Proposed
mechanism for rapid transfer of
substrate from
enzyme to enzyme. There are three main types:
- Tight channeling.
- Electrostatic channeling.
- Transient channeling.
The first type has been observed in
tryptophan synthase and carbamoyl phosphate synthase. These enzymes have
diffusable and reactive intermediates, respectively. The
crystal structures of both have shown a distinct tunnel running through the complex, between the
active sites.
However, electrostatic channels (in a Malate Dehydrogenase/Citrate Synthase chimera for example) are somewhat less certain. This involves the substrate travelling across a charged path on the surface of a complex. It is also thought to occur between the two main drug targets in malaria, which channel folate between themselves.
Interestingly, a report on formiminotransferase -cyclodeaminase suggests that a combination of these first two mechanisms (a charged tunnel) occurs.
However, the final type is the least recognised. Evidence for lactate dehydrogenase / glyceraldehyde-3-phosphate dehydrogenase chanelling their cofactor, NAD has been scarce. Other examples of ternary complex formation before channeling (especially in the TCA cycle, realated to the metabolon theories) have been equally scarce.