Substrate channeling is the passing of the intermediary metabolic product of one
enzyme directly to another enzyme or
active site
In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...
without its release into solution. When several consecutive enzymes of a metabolic pathway channel substrates between themselves, this is called a
metabolon In biochemistry, a metabolon is a temporary structural-functional complex formed between sequential enzymes of a metabolic pathway, held together both by non-covalent interactions and by structural elements of the cell, such as integral membrane pro ...
. Channeling can make a
metabolic pathway more rapid and efficient than it would be if the enzymes were randomly distributed in the
cytosol, or prevent the release of unstable intermediates.
It can also protect an intermediate from being consumed by competing reactions catalyzed by other enzymes.
Channeling can occur in several ways. One possibility, which occurs in the
pyruvate dehydrogenase complex
Pyruvate dehydrogenase complex (PDC) is a complex of three enzymes that converts pyruvate into acetyl-CoA by a process called pyruvate decarboxylation. Acetyl-CoA may then be used in the citric acid cycle to carry out cellular respiration, and t ...
, is by a substrate being attached to a flexible arm that moves between several active sites (not very likely). Another possibility is by two active sites being connected by a tunnel through the protein and the substrate moving through the tunnel; this is seen in
tryptophan synthase
Tryptophan synthase or tryptophan synthetase is an enzyme () that catalyses the final two steps in the biosynthesis of tryptophan. It is commonly found in Eubacteria, Archaebacteria, Protista, Fungi, and Plantae. However, it is absent from Animali ...
.
[ A third possibility is by a charged region on the surface of the enzyme acting as a pathway or "electrostatic highway" to guide a substrate that has the opposite charge from one active site to another. This is seen in the bifunctional enzyme ]dihydrofolate reductase
Dihydrofolate reductase, or DHFR, is an enzyme that reduces dihydrofolic acid to tetrahydrofolic acid, using NADPH as an electron donor, which can be converted to the kinds of tetrahydrofolate cofactors used in 1-carbon transfer chemistry. In ...
- thymidylate synthase. The channeling of aminoacyl-tRNA for protein synthesis in vivo has been also reported.
A presence of channel in enzyme structure is rather common feature as more than 68% of enzymes have active site access channels.
MOLEonline
software.
See also
* Enzyme kinetics
* Enzyme assay
* Enzyme catalysis
References
Enzyme kinetics
{{enzyme-stub