What is the function of factor B

Factor B plays different roles in various fields. In immunology, it is a key component of the complement pathway activation pathway and participates in the body's defense; In protein science, it is a measure that describes the thermal motion or positional uncertainty of atoms, used to analyze the stability and dynamics of protein structures. Here is a specific explanation:

The role of factor B in immunology

Factor B is a heat-resistant globulin mainly synthesized by the liver and macrophages, and is an important factor in the complement pathway activation pathway. Its mechanism of action is as follows:

Complement pathway activation: Factor B, as a precursor of C3 activator, binds to C3b without antibody involvement and is cleaved by Factor D into Ba and Bb fragments. Bb and C3b form C3 convertase (C3bRb), which continuously cleaves C3, amplifies the complement cascade reaction, and ultimately promotes pathogen clearance (such as bacteria, viruses) or cell damage. Immune regulation: Recent studies have found that two cleaved fragments of factor B also have immune regulatory effects. Among them, Bb can promote the proliferation of activated B cells stimulated by Staphylococcus aureus Cowan I strain (SAC), while Ba has a significant inhibitory effect on the proliferation of activated B cells induced by B cell growth factor (BCGF), and it is concentration dependent.

Clinical significance:

Elevation: seen in malignant tumors such as ear malignant tumors, seminal vesicle malignant tumors, vaginal malignant tumors, etc. This may be due to the increased activity of the monoclonal antibody macrophage system and the enhanced ability to synthesize factor B in tumor patients, which is a non-specific immune response of the body against tumors.

. In addition, during the acute phase of recurrent respiratory infections, factor B is also significantly elevated. Reduced: seen in autoimmune hemolytic anemia, cirrhosis, chronic active hepatitis, acute glomerulonephritis, systemic lupus erythematosus and other diseases. In these diseases, the activation of the complement pathway leads to the depletion of factor B.

The role in protein science

Factor B (also known as temperature factor or Debye Waller factor) is a measure used to describe the thermal motion or positional uncertainty of atoms in protein or other molecular crystal structures.

. The mechanism of action is as follows:

Describing atomic thermal motion: In the molecular structure, atoms are not stationary and are affected by various thermal disturbances, vibrations, or other factors, causing them to deviate from their average position. The higher the B-factor, the greater the mobility or uncertainty of the atom in the crystal structure, in other words, the position of the atom is more unstable or the resolution of the analysis is poorer. Analyzing protein structure: Factor B can help researchers evaluate which parts of the structure are more flexible or dynamic, and which parts are relatively stable. The atoms with stable secondary structures such as alpha helices and beta folds in the main chain have a relatively low B factor; Atoms in side chains or disordered regions (such as cyclotrons or near active sites) have a relatively high B-factor.

Engineering enzyme applications: By utilizing factor B to recognize and interpret the rigidity, flexibility, and dynamics in proteins, the directed evolution during B-FIT can enhance protein thermal stability.

. For example, lipase from Bacillus subtilis (Lip A), which consists of 181 amino acids and was previously characterized by X-ray crystallography. Using a UV vis reader to monitor the Lip catalyzed hydrolysis of p-nitrophenyl octanoate and perform rapid screening of its activity after heat treatment, it can release yellow UV reactive p-nitrophenol anions. In this initial step, a 15 minute heat treatment is applied to determine the T5015 value, which is the temperature required to reduce the initial enzyme activity by 50% within this time span. Six of the samples showed improved variants. Then, by repeating heat treatment for 60 minutes (mutant X, T5060=89 ℃; Variant XI, T5060=93 ℃), while for variant XI, the Δ Δ G ‡ has been proven to be 4.0 kcal/mol. In the subsequent theoretical study of Lip A variant XI, molecular dynamics (MD) identified the formation of H-bonds due to mutations, which helped to suppress folding during heat treatment.