Transferrin in Cell Culture: A Key Iron Carrier for Serum‑Free Systems

I. Structure of Transferrin (TF)

TF contains 679 amino acid residues, with a molecular weight of approximately 79 kDa and an isoelectric point of about 6.6. Its amino acid sequence includes 38 cysteine residues capable of forming 19 pairs of disulfide bonds, which are critical for stabilizing the protein structure, and it possesses three N‑glycosylation sites. TF consists of two structurally similar iron‑binding domains: an N‑terminal domain (336 aa) and a C‑terminal globular domain (343 aa). The two domains are connected by a short spacer sequence.

Each Fe³⁺‑binding site in the domains contains four conserved amino acids, including two tyrosines, one aspartic acid and one histidine, and these residues are arranged in an octahedral geometry. In addition, two oxygen atoms provided by carbonate ions are required at the Fe³⁺‑binding sites to stabilize iron atoms. Near the TF binding sites, Gly‑65, Glu‑83, Tyr‑85, Arg‑124, Lys‑206, Ser‑248 and Lys‑296 play key roles in iron release. The protonation effect of the Lys‑206‑Lys‑296 base pair located in the domain opposite to the N‑terminus can induce the open or closed conformation of TF.

II. Functions of Transferrin (TF)

Iron is one of the essential elements for sustaining cell growth, proliferation and metabolic activities. Most free iron in the body is transported and delivered by transferrin. Transferrin (TF), the major iron‑binding and iron‑transporting β‑globulin in plasma, is primarily responsible for carrying iron absorbed from the gastrointestinal tract and iron released from erythrocyte degradation. In human serum, the concentration of TF is approximately 2.5 g/L, 30% of which is iron‑occupied. TF mainly exists in three forms: holo‑transferrin (holo‑TF), partially saturated transferrin (sidero‑TF), and apo‑transferrin (apo‑TF).

In the extracellular compartment (pH ≈ 7.5), transferrin receptor (TFR) has a higher binding affinity for iron‑bound TF (holo‑TF) than for the iron‑free form (apo‑TF). As a result, iron‑bound TF is subsequently internalized, whereas apo‑TF is released at the cell surface. In endosomes (pH ≈ 5.6), TFR preferentially binds to apo‑TF over holo‑TF, mediating the transport of apo‑TF from endosomes back to the plasma membrane. Through this process, iron is delivered into cells and TF is recycled.

Iron is critical for in‑vitro cell culture. As a cofactor of various enzymes, transferrin participates in multiple cellular physiological functions, three key aspects of which are described below:

1. Participation in cellular respiratory metabolism
Mitochondria are the site of cellular oxidative metabolism and the energy center of cells. Succinate dehydrogenase on the mitochondrial membrane plays a vital role in this process. As a component of succinate dehydrogenase, iron is involved in cellular oxidative metabolism and energy production.

2. Protection of cells against oxidative damage
As the core of the iron‑porphyrin structure, iron is a key constituent of enzymes such as catalase and peroxidase. It eliminates the toxicity of hydrogen peroxide, phenols, amines and aldehydes, protecting cells from damage caused by peroxides including H₂O₂. It provides antioxidant defense for cells and improves cellular health.

3. Improvement of cell density and viability
Iron is an essential element for cell proliferation. Without iron, cells cannot progress from the G1 phase to the S phase during proliferation, and iron deficiency induces cellular apoptosis and death. Iron deficiency impairs DNA synthesis during cell replication, because iron is a component of ribonucleotide reductase — the rate‑limiting enzyme that catalyzes the conversion of ribonucleotides to deoxyribonucleotides in DNA synthesis.

III. Applications of Transferrin in Serum‑Free Culture

Transferrin (TF) is an indispensable component in cell culture. Especially in serum‑free media (SFM), TF has been widely applied on a large scale in biomanufacturing, such as monoclonal antibody production, recombinant protein synthesis, as well as the culture of immune cells and stem cells.

1.Enhancement of Cell Growth and Product Yield
Supplementation of TF in serum‑free media (SFM) supports high‑density culture of CHO cells, prolongs production cycles and boosts protein yields.
Adding TF to serum‑free media for hybridoma cells improves cell proliferation rate and viability.
By regulating iron homeostasis, transferrin reduces protein glycoform heterogeneity and ensures batch‑to‑batch consistency of pharmaceuticals.
When applied to serum‑free culture of CAR‑T cells, NK cells and mesenchymal stem cells (MSCs), TF maintains cell viability and functional characteristics.

2.Formulation Simplification and Standardization
As a core component of supplements including ITS (insulin‑transferrin‑selenium) and SPIT/SPITE, TF can substitute serum to achieve animal‑free, chemically defined formulations and reduce batch‑to‑batch variations.

3.Substitution for Chemical Chelators
Chemical chelators (e.g., EDTA, citrate) are hard to regulate redox cycles and prone to producing free radicals. The advantage of iron binding by TF is that it relies on natural receptor‑mediated pathways without generating free radical by‑products.

Beijing Anrate Biotechnology Co., Ltd. provides industrially produced animal‑free recombinant human transferrin derived from yeast and CHO cells. Inquiries and trial tests are welcome.

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