Figure 1.
Multiplicity of steady states in kinetics of fructose-6-phosphate node (F6P-node).
(A) Feedback activation of phosphofructokinase (PFK) by fructose-1,6-bisphosphate (F16BP) (B) Bistability in the kinetics of PFK due to feedback regulation by F16BP. The simulation was performed using only two enzymes PFK and aldolase (ALDO). F6P was varied and the steady state PFK flux was solved algebraically. Activation of PFK by F16BP was set at KPFK,fbp = 0.65 mM. (C) Allosteric regulations in the F6P-node. (D) Modulation of bistability span by K/P ratio of PFKFB. The bifunctional enzyme PFKFB was integrated with PFK and ALDO to construct a three enzyme system. Simulations were performed at varying K/P value of PFKFB while keeping all other conditions the same as in (B).
Table 1.
Kinetic properties and the allosteric regulations of glycolytic enzymes expressed in mammalian cells.
Figure 2.
Multiplicity of steady states in the glycolysis flux.
(A) Steady state glycolysis flux with neither Loop 1 nor Loop 2 active. The isozyme set consisting of PFKP (no activation by F16BP) and PKM1 (no activation by F16BP) were used in the simulation. The steady state flux exhibits classical Michaelis-Menten kinetics. (B) Steady state glycolysis flux with only Loop 1 active. When only Loop 1 is active, the glycolysis flux exhibits bistability. The isozyme set PFKL (KPFK,fbp = 0.65 mM) and PKM1 were used in the simulation. (C) Steady state glycolysis flux with only Loop 2 active. When only Loop 2 is active, the glycolysis flux also exhibits bistability. The isozyme set PFKP and PKM2 (KPK,fbp = 0.04 mM) were used in the simulation. (D) Steady state glycolysis flux with both Loop 1 and Loop 2 active. The isozyme set PFKL and PKM2 were used in the simulation. When both Loop 1 and Loop 2 are active, multiplicity of steady state resembling superposition of (B) and (C) is observed. For A–D the K/P of PFKFB was set at 10. (E) The effect of K/P modulation on the multiplicity of steady state in glycolysis. In this simulation PFKL and PKM2 were used, while K/P was varied.
Figure 3.
Bistability in cultured HeLa cells.
Cells initially cultured in high glucose (♦) or low glucose (□) exhibit different rates of (A) specific glucose consumption and (B) specific lactate production when exposed to varying glucose concentrations.
Figure 4.
Effect of PFKFB levels on the transient response of glycolysis activity to pulse input in glucose concentration.
(A) Steady state behavior of glycolysis at different levels of PFKFB. All steady state glycolytic plots for different PFKFB levels including 100%, 50%, 20% and 10% superimpose on each other. (B) Glucose pulse input. This figure shows the pulse input in glucose concentration made at 50h for duration of 1.5h and an amplitude sufficient enough to increase glucose concentration from low flux state (a) to high flux region (b). (C) Response of the glycolysis to pulse input in glucose concentration for systems with different PFKFB levels. Systems were stationed at low flux state (a) when the pulse input was initiated. Systems with 100% and 50% PFKFB switch to the high state (b) in the given glucose pulse time. On the contrary, the response of the systems with 20% and 10% PFKFB is not fast enough to switch to the high state and therefore return to the low state when glucose is switched back to original concentration.
Figure 5.
Glycolytic behaviors observed in mammalian cells.
Two types of glycolytic steady state kinetics were observed. These include steady state behavior with no multiplicity of states or those with multiplicity of states. The type of glycolytic isozymes expressed forms the basis for presence or absence of multiplicity of states in glycolysis activity. In case of non-proliferating cell, isozyme combination comprising of PFKP, PKM1 and PFKFB with K/P = 0.5 was used whereas in case of proliferating cell, isozyme combination including PFKM, PKM2 and PFKFB with K/P = 10 was employed.