Hemoglobin is made up of two main components: four protein chains called globins and four small iron-containing molecules called heme groups. Together, these parts form a single hemoglobin molecule capable of carrying up to four oxygen molecules at once. Every red blood cell contains roughly 270 million hemoglobin molecules, making it the most abundant protein in your blood by a wide margin.
The Four Protein Chains
The protein portion of hemoglobin is a tetramer, meaning it’s built from four smaller protein subunits bundled together. In normal adult hemoglobin (called HbA), those four subunits are two alpha chains and two beta chains. The alpha and beta chains are similar in size but differ in their exact amino acid sequences, which matters for how the molecule picks up and releases oxygen.
Each of the four chains folds into a compact, roughly spherical shape with a pocket in the middle. That pocket is where the heme group sits. The folding pattern is specific: if the chain doesn’t fold correctly, the heme group can’t nestle into position, and the molecule won’t function.
The Heme Group and Its Iron Atom
Each of the four protein chains holds one heme group, so a complete hemoglobin molecule contains four hemes total. A heme group is a flat, ring-shaped structure called a porphyrin ring with a single iron atom sitting at its center. The porphyrin ring is built from four smaller rings (called pyrroles) linked together, creating a disc-like platform that cradles the iron.
The iron atom is the actual site where oxygen attaches. For oxygen binding to work, the iron must be in its reduced form, carrying a 2+ charge. Red blood cells contain special enzymes that keep the iron in this reduced state. When iron loses an electron and shifts to a 3+ charge, the hemoglobin can no longer carry oxygen. This non-functional form is called methemoglobin, and your body constantly works to prevent it from accumulating.
Building heme is an eight-step process that bounces between two parts of the cell. It starts inside the mitochondria, where a compound from the cell’s energy cycle combines with the amino acid glycine. The intermediate products then move out into the cell’s main fluid compartment for several additional steps before the nearly finished molecule returns to the mitochondria for the final reactions, including the insertion of the iron atom.
How the Parts Work Together
Hemoglobin’s power comes not just from its components but from the way those components communicate with each other. When the first oxygen molecule binds to one of the four heme groups, it triggers a physical shape change in that subunit. The iron atom shifts slightly, pulling on the protein chain attached to it. That tug ripples outward to the neighboring subunits, loosening them and making it easier for the next oxygen molecule to attach.
This cascading effect is called cooperative binding. The molecule essentially switches between two states: a “tense” state with low oxygen affinity and a “relaxed” state with high affinity. In your lungs, where oxygen is plentiful, this means hemoglobin loads up quickly. In your tissues, where oxygen levels drop, the process reverses, and hemoglobin releases its cargo efficiently. The alpha subunits play a particularly important role in triggering the switch from the tense to the relaxed shape. When oxygen binds to an alpha chain first, it changes the overall structure of the tetramer and raises the affinity of the remaining subunits.
Carbon Dioxide Transport
Oxygen isn’t the only gas hemoglobin carries. On the return trip from your tissues to your lungs, hemoglobin picks up carbon dioxide, the waste product of metabolism. Carbon dioxide doesn’t bind at the same spot as oxygen. Instead, it attaches to the exposed ends of the four protein chains (the amino-terminal groups), forming what’s called carbaminohemoglobin. This accounts for a meaningful fraction of the carbon dioxide your blood transports back to the lungs for exhaling, with the rest dissolved in blood plasma or converted to bicarbonate.
Hemoglobin Variations
Not all hemoglobin uses the same protein chains. Fetal hemoglobin (HbF) swaps the two beta chains for two gamma chains, keeping the same two alpha chains. This substitution gives fetal hemoglobin a higher oxygen affinity than adult hemoglobin, which allows a developing baby to pull oxygen from the mother’s blood across the placenta. After birth, production gradually shifts from gamma to beta chains, and by about six months of age, most of a baby’s hemoglobin is the adult HbA form.
Genetic mutations in the globin chains produce hemoglobin variants that can cause disease. Sickle cell disease, for example, results from a single amino acid change in the beta chain. That one swap causes hemoglobin molecules to stick together under low-oxygen conditions, distorting red blood cells into rigid, crescent shapes. Thalassemias arise when the body produces too little of either the alpha or beta chains, leaving an imbalance that damages red blood cells.
Normal Hemoglobin Levels
A standard blood test measures how much hemoglobin you have in grams per deciliter. Healthy ranges differ by sex: 13.2 to 16.6 g/dL for men and 11.6 to 15 g/dL for women. Children’s ranges vary by age. Levels below these ranges indicate anemia, which can stem from iron deficiency (not enough raw material for heme), blood loss, or problems with red blood cell production. Levels above the range can signal dehydration or conditions where the body overproduces red blood cells.
Because iron sits at the heart of every heme group, your dietary iron intake directly affects how much functional hemoglobin your body can build. Each hemoglobin molecule requires four iron atoms, one per heme. When iron stores run low, your body can still make the protein chains, but without the completed heme groups to fill them, the hemoglobin molecules can’t do their job.