Enzymes lose activity before they look degraded

Catalysis can fail while the polypeptide chain is still the length the gel expects. A Coomassie band that looks as strong as last month's does not carry last month's specific activity with it. Active-site oxidation, inhibitor left over from the purification, a cofactor you forgot to add, and a partial unfold are common ways an enzyme goes quiet without looking degraded. This overview is the decision path for that disagreement. How mass and rate share a sentence is continued in activity assays versus abundance. Inhibitor carryover is named in protease inhibitors and when they interfere. The stained lane itself is interpreted in reading a protein gel.

The band is abundance of stain, not a rate

SDS-PAGE with a total-protein stain shows chains that are present and soluble in sample buffer. It is almost silent about the geometry of an active site. A nick that removes three residues from a terminus may not move the band enough to notice, and a chemical change to one cysteine may not move it at all. People still write "the protein looks fine" under a photo and shelve the assay. The photo answered a mass question. The experiment asked a catalytic one.

Specific activity puts them together: product formed per time, divided by the mass of enzyme, under stated conditions. If the mass is an albumin-equivalent Bradford number of a mixture, say so. If the mass is from a coefficient, the coefficient's assumptions apply. Either way, a drop in specific activity with an unchanged band is a real result. It means something in the catalytic machinery, the assay, or the soluble fraction changed.

Four mechanisms that spare the gel

Oxidation is the quiet one. Catalytic cysteines, iron-sulfur chemistry and some other centres lose function when they oxidise. The backbone is intact. A freshly prepared enzyme and one that sat in a thin film of liquid overnight can share a gel and not share a rate. Sometimes a reductant restores the cysteine enzyme. Sometimes the damage is past that. Add reductant only if the enzyme's instructions allow it, because a reductant is also an assay interference for many copper methods and for some substrates.

Inhibitor carryover is the rude one. PMSF or AEBSF left in a serine protease, leupeptin left in a cysteine protease, or EDTA left in a zinc enzyme will suppress activity. The gel remains flattering. Buffer exchange, done until conductivity and the assay agree, is the test. A spike of uninhibited control enzyme into the same buffer shows whether the buffer itself is poisonous.

Missing cofactor is the embarrassing one. Kinases need the right nucleotide. Many hydrolases and oxidoreductases need a metal or a prosthetic group that purification removed. UniProt often lists cofactors for a characterised enzyme. Add back only what that enzyme is known to require, at a range from its protocol, and include a no-enzyme blank so the cofactor is not mistaken for activity. Do not dump a random metal mix into the well.

Partial unfolding is the structural one. The chain is complete, the hydrodynamic behaviour may already be wrong, and the active site no longer forms. A cloudy sample, a void peak, or a protein that has been through too many freeze-thaws belongs in this bin. Clarifying and assaying the supernatant tells you whether any folded material remains. A band in the well after a native handling, compared with a normal SDS band, supports the picture. It still does not replace the rate measurement.

An assay at decision level

Define the product. If the reaction releases a coloured or fluorescent species, build a standard curve of that product in the same buffer. Read unknowns inside the curve. A blank without enzyme catches substrate breakdown and instrument drift. A blank without substrate catches enzyme-only colour. Time points, or a continuous rate in the linear window, beat a single endpoint that may already have plateaued.

Temperature and pH are part of the unit. An assay run at an unspecified "room temperature" in a hot laboratory is not comparable to one run at a controlled 25 or 37 Celsius. Use the range the enzyme's instructions give, and write the number you held. Substrate concentration relative to the enzyme's typical working range matters. Too little substrate makes every enzyme look slow. This article does not give a microlitre recipe. Follow the source protocol for volumes, and keep the logic of blank, standard and linear window.

If the first assay is zero, branch. Confirm the gel still shows intact protein in that aliquot. Confirm the blank is low. Then test carryover, cofactor and a fresh dilution. A zero from an assay with a high blank is an uninterpretable assay, not a dead enzyme. A zero from a clear assay, an intact band, no inhibitor and the right cofactor is when you earn the right to discuss unfolding or a failed refold.

Failure modes that masquerade as biochemistry

An expired substrate looks like a dead enzyme. Test the substrate with a trusted aliquot if you have one. A coupling enzyme that has itself died looks like the primary enzyme died. Check the coupling step alone. Evaporation in a warm plate raises concentrations mid-assay and bends the curve. Seal the plate and control the temperature. Calculating a rate from a single saturated endpoint makes two different enzymes look identical. Stay in the linear window.

Densitometry of the band cannot rescue a missing standard curve. Neither can a more sensitive stain. Sensitivity is not specificity of the reaction.

Research limits, climate and enquiry

Enzyme assays in this sense are research measurements. They are not clinical enzyme tests and not a diagnostic approval. Hazardous substrates and coupling enzymes follow their own safety notes. Biosafety of the source material is institutional.

Ice buckets in a hot room are a short story. An assay that says "on ice" and then waits an hour in meltwater has left the condition you think you held. Power cuts stop plate readers mid-kinetic. Discard that curve and rerun it. Record the room and the instrument temperature when the rate is temperature-sensitive. A frost-free freezer that cycled all week can be the entire explanation of a lower specific activity. Check storage history before you redesign the enzyme.

An enquiry about assay reagents should name the enzyme class, the product you will detect, any cofactor, and the inhibitor classes the sample may still contain. Components sit in the reagents and chemicals catalogue. Use a quote for that specification. If the inactive preparation is part of a broader expression question, the custom protein expression and purification page is an enquiry reference where the method can be discussed. It is not a claim that an enzyme is being produced or assayed as a service.