Recombinant proteins: choosing between recombinant protein suppliers, what recombinant protein services quote for when custom proteins are ordered, when a custom recombinant protein or a purified recombinant protein beats a catalogue protein reagent, and where recombinant protein a sits as a reagent of its own

The decisions behind the quote

1. The expression system decides the protein. Bacterial expression is cheap, fast and gives no mammalian glycosylation or complex folding. Insect and mammalian systems cost considerably more and produce material that behaves like the native protein. A quote is only meaningful once the system is fixed, because it is the largest single price driver and the largest single quality driver.
2. Tags are a trade, not a free addition. An affinity tag makes purification straightforward and can change folding, activity and immunogenicity. Decide whether the tag is cleaved, what the residual scar is, and whether you need a tag-free preparation, because tag removal adds a step and a yield loss that must be in the quote.
3. Purity is a method, not a number. A purity claim means nothing until you know how it was measured. Ask which method produced the figure and ask for the trace. Two suppliers claiming the same purity by different methods are not making the same claim.
4. Activity is the specification that matters. For an enzyme or a ligand, ask what activity assay is run, on what schedule, and whether the result is lot-specific or a generic statement. A protein certified only for purity can be pure and inactive, which is the most common disappointment in this category.
5. Scale and repeat supply. Custom production is quoted per batch and the unit price falls sharply with scale. If the programme will need more, negotiate the repeat batch and the cell bank retention at the same time as the first order, when you have the most leverage.

Catalogue against custom

A catalogue protein is available now, lot-certified and cheaper, and you accept the supplier's construct, tag and system. A custom order gives you the construct you want and costs more, takes longer and carries development risk that should be staged rather than paid for up front.

The honest test is whether the catalogue construct differs from what your experiment needs in a way that changes the answer. Often it does not, and the custom order is buying precision the assay cannot see.

What to put in a request for quotation

Send the sequence or the accession, the expression system you require or will accept, the tag strategy including whether it is cleaved, the quantity and the buffer and format you need it in, the purity method and threshold, the activity assay, and the endotoxin limit if the protein goes near cells.

Endotoxin is the specification most often omitted and most often responsible for confusing cell-based results. If the protein touches cells, put a limit in the request rather than hoping.

Common questions

What is a recombinant protein?

A protein produced by expressing its gene in a host organism rather than purifying it from its natural source. The host, the construct and the purification together determine whether it behaves like the native protein.

Which expression system should I choose?

Bacterial for simple, non-glycosylated proteins where cost and speed matter; insect or mammalian where folding, post-translational modification or native activity matter. The choice dominates both price and behaviour, so fix it before comparing quotes.

Do I need a tag-free protein?

For structural work, for immunisation and for anything where the tag could interfere with binding or activity, usually yes. For routine pull-downs and detection the tag is often useful. Removing it costs a step and some yield.

Why does endotoxin matter?

Bacterially expressed protein carries endotoxin that activates immune cells at low levels, so it confounds any cell-based or in vivo experiment. Specify a limit and require a lot-specific result.

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