While at first it might seem desirable to have metabolic enzymes normally operate at their maximal rate, this leaves little room to adjust throughput or flux to changes in substrate availability. When an enzyme is saturated by substrate, the rate of product formation cannot increase to accommodate a surge in substrate availability (see Figure 1). Moreover, even relatively large decreases in substrate availability will produce only disproportionately small decreases in rate. Consequently, most enzymes have evolved such that their Km values for substrates tend to be close to the latter’s average intracellular concentrations, so that changes in substrate concentration generate corresponding changes in metabolite flux (Figure 1). Responses to changes in substrate level represent an important but passive means for the intercellular coordination of metabolite flow. Adaptation to extracellular signals requires mechanisms for regulating enzyme efficiency in an active manner.
Fig1. Differential response of the rate of an enzyme catalyzed reaction, ΔV, to the same incremental change in substrate concentration at a substrate concentration close to Km (ΔVA ) or far above Km (ΔVB ).
Metabolite Flow Tends to Be Unidirectional
Normally, living cells exist in a dynamic steady state in which the mean concentrations of metabolic intermediates remain relatively constant over time. This reflects the fact that, in living cells, the reaction products of one enzyme-catalyzed reaction serve as substrates for, and are rapidly removed by, other enzyme catalyzed reactions (Figure 2). Under these circumstances flux through many nominally reversible enzyme-catalyzed reactions occurs unidirectionally. In a pathway comprised of a succession of coupled enzyme-catalyzed reactions, the continual depletion of the pathway intermediates pulls the equilibrium for even those reactions characterized by small or even slightly unfavorable changes in free energy in favor of product formation, because the overall change in free energy that favors unidirectional metabolite flow. This situation is somewhat analogous to the flow of water through a pipe in which one end is lower than the other. Flow of water through the pipe remains unidirectional despite the presence of bends or kinks, due to the overall change in height, which corresponds to the pathway’s overall change in free energy (Figure 3).
Fig2. An idealized cell in steady state. Note that metabolite flow is unidirectional.
Fig3. Hydrostatic analogy for a pathway with a rate limiting step (A) and a step with a ΔG value near 0 (B).
