Multienzyme

Multienzyme refers to a system in which two or more enzymes participate in a sequence of chemical transformations, often acting in a coordinated manner to convert substrates into products. The term can describe either a single enzyme that contains multiple catalytic domains or a physical assembly of separate enzymes that function together as a unit.

In single-molecule multienzyme proteins, multiple active sites within one polypeptide chain enable sequential reactions without releasing intermediates, a phenomenon known as substrate channeling. In multienzyme complexes, distinct enzymes associate into a supramolecular assembly that channels substrates and intermediates along a metabolic pathway. Well-known examples include the pyruvate dehydrogenase complex, which converts pyruvate to acetyl-CoA through several steps within a single complex; fatty acid synthase, which orchestrates successive chain elongation steps; and the purinosome, a proposed transient assembly involved in purine biosynthesis.

Organizational strategies vary: some systems are organized around a rigid scaffold protein that holds enzymes in place; others are dynamic, forming and dissolving in response to metabolic state. Benefits include increased efficiency by reducing diffusion losses, coordinated regulation, and protection of labile intermediates. Substrate channeling can also facilitate coupling of exergonic and endergonic steps and minimize side reactions.

Research methods span biochemistry, structural biology, and cell biology. Techniques include enzyme assays, X-ray crystallography and cryo-electron microscopy, cross-linking and mass spectrometry, and advanced imaging to observe interactions in living cells. Multienzyme systems are widespread in metabolism and are a focus in biochemistry, systems biology, and synthetic biology for understanding regulation and for engineering new catalytic pathways.