Proteins containing iron (Fe) are essential for a wide range of biological functions,
primarily due to iron's ability to exist in multiple oxidation states (Fe²⁺ and Fe³⁺). This
property makes them perfect for facilitating electron transfer and oxygen transport.
Types of Fe-Proteins
Fe-proteins can be broadly classified based on the form in which iron is incorporated
into the protein structure.
• Iron-Sulfur (Fe-S) Proteins: These are a diverse class of proteins that contain
iron-sulfur clusters. These clusters, such as [2Fe-2S], [3Fe-4S], and [4Fe-4S], act
as electron carriers in many vital metabolic processes, including:
o Cellular Respiration: They are key components of the electron transport
chain in mitochondria (e.g., in Complexes I, II, and III).
o Photosynthesis: They play a role in the electron transport chain in
chloroplasts.
o Nitrogen Fixation: A well-known example is the Fe protein (NifH) of
nitrogenase, which acts as an obligate electron donor to the molybdenum-
iron (MoFe) protein to reduce dinitrogen to ammonia.
o Gene Regulation: Some Fe-S proteins, like aconitase, act as sensors for
iron levels, regulating gene expression to control iron homeostasis.
• Heme-Containing Proteins: These proteins contain a heme group, a complex
porphyrin ring with a central iron atom. The iron in the heme group can bind and
release oxygen, carbon monoxide, or other small molecules.
o Hemoglobin: Found in red blood cells, its primary function is to transport
oxygen from the lungs to the tissues. It has four heme groups, allowing it
to bind four oxygen molecules.
o Myoglobin: Found in muscle cells, it stores oxygen and facilitates its
diffusion within the muscle tissue. It contains a single heme group.
o Cytochromes: Found in mitochondria and other cellular compartments,
they are crucial for electron transport and redox reactions.
• Iron-Storage and Transport Proteins: These proteins are responsible for
managing the body's iron supply to prevent iron overload, which can be toxic.
o Ferritin: An intracellular protein that stores iron in a non-toxic form. It's
a spherical protein shell that can hold up to 4,500 iron atoms. It releases
iron when the body needs it, acting as an iron buffer.
o Transferrin: A glycoprotein found in blood plasma that transports iron. It
binds to two ferric (Fe³⁺) ions and delivers them to cells throughout the
body via transferrin receptors.
o Ferroportin: The only known cellular iron exporter. It facilitates the
movement of iron from cells into the bloodstream and is a key regulator of
systemic iron balance.
