Substrate channeling is when the intermediary metabolic product of one enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 is passed directly to another enzyme or active site

Active site

In biology the active site is part of an enzyme where substrates bind and undergo a chemical reaction. The majority of enzymes are proteins but RNA enzymes called ribozymes also exist. The active site of an enzyme is usually found in a cleft or pocket that is lined by amino acid residues that...

 without being released into solution. When several consecutive enzymes of a metabolic pathway channel substrates between themselves, this is called a metabolon

Metabolon

A metabolon is a temporary structural-functional complex formed between sequential enzymes of a metabolic pathway, held together by noncovalent interactions, and structural elements of the cell such as integral membrane proteins and proteins of the cytoskeleton....

. Channeling can make a metabolic pathway

Metabolic pathway

In biochemistry, metabolic pathways are series of chemical reactions occurring within a cell. In each pathway, a principal chemical is modified by a series of chemical reactions. Enzymes catalyze these reactions, and often require dietary minerals, vitamins, and other cofactors in order to function...

 more rapid and efficient than it would be if the enzymes were randomly distributed in the cytosol

Cytosol

The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....

, 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, is when a substrate is attached to a flexible arm that moves between several active sites. Another possibility is when two active sites being connected by a tunnel through the protein and the substrate moves through the tunnel, this is seen in tryptophan synthase

Tryptophan synthase

Tryptophan synthase or tryptophan synthetase is an enzyme that catalyzes 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 animalia. It is typically found as an α2β2 tetramer...

. A third possibility is when a charged region on the surface of the enzyme acts 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

- Function :Dihydrofolate reductase converts dihydrofolate into tetrahydrofolate, a methyl group shuttle required for the de novo synthesis of purines, thymidylic acid, and certain amino acids...

-thymidylate synthase

Thymidylate synthase

Thymidylate synthetase is the enzyme used to generate thymidine monophosphate , which is subsequently phosphorylated to thymidine triphosphate for use in DNA synthesis and repair....

. The channeling of aminoacyl-tRNA for protein synthesis in vivo has been also reported.