The phenomenon in which a metabolite 'I', product of an enzyme 'E1', is directly transferred to the next enzyme 'E2', which uses it as a substrate. Thus in such a short pathway, the complex E1.I is a better substrate for E2, compared to the freely diffusible small molecule 'I'. Both the following reactions may occur and compete: The occurence of channeling or direct transfer through transient association of enzymes during activity has important kinetic as well as regulatory implications. Indeed, in contrast with the 'normal mechanism', the metabolite 'I' is forced to go on through E2 rather than being available as a substrate to another enzyme 'En' in a different (competing?) pathway.
An extreme case for channeling is the permanent organization of enzymes in a pathway into a 'multi-enzyme complex', in which the intermediates are restricted into a common internal active locus. The complex represents a black box into which a substrate enters, is sequentially processed in isolation from the medium, and only the final product comes out and become available to further catalysis by other systems. A famous example for that is the pyruvate dehydrogenase complex of bacteria and mitochondria.
last update, Dec 1999 - Claude Aflalo - suggestions are welcome...