The Modification of Regulatory Circuits Involved in the Control of Polyhydroxyalkanoates Metabolism to Improve Their Production

Abstract

Poly-(3-hydroxyalkanoates) (PHAs) are bacterial carbon and energy storage compounds. These polymers are synthesized under conditions of nutritional imbalance, where a nutrient is growth-limiting while there is still enough carbon source in the medium. On the other side, the accumulated polymer is mobilized under conditions of nutrient accessibility or by limitation of the carbon source. Thus, it is well known that the accumulation of PHAs is affected by the availability of nutritional resources and this knowledge has been used to establish culture conditions favoring high productivities. In addition to this effect of the metabolic status on PHAs accumulation, several genetic regulatory networks have been shown to drive PHAs metabolism, so the expression of the PHAs genes is under the influence of global or specific regulators. These regulators are thought to coordinate PHAs synthesis and mobilization with the rest of bacterial physiology. While the metabolic and biochemical knowledge related to the biosynthesis of these polymers has led to the development of processes in bioreactors for high-level production and also to the establishment of strategies for metabolic engineering for the synthesis of modified biopolymers, the use of knowledge related to the regulatory circuits controlling PHAs metabolism for strain improvement is scarce. A better understanding of the genetic control systems involved could serve as the foundation for new strategies for strain modification in order to increase PHAs production or to adjust the chemical structure of these biopolymers. In this review, the regulatory systems involved in the control of PHAs metabolism are examined, with emphasis on those acting at the level of expression of the enzymes involved and their potential modification for strain improvement, both for higher titers, or manipulation of polymer properties. The case of the PHAs producer Azotobacter vinelandii is taken as an example of the complexity and variety of systems controlling the accumulation of these interesting polymers in response to diverse situations, many of which could be engineered to improve PHAs production.

Investigation of the Physiology of the Obligate Alkaliphilic Bacillus marmarensis GMBE 72T Considering Its Alkaline Adaptation Mechanism for Poly(3-hydroxybutyrate) Synthesis

“…To increase productivity, one of the most effective approaches is to identify metabolic pathways using the complete genome sequences of microorganisms and develop a metabolic network to design cost-effective manufacturing processes [ 29 ]. Additionally, it is very important to know the regulatory circuits that control PHB metabolism [ 30 ]. The production of PHB generally increased under stress conditions in the presence of carbon sources, but interestingly, B. marmarensis produced PHB in the absence of carbon sources, and PHB titer was reduced by the addition of glucose [ 17 ].…”

“…Bacteria synthesize PHB by the conversion of acetyl CoA to acetoacetyl-CoA and later to β-hydroxybutyryl-CoA (3-hydroxybutyryl-coenzyme A). The concentration of dihydronicotinamide adenine dinucleotide phosphate (NADPH) and a high ratio of NADPH/nicotinamide adenine dinucleotide phosphate (NADP) are crucial to stimulate PHB synthesis [ 30 ]. The enhancement of the PHB synthesis by lactose might be due to the production of NADPH at the desired level for the activity of 3-hydroxybutyryl-CoA dehydrogenase (1.1.1.157).…”

“…The production of PHB is achieved through the NADPH-dependent pathway by converting acetyl-CoA to PHB [ 30 ]. Since succinate and lactate are produced during cultivation [ 16 ], B. marmarensis prefers either the citric acid cycle (the TCA cycle) or PHB synthesis.…”

“…Acetyl-CoA, a branch point for PHA synthesis, is expressed through fatty acid biosynthesis and glycolysis. Velázquez-Sánchez et al [ 30 ] reported that polyhydroxyalkanoic acid synthase (PhaC), which catalyzes the terminal step of the PHA synthesis, is inhibited by the ratio of R-3-hydroxyacyl-CoA/CoA and free CoA. Additionally, the oxidation and the accumulation of 3-hydroxyacyl-CoA are affected by the acetyl CoA/CoA and NADH/NAD ratios that regulate the PHA accumulation.…”

“…Additionally, the oxidation and the accumulation of 3-hydroxyacyl-CoA are affected by the acetyl CoA/CoA and NADH/NAD ratios that regulate the PHA accumulation. The high ratio of acetyl CoA/CoA and NADH/NAD results in PHA accumulation in the cells [ 30 ]. In the case of B. marmarensis , efficient PHA accumulation was not found in the cells of B. marmarensis growing in the medium including olive oil, which contains a high amount of oleic acid.…”