“…There is some disagreement about whether fusion proteins with simple linker sequences intended to provide only proximity and/or orientation of the enzymes constitute substrate channeling in a formal sense . In systems such as those which employ fusion proteins, , protein/DNA scaffolds, − or surface coimmobilization of constituents, , significant catalytic rate enhancements have been observed which exceed expectations when proximity alone is considered; Sigman et al illustrated that, for a typical enzyme (e.g., turnover frequency, k , of 10 s –1 ) and a typical aqueous-phase small molecule (e.g., diffusion coefficient, D , of 10 –5 cm 2 s –1 ), there is no appreciable enhancement of local concentrations of the intermediate species at subsequent reaction centers compared to the bulk solution . That is, diffusion typically occurs at time scales that are orders of magnitude faster than enzyme turnover events, so proximity alone should not produce an observable overall rate increase for most cascade reactions.…”