What Is the Molecular Weight of Ferritin?

Ferritin is a protein found in nearly all living organisms that is responsible for storing and releasing iron in a controlled manner. This function allows the body to maintain a healthy balance of iron, preventing both deficiency and overload. The protein’s unique structure and large size are directly related to its capacity to safely manage this reactive element.

Understanding Molecular Weight in Proteins

The molecular weight of any molecule, including a protein, represents the total mass of all the atoms that compose it. For large biological molecules, this value is often expressed in units called Daltons (Da) or, more commonly, kilodaltons (kDa), where one kilodalton is equal to 1,000 Daltons. A Dalton is an alternate name for the atomic mass unit, and using these units makes it easier to discuss the mass of enormous structures like proteins.

Proteins are constructed from smaller building blocks called amino acids, and the total molecular weight is the sum of the weights of all these constituent amino acids. The average molecular weight of a single amino acid is about 110 Daltons, so a protein’s final mass is directly influenced by the number and type of amino acids in its sequence. This measurement is a characteristic used by scientists to identify and purify proteins.

The Architecture of Ferritin

Ferritin is a complex, multi-subunit protein that forms a hollow, spherical shell assembled from 24 individual protein subunits. This empty protein shell is known as apoferritin. The assembly results in a nanocage with an external diameter of about 12 nanometers and an internal cavity of about 8 nanometers, where iron is stored.

These 24 subunits are of two different types, known as the H (heavy) and L (light) chains. The H-chain is slightly larger than the L-chain. The two subunit types are homologous, sharing about 50% of the same amino acid sequence, which allows them to fit together to form a stable, spherical structure.

The different subunits also have distinct functions. The H-chain subunit has ferroxidase activity, which converts iron from its soluble ferrous (Fe2+) form into the ferric (Fe3+) form for storage. The L-chain subunits provide a surface that helps in the nucleation of the iron core, promoting the safe storage of iron inside the protein’s cavity.

Ferritin’s Molecular Weight Values

The molecular weight of the iron-free apoferritin shell is approximately 450 to 480 kilodaltons (kDa). This value varies because different tissues produce ferritin with different ratios of H and L subunits. The H-subunit has a molecular weight of about 21 kDa, while the L-subunit is smaller at around 19 kDa.

A significant change in molecular weight occurs when ferritin stores iron, at which point it is called holoferritin. The hollow core can store up to 4,500 iron atoms, which dramatically increases the total molecular weight. For example, holoferritin can be about 55% more massive than its iron-free counterpart. While apoferritin has a relatively stable molecular weight, holoferritin’s is highly variable and depends on the amount of iron it holds.

Why Ferritin’s Molecular Weight Matters

The large size and molecular weight of the ferritin molecule are directly linked to its function of iron sequestration. Its substantial mass and spherical structure ensure it remains within cells, providing a stable environment for iron storage. This prevents potentially toxic iron from participating in harmful chemical reactions, such as the generation of reactive oxygen species.

The composition of ferritin, known as isoferritins, differs between tissues. These variations have different ratios of H and L subunits tailored to specific needs. For example, tissues with high metabolic activity like the heart have ferritin rich in H-subunits. In contrast, tissues focused on long-term iron storage, such as the liver and spleen, produce ferritin with a higher proportion of L-subunits.

The large size of the ferritin complex is also a factor in how it is processed and degraded. When ferritin aggregates, it can form an insoluble iron storage complex called hemosiderin, which is often found when iron levels are excessively high.