Heme is a specialized molecule built around a central iron atom, forming a complex organic ring structure. It is a component of several important proteins involved in functions ranging from gas transport to chemical catalysis. The iron atom at its core gives heme unique properties for life-sustaining biological activities. The molecule’s name is derived from the Greek word for blood, where it was first identified.
Heme’s Function in Oxygen Transport
Heme’s most recognized role is in the transport of oxygen throughout the body, a task carried out by the protein hemoglobin. Found within red blood cells, hemoglobin contains heme groups that act as the binding sites for oxygen. The iron atom within each heme group can reversibly bind to an oxygen molecule, meaning it can pick it up and release it as needed. This process begins in the lungs, where the high concentration of oxygen facilitates its binding to hemoglobin.
As blood circulates, these oxygen-laden red blood cells travel to tissues with lower oxygen levels. In these areas, the oxygen is released from the heme groups to supply the body’s cells.
A related protein, myoglobin, performs a similar function but on a more localized scale within muscle tissue. Myoglobin also contains a heme group and binds oxygen, but it acts as a storage reservoir rather than a long-distance transporter. It holds onto oxygen within muscle cells, ensuring a supply is available during periods of high metabolic demand, such as intense physical activity. This localized storage helps muscles maintain performance when the immediate oxygen delivery from hemoglobin is insufficient.
Heme’s Other Biological Roles
Beyond its role in moving oxygen, heme is a component of proteins involved in cellular energy production. Specifically, it is found in cytochromes, which are proteins embedded in the inner membranes of mitochondria. These cytochromes are part of the electron transport chain, a series of chemical reactions that generate adenosine triphosphate (ATP), the main energy currency of the cell.
The iron atom in heme facilitates this process by switching between different oxidative states, which allows it to accept and then donate electrons. This electron-shuttling capability is fundamental to the step-by-step process of the electron transport chain, which ultimately drives the synthesis of ATP.
Heme also serves as a component of certain enzymes, such as catalase. Catalase plays a protective role within cells by neutralizing hydrogen peroxide, a potentially harmful byproduct of metabolic processes. Hydrogen peroxide can damage cellular structures if allowed to accumulate. Catalase, using its heme group, efficiently breaks down hydrogen peroxide into harmless water and oxygen, functioning as a cellular cleanup system.
Heme Synthesis and Breakdown
The body produces heme through a complex, multi-step enzymatic pathway that takes place in both the mitochondria and the cytosol of cells. The primary sites for this synthesis are the liver and the bone marrow. The process involves several enzymes that together construct the porphyrin ring and ultimately insert an iron atom to form the final heme molecule.
Defects in the genes that code for the enzymes in this synthesis pathway can lead to a group of genetic disorders known as porphyrias. These errors can cause the accumulation of precursor molecules, leading to a variety of symptoms depending on the specific enzyme affected. The regulation of this pathway is tightly controlled, as an overproduction of heme or its precursors can be toxic to cells.
The lifecycle of heme concludes with its breakdown and recycling, primarily occurring when old red blood cells are removed from circulation. The heme molecule is broken down, a process that releases the iron so it can be stored and reused. During this degradation, heme is first converted into a green pigment called biliverdin, and then into a yellow-orange pigment called bilirubin. These breakdown products are responsible for the color changes seen in a healing bruise and can cause the yellowing of the skin and eyes known as jaundice.
Dietary Heme and Iron Absorption
Iron is obtained from the diet in two primary forms: heme iron and non-heme iron. Heme iron is found exclusively in animal-based foods such as red meat, poultry, and fish, where it is derived from the hemoglobin and myoglobin in animal tissues.
Non-heme iron is found in plant-based foods like leafy green vegetables, beans, and whole grains, though animal products also contain some. This form of iron is not part of a heme molecule and exists as an iron salt.
The body absorbs these two forms of iron differently. Heme iron is much more readily absorbed in the intestines than non-heme iron, and its absorption is less affected by other dietary components. The body’s ability to absorb non-heme iron can be influenced by other foods consumed at the same time. For instance, vitamin C can enhance its absorption, while compounds like tannins in tea and coffee can interfere with it.