SUBTLETIES IN CONTROL BY METABOLIC CHANNELING AND ENZYME ORGANIZATION

Because of its importance to cell function, the free-energy metabolism of the living cell is subtly and homeostatically controlled. Metaboli c control analysis enables a quantitative determination of what contro ls the relevant fluxes, However, the original metabolic control analys is was developed for idealized metabolic systems, which were assumed t o lack enzyme-enzyme association and direct metabolite transfer betwee n enzymes (channelling). We here review the recently developed molecul ar control analysis, which makes it possible to study non-ideal (chann elled, organized) systems quantitatively in terms of what controls the fluxes, concentrations, and transit times. We show that in real, non- ideal pathways, the central control laws, such as the summation theore m for flux control, are richer than in ideal systems: the sum of the c ontrol of the enzymes participating in a non-ideal pathway may well ex ceed one (the number expected in the ideal pathways), but may also dro p to values below one. Precise expressions indicate how total control is determined by non-ideal phenomena such as ternary complex formation (two enzymes, one metabolite), and enzyme sequestration. The bacteria l phosphotransferase system (PTS), which catalyses the uptake and conc omitant phosphorylation of glucose (and also regulates catabolite repr ession) is analyzed as an experimental example of a non-ideal pathway. Here, the phosphoryl group is channelled between enzymes, which could increase the sum of the enzyme control coefficients to two, whereas t he formation of ternary complexes could decrease the sum of the enzyme control coefficients to below one. Experimental studies have recently confirmed this identification, as well as theoretically predicted val ues for the total control. Macromolecular crowding was shown to be a m ajor candidate for the factor that modulates the non-ideal behaviour o f the PTS pathway and the sum of the enzyme control coefficients.