Serpin A-7 (Recombinant)

Frequently Asked Questions

What is the expected molecular weight of recombinant Serpin A-7 on SDS-PAGE and Western blot?

The mature Serpin A-7 polypeptide is 415 amino acids, giving a predicted unglycosylated MW of approximately 46 kDa. Because this recombinant is expressed in HEK293 cells and carries native-like N-linked glycosylation, the apparent MW on reducing SDS-PAGE typically runs between 54–60 kDa — a shift of roughly 8–14 kDa attributable to glycan addition. Expect a single diffuse band in that range; heterogeneous glycosylation accounts for slight band broadening. Deglycosylation with PNGase F collapses the band to ~46 kDa, which can serve as a useful confirmation step.

Is recombinant Serpin A-7 a classical serine protease inhibitor or purely a carrier protein — does it have inhibitory activity?

Serpin A-7 (Thyroxine-Binding Globulin, TBG) is structurally a serpin but is not a functional serine protease inhibitor under physiological conditions. Its reactive-center loop does not efficiently form stable acyl-enzyme complexes with canonical serpin targets. Its primary role is high-affinity binding of thyroxine (T4, Kd ~10⁻¹⁰ M) and, with roughly 10-fold lower affinity, T3. This recombinant is therefore appropriate for hormone-binding assays, carrier-protein functional studies, and antibody validation — not for classical serpin inhibitory activity assays such as elastase or thrombin inhibition.

How do I set up a T4 hormone-binding assay using recombinant Serpin A-7, and what starting concentration is recommended?

For equilibrium binding or competitive displacement assays using radiolabeled or fluorescent T4 analogs, a starting concentration of 50–200 nM recombinant Serpin A-7 in binding buffer (50 mM Tris-HCl pH 7.5, 150 mM NaCl, 0.1% BSA) is a practical starting range. Binding reactions are typically equilibrated at 37°C for 30–60 minutes. Because T4 binding is strongly pH-sensitive (affinity drops sharply below pH 7.0), maintain buffer pH carefully. Include a no-protein background control and a known T4 standard curve. Confirm protein concentration by A280 or BCA before each assay run.

What buffer conditions are optimal for storing and diluting recombinant Serpin A-7 to preserve T4-binding activity?

This recombinant is supplied in 50 mM Tris-HCl pH 7.5, 150 mM NaCl, 10% glycerol — conditions compatible with direct use in most binding and immunoassay formats. For T4-binding experiments, dilute into assay buffer (50 mM Tris-HCl pH 7.4–7.5, 150 mM NaCl, 0.1% BSA) rather than PBS alone; BSA reduces non-specific T4 adsorption to tube walls. Avoid Tween-20 concentrations above 0.05%, which can disrupt hydrophobic hormone interactions. Working dilutions are stable at 4°C for up to 48 hours; do not refreeze working stocks.

Can I use recombinant Serpin A-7 as a Western blot positive control with the matched anti-Serpin A-7 antibody RP-SerpinA7?

Yes — this is one of the primary intended uses. RP-SerpinA7 (/anti-serpin-a7-rabbit-polyclonal-antibody) was raised and validated against the same full-length mature sequence expressed here, so antigen–antibody compatibility is confirmed. Load 20–50 ng of recombinant Serpin A-7 per lane alongside your cell lysate or serum samples. Under reducing conditions, expect a band at 54–60 kDa. This loading range produces a clean signal without saturation at typical primary antibody dilutions of 1:500–1:2,000. Running the recombinant in an adjacent lane removes ambiguity about band identity in complex samples.

How much recombinant Serpin A-7 should I load for Western blot to avoid signal saturation with RP-SerpinA7 antibody?

For routine positive-control lanes, 25–50 ng per lane is a reliable starting point when using RP-SerpinA7 at 1:1,000 dilution with standard HRP-conjugated secondary antibodies and ECL detection. If you are running a dilution series to bracket dynamic range, test 10, 25, 50, and 100 ng. At 100 ng, some saturation is possible with highly sensitive detection substrates. Because the recombinant runs at 54–60 kDa (glycosylated), confirm band position matches your experimental sample before interpreting results from cell lines with differing glycosylation states.

How should I handle, aliquot, and store recombinant Serpin A-7 to maximize shelf life?

Upon receipt, spin briefly at 400 × g to collect any condensate, then aliquot into single-use volumes before the first freeze. Store at −20°C; avoid repeated freeze-thaw cycles, which progressively reduce both protein integrity and T4-binding capacity. Purity is >90–95% by SDS-PAGE, and endotoxin is <0.1 EU/µg by LAL assay. Properly stored aliquots are stable for at least 12 months. Do not store working dilutions long-term — prepare fresh from frozen stock for each experiment. Adding carrier BSA (0.1%) to diluted working stocks can reduce adsorptive losses when working at sub-nanomolar concentrations.

Why use HEK293-expressed recombinant Serpin A-7 instead of E. coli-expressed protein for functional assays?

Serpin A-7 carries multiple N-linked glycosylation sites that contribute to correct folding and full hormone-binding capacity. Prokaryotic expression systems lack the glycosylation machinery to reproduce this, and bacterially expressed TBG frequently exhibits reduced T4-binding affinity and aggregation propensity. HEK293 expression yields a glycoprotein with glycan patterns close to the endogenous serum protein, making this recombinant a more reliable functional standard for binding assays and a more structurally representative antigen for antibody validation by ELISA or Western blot.