processivité

Processivité is a term used in biochemistry to describe the ability of an enzyme to catalyze multiple rounds of a chemical transformation without dissociating from its substrate. In the context of nucleic acid–processing enzymes, it is often defined as the average number of monomer units added (for example nucleotides by a DNA polymerase) before the enzyme dissociates from the substrate, or equivalently the length of the product produced per binding event.

Practically, processivity can be viewed as the likelihood that an enzyme will continue to act on the substrate after each catalytic step. If the probability of dissociation after a single step is p, the average number of steps before dissociation is 1/p. Thus, high processivity corresponds to a low dissociation probability and to long, continuous products. Processivity is distinct from catalytic rate or velocity, which describe the speed of each individual cycle; a highly processive enzyme can still act slowly per step, but it remains bound for many steps.

Determinants of processivity include direct enzyme–substrate interactions, the presence of accessory factors, and structural features such as sliding clamps or processivity factors. In DNA replication, sliding clamps (for example PCNA in eukaryotes or the beta clamp in bacteria) and clamp-loader complexes greatly increase processivity by keeping the polymerase bound to the DNA. Other enzymes, such as telomerases or certain polymerases, achieve high processivity through specialized subunits or domains.

Methods to assess processivity range from bulk assays measuring product length distributions to single-molecule techniques that monitor real-time catalytic runs. Understanding processivity helps explain how enzymes efficiently process long substrates and how cellular cofactors modulate replication, repair, and transcription machinery.