Intracellular convection, homeostasis and metabolic regulation.
Two views currently dominate experimental approaches to metabolic regulation. The first, let us call it Model 1, assumes that cells behave like a watery bag of enzymes. The alternative Model 2, however, assumes that 3-dimensional order and structure constrain metabolite behavior. A major problem in cell metabolism is determining why essentially all metabolite concentrations are remarkably stable (homeostatic) over large changes in pathway fluxes-for convenience, this is termed the [s] stability paradox. During large-scale transitions from maintenance metabolic rates to maximally activated work, contrasting demands of intracellular homeostasis versus metabolic regulation obviously arise. Data accumulated over the last 3-4 decades now make it clear that the demands of homeostasis prevail: during rest-work transitions, metabolites such as ATP and O(2) are notably and rigorously homeostatic; other intermediates usually do not vary by more than 0.5- to threefold over the resting condition. This impressive homeostasis is maintained despite changes in pathway fluxes that can exceed two orders of magnitude. Classical or Model 1 approaches to this problem can explain metabolite homeostasis, but the mechanisms for each metabolite, each enzyme locus, are necessarily specific. Thus Model 1 approaches basically do not provide a global explanation for the [s] stability paradox. Model 2 takes a different tack and assumes that an intracellular convection system acts as an over-riding 'assist' mechanism for facilitating enzyme-substrate encounter. Model 2 postulates that intracellular movement and convection are powered by macromolecular motors (unconventional myosins, dyneins, kinesin) running on actin or tubulin tracks. For fast and slow muscle fibers, microfilaments are concentrated near the periphery (where convection may be most important), but also extend throughout the actomyosin contractile apparatus both in horizontal and vertical dimensions. To this point in the development of the field, Model 1 and Model 2 approaches have operated as 'two solitudes', each considering the other incompatible with its own experimental modus operandi. In order to finally assemble a model that can sensibly explain a realistic working range of metabolic systems, opening of channels of communication between the above two very differing views of metabolic regulation would seem to be the requirement for the future.