Nonprocessive

Nonprocessive describes enzymes or molecular motors that, after binding to their substrate, catalyze only a limited number of reactions before dissociating. This is in contrast to processive catalysts or motors, which can perform many consecutive catalytic events or mechanical steps without releasing the substrate. The term is used across biochemistry and cell biology to distinguish modes of action that affect efficiency, regulation, and coordination with other cellular processes.

In enzymes, nonprocessivity means the enzyme often dissociates after incorporating a single or a few catalytic turnovers. For example, bacterial DNA polymerase I is nonprocessive without additional factors and tends to perform nick translation or short synthesis, whereas the Pol III holoenzyme is highly processive due to a sliding clamp that tethers it to DNA and enables long synthesis runs.

In molecular motors, nonprocessive motors take one or a few steps per binding event. Myosin II, which drives muscle contraction, is nonprocessive at the single-molecule level, with many such molecules working in concert to produce continuous force. By contrast, myosin V is processive, capable of many successive steps before dissociating.

Measurement and relevance: Processivity is often quantified as the average number of turnovers or steps per binding event, or as a run length. It is assessed with single-molecule techniques such as fluorescence imaging or force spectroscopy by analyzing dwell times, stepping probabilities, and dissociation events. Processivity is influenced by substrate affinity, the presence of accessory subunits or cofactors, temperature, ionic conditions, nucleotide concentration, and mechanical load. See also processivity and sliding clamp.