue

It is this iron that binds to oxygen to take it to the blood. When it is in a high oxygen environment (like in the lungs) it is able to bind oxygen much easier (because it is in the R-structure) but when it is in a low oxygen environment (like in the tissues) it does not bind oxygen as well (because it is in the T-structure) so the oxygen can be released into the tissue.

Hemoglobin is so efficient at bringing oxygen to the tissues that even people who are anemic (so they have less hemoglobin) to the point of having half the normal amount of hemoglobin they can still function fairly well. They will just get more tired faster.

The R-structure

The entire hemoglobin unit is actually made up of four of the units that were shown above. These units are connected with a protein chain. In the R-structure these four units are drawn closer together in the center. When there is a high concentration of oxygen then even if hemoglobin is in the T-structure at least one of the units can bind with oxygen. As soon as one binds it pulls the other units closer. This makes it easier for further oxygen to bind to the other units. This continues until all four until are filled and the hemoglobin has pulled into the center.

Carbon Monoxide Warning

Oxygen isn't the only thing that can bind to the hemoglobin. Carbon monoxide is actually the preferred ligand (a ligand is something that is attached to the hemoglobin) and it binds 250 times better than oxygen. So if the lungs have carbon monoxide in them then the hemoglobin will bind to the carbon monoxide before it will bind to the oxygen. This prevents the oxygen from being brought to the tissue.

We are exposed to small amounts of carbon monoxide on a daily basis, but it is such a small amount that the typical person only has about 1% of their hemoglobin units occupied by carbon monoxide. Since we still have plenty of hemoglobin units to bring oxygen to the tissues we don't feel any ill effects from the carbon monoxide.

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