Microcompartmentation, metabolic channelling and carbohydrate metabolism.

The inter-organelle cytoplasm of eukaryotic cells was once considered to be a homogeneous solution in which many of the enzymes of intermediary metabolism are soluble; however, advances in cell biology have revealed an intricate picture at the microscopic level of cytoplasm structure. Consequently, a great deal of constraint is required when extrapolating to the intact cell from enzyme studies in vitro, a point made frequently in the literature of the last decade or so. The idea of spatial organization is now accepted and covers a wide variety of local microenvironments and possibly localized metabolic channelling. The latter, although accepted as a phenomenon, is controversial in terms of its physiological significance. This review covers evidences showing that both glycolytic and glycogenolytic enzymes are microcompartmentalized. The potential significance of this compartmentation appears to involve metabolic chanelling, a process by which rearrangement of enzymes on a dynamic cytomatrix leads to "channels" in which metabolic substrates are passed from one enzyme to the next. The combined effects of such enzyme proximity and their activation as a result of the altered kinetic properties conferred upon the enzymes by their cytoskeletal associations favours maximal rate of reaction. These and other aspects of microcompartmentation and metabolic channelling are discussed.