| Abstract |
Microorganisms differ in their effectiveness in uptake and selection of substances that they bring in from the environment. They also differ in how they balance the allocation of nutrients for immediate and for delayed use. Moreover, they may not take up resources as fast as they seemingly could, and they may extrude derivatives of substances just pumped in. A good deal of these apparent choices must reside in the uptake systems and the linkage of these with the cell’s intermediate metabolism. An important feature is that a resource may vary in concentration from time to time, nutrient to nutrient, and habitat to habitat. This variation must have been critical to the evolution of regulatory processes. Some possibilities for the combined uptake and consumption are considered for substrates serving the same (homologous) and different (heterologous) roles for the bacterium. From the membrane transport processes diagrammed in Fig. 1c and Fig. 2 and corresponding computer program given in Appendix A, the combined effect of uptake processes and cell growth can be studied. The model can be modified for various alternate models to study the possible control of cellular uptake and metabolism for the range of ecological roles of the bacterium. Figure 1Models for bacterial growth. (a) The Monod model. Monod postulated, analogously to Michaelis–Menten enzyme kinetics, that a reversible complex is formed between the substrate, S, and the bacterium, B. This complex is resolves to recover the original catalyst, i.e., the bacterium, and the product, which is a second bacterium. (b) The Best model. The external substrate, SE, diffuses reversibly through the cell membrane. When internal to the cell, where it is designated as SI, it reacts with a cell enzyme, E. This is the first step in its metabolism. The complex, in an irreversible reaction, regenerates the enzyme, produces a product, P, that then is used variously to supports different aspects of cell growth. (c) Linkage model. This model links uptake with consumption. A pathway for facilitated diffusion acts to bring the substrate into the cell via a membrane-bound carrier, T. All stages (binding, dissociating, traversing the membrane) are reversible. The substrate, on dissociation, reacts internally with the first enzyme in the intermediary metabolism pathway according to the usual kind of enzyme mechanism with a Km and a V′max. Three regulatory mechanisms are shown: (i) by combination with a phosphate group by a two-component system; (ii) by overflow metabolism as the internal substrate is ejected from the cell in a transformed state, X; (iii) by storage within the cell as compound Y. Figure 2Metabolic scheme for the linkage of uptake of two growth substrates. The transport system for both substrates was modeled from an earlier review [30]. It has been outlined in Fig. 1c. The symbols S, T, E, I stand for substrate, transporter, external, and internal substrate. These symbols have an A or B added to them depending on which transport system is involved. Rate constants have been designated, for example, by K13B instead of K13B. Also KMB is used instead of KMB and VmaxB instead of VmaxB, to designate Michaelis–Menten constants and maximum velocities. In this scheme there are four forms of the free substrates and four forms of the transporter for each substrate. The 12 kinetic rate constants for each substrate should be considered the equivalents of the rate constants in chemical kinetics or in enzymology. The internal form of each kind of substrate, SIA and SIB, feeds into central metabolism at a maximum rate dependent on EA*K13A and EB*K13B when SIA or SIB are large relative to the binding constants, KMA or KMB (i.e., at saturation condition). The internal substrate may also be extruded in another form, XA and XB, or stored as a reserve material inside the cell, YA and YB. These phenomenological constants correspond to different kinetic forms for homologous and heterologous substrates as described in the text. The open-headed arrows present a possible control of the internal pools on that function by regulating TIA and TIB. |
