MICROCOMPARTMENTATION, METABOLIC CHANNELING 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 intermedi ary metabolism are soluble; however, advances in cell biology have rev ealed an intricate picture at the microscopic level of cytoplasm struc ture. Consequently, a great deal of constraint is required when extrap olating 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 spa tial organization is now accepted and covers a wide variety of local m icroenvironments and possibly localized metabolic channelling. The lat ter, although accepted as a phenomenon, is controversial in terms of i ts physiological significance. This review covers evidences showing th at both glycolytic and glycogenolytic enzymes are microcompartmentaliz ed. The potential significance of this compartmentation appears to inv olve metabolic chanelling, a process by which rearrangement of enzymes on a dynamic cytomatrix leads to ''channels'' in which metabolic subs trates 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 a ssociations favours maximal rate of reaction. These and other aspects of microcompartmentation and metabolic channelling are discussed.