native PAGE

Meaning: Native (nondenaturing) PAGE runs proteins through a polyacrylamide gel without SDS, heat, or reducing agents such as β-mercaptoethanol. Folded structure, including quaternary structure, is generally preserved, so multisubunit proteins usually migrate as intact complexes and many enzymes stay active. Migration depends on net charge at the buffer pH, size, and shape. The gel acts as a sieve, so compact, highly charged proteins tend to move faster.

On the MCAT: A passage compares native PAGE and SDS-PAGE of the same sample. A protein made of two identical subunits gives one band at the dimer position on native PAGE but one band at the monomer mass on SDS-PAGE. Native PAGE is chosen to keep activity or to test whether proteins associate. Direction follows charge: if the buffer pH is above the protein's pI, the protein is net negative and runs toward the anode (+); if below, it tends to run toward the cathode (−).

Common confusion: Students read native bands as a measure of molecular mass. Two proteins of equal mass can separate by charge or shape, and a larger protein can outrun a smaller one. SDS gives a roughly uniform negative charge per unit mass, which is what makes SDS-PAGE mass-based.