processivity

Processivity is a property of certain enzymes and molecular machines describing their ability to catalyze multiple rounds of a reaction on a substrate without dissociating. It is often expressed as the average number of catalytic events per binding event, or, for processive motors, the distance moved or number of steps taken before detachment.

In DNA replication, DNA polymerases can synthesize long stretches of DNA without falling off. Their processivity is greatly increased by accessory factors such as sliding clamps (e.g., PCNA in eukaryotes, the β-clamp in bacteria) that tether the polymerase to the template. The holoenzyme composition and template structure influence processivity, which is typically measured as nucleotides added per binding event or as the length of DNA synthesized per binding.

In transcription, RNA polymerases generally show substantial processivity along DNA; elongation factors and chromatin context modulate it. Processivity can be reduced by obstacles such as DNA damage or strong secondary structures; certain termination mechanisms act by reducing processivity.

For molecular motors, processivity describes how far the motor travels along a filament on a single binding event, typically reported as the number of steps or the distance moved before detachment. Kinesins and myosins often exhibit high processivity necessary for long-range transport.

Processivity is distinct from turnover rate, which measures catalytic speed per event. It depends on enzyme structure, substrate sequence, cofactors, and environmental conditions. High processivity is advantageous for rapid synthesis of long polymers, while low processivity can be relevant for proofreading or regulated stopping.