Heme is not a protein. It is a small, complex molecule that serves as a necessary component for many proteins to perform their biological work. Heme is a type of prosthetic group, meaning it is a non-amino acid structure tightly bound to a protein required for the protein’s function. This arrangement creates a functional unit where the protein provides the structure, and the heme provides the site of action. These compounds are known broadly as hemoproteins.
Heme: The Molecule That Isn’t a Protein
Heme is defined by its chemical structure, which differs from the long chains that characterize proteins. A protein is a large biological molecule made up of long chains of amino acids, folded into a specific three-dimensional shape. Heme, conversely, is a coordination complex, consisting of a ring-shaped organic compound known as a porphyrin, with a single iron atom held at its center.
The porphyrin ring is built from four smaller pyrrole rings linked together, creating a structure that strongly binds the iron ion. The iron atom gives heme its function, as it is the site capable of reversibly binding to small molecules like oxygen. Heme is synthesized primarily in the bone marrow and the liver through a series of enzyme-catalyzed reactions.
The relationship between the heme molecule and the protein it attaches to can be thought of as a specialized tool within a larger machine. The protein provides the scaffold, protecting the heme and controlling its environment, while the heme acts as the active tool. This partnership allows hemoproteins to achieve a diverse range of functions, including oxygen transport, electron transfer, and chemical catalysis.
Biological Partnership: Heme’s Function Within Proteins
The most recognized role of heme is enabling the transport of oxygen throughout the body as part of the protein hemoglobin. Hemoglobin is found within red blood cells, and each molecule is a complex of four protein subunits, with each subunit containing one heme group. A single hemoglobin molecule can bind up to four oxygen molecules, one at each heme iron site.
The protein part of hemoglobin creates a pocket for the heme, ensuring the iron atom can bind oxygen effectively without being permanently oxidized. When blood passes through the lungs, oxygen binds to the ferrous iron in the heme, causing a slight shape change. This change is communicated to the surrounding protein, increasing the oxygen-binding affinity of the other three heme groups in the molecule.
Heme also plays a role in muscle tissue as part of the protein myoglobin, which functions to store oxygen. Myoglobin is a simpler protein structure containing only one heme group, allowing it to hold a reserve of oxygen for immediate use by muscle cells during intense activity.
Heme is also an integral component of cytochromes, a class of hemoproteins involved in the electron transport chain, which generates cellular energy. In this function, the heme iron cycles between different oxidation states to shuttle electrons, rather than binding oxygen.
Understanding Heme Iron in the Diet
The term “heme” is often encountered in a nutritional context, referring to the iron consumed specifically from animal-based foods. Heme iron in the diet comes from the hemoglobin and myoglobin present in meat, poultry, and fish. It is consumed as part of these larger hemoproteins, which are broken down during digestion.
The body absorbs iron from the heme molecule much more efficiently than it absorbs non-heme iron, which is found in plant sources and fortified foods. Heme iron has a higher rate of absorption, typically ranging from 15% to 35%, because it is absorbed directly into the intestinal cells as an intact complex. Non-heme iron must be converted into a more soluble form before absorption, a process easily affected by other foods.
Compounds like phytates in grains and legumes, or tannins in coffee and tea, can inhibit the absorption of non-heme iron. Although heme iron usually represents only a small portion of total iron intake in Western diets, its high bioavailability means it accounts for a large amount of the iron absorbed by the body. The absorption of heme iron is also less regulated by the body’s current iron stores compared to non-heme iron.