Vitreoscilla Haemoglobin: A Tool to Reduce Overflow Metabolism

Abstract

Overflow metabolism is a phenomenon extended in nature, ranging from microbial to cancer cells. Accumulation of overflow metabolites pose a challenge for large-scale bioprocesses. Yet, the causes of overflow metabolism are not fully clarified. In this work, the underlying mechanisms, reasons and consequences of overflow metabolism in different organisms have been summarized. The reported effect of aerobic expression of Vitreoscilla haemoglobin (VHb) in different organisms are revised. The use of VHb to reduce overflow metabolism is proposed and studied through flux balance analysis in E. coli at a fixed maximum substrate and oxygen uptake rates. Simulations showed that the presence of VHb increases the growth rate, while decreasing acetate production, in line with the experimental measurements. Therefore, aerobic VHb expression is considered a potential tool to reduce overflow metabolism in cells.

Keywords: P/O ratio; Vitreoscilla haemoglobin; flux balance analysis; overflow metabolism.

Figure 1

Overview of Overflow Metabolism as Originated from a Metabolic Imbalance. (A): NADH regeneration rate can be insufficient for given substrate and oxygen uptake rates (q_S and q_O2, respectively) to fully oxidize the carbon source to CO₂ (q_CO2). Therefore, overflow metabolites are produced (q_overflow) to contribute to NADH regeneration. (B): Initially q_O2 displays a linear correlation with q_S. However, at some point q_O2 reaches a maximum (q_O2, _crit) and q_S continues increasing, with the concomitant production of overflow metabolites.

Figure 2

Distribution of Fluxes with Varying P/O ratio. Biomass production is Maximized, Constraints q_S = −11 mmol g⁻¹h⁻¹ [88], q_O2 = −8 mmol g⁻¹h⁻¹ for wild type (blue) and −8 × 1.25 mmol g⁻¹h⁻¹ in the presence of VHb (red).

Figure 3

Distribution of fluxes with varying P/O ratio. Biomass production is maximized, constraints q_S = −11 mmol g⁻¹h⁻¹, q_O2 = −15 mmol g⁻¹h⁻¹ [89] for wild type (blue) and −15 × 1.25 mmol g⁻¹h⁻¹ in the presence of VHb (red).