Propionyl Coenzyme A (Propionyl-CoA) Carboxylase in Haloferax mediterranei: Indispensability for Propionyl-CoA Assimilation and Impacts on Global Metabolism

Abstract

Haloarchaea represent a distinct group of Archaea that typically inhabit hypersaline environments, in which nutrient supplies could vary considerably over time. Therefore, most of these extremophiles have developed the adaptation mechanism of depositing poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) intracellularly to store carbon and energy when carbon sources are oversupplied and utilizing PHBV in the absence of exogenous carbon sources (1, 2). During the process of PHBV biosynthesis and utilization, propionyl coenzyme A (propionyl-CoA) is an important intermediate metabolite. In addition, propionyl-CoA is also an essential intermediate of the methylaspartate cycle, a pathway for acetate assimilation in haloarchaea (3). In Bacteria and Eukarya, propionyl-CoA metabolism has been extensively studied, as excess propionyl-CoA inside the cell causes toxic effects (4, 5). For example, deficiencies in propionyl-CoA utilization affect polyketide synthesis, cell growth, and morphology of conidia of Aspergillus fumigatus (5) or lead to the serious disease propionic acidemia in humans (4). However, the enzymes for propionylCoA metabolism as well as the physiological roles of propionylCoA metabolism remain unclear for haloarchaea.Propionyl-CoA carboxylase (PCC) is the key enzyme for propionyl-CoA metabolism by catalyzing the carboxylation of propionyl-CoA to methylmalonyl-CoA. It is widely distributed in the three domains of life. PCC is typically composed of three functional components: the biotin carboxylase (BC), the biotin-carboxyl carrier protein (BCCP), and the carboxyltransferase (CT) (6). The crystal structure of a bacterial PCC holoenzyme has been reported, and a similar structure for human PCC has been obtained by using cryo-electron microscopy reconstruction (7). Recently, an acyl-CoA carboxylase with almost equal acetyl-CoA carboxylase (ACC) and PCC activities was characterized for the thermophilic archaeon Metallosphaera sedula (8). The ACC/PCC enzyme is responsible for two important carboxylation steps in the autotrophic carbon fixation cycle of the 3-hydroxypropionate/ 4-hydroxybutyrate cycle (9). Whether a similar ACC/PCC enzyme occurred in haloarchaea remained to be determined.Haloferax mediterranei is a metabolically versatile haloarchaeon and has been used as a model for studies of haloarchaeal metabolism, and especially PHBV biosynthesis, for decades (10). This strain can accumulate a large amount of PHBV with a high 3-hydroxyvalerate (3HV) content (ϳ10 mol%) from many unrelated carbon sources (11). In the absence of an exogenous carbon source, PHBV is degraded for nutritional purposes, and a large quantity of propionyl-CoA was able to be produced. Bioinformatic analysis revealed that potential genes encoding propionyl-

Enoyl-CoA hydratase mediates polyhydroxyalkanoate mobilization in Haloferax mediterranei

“…Furthermore, the genome analysis of the sequenced haloarchaea species implied that PHA-accumulating haloarchaeal strains may mobilize the storage compound during carbon starvation through the β-oxidation cycle as mediated by PhaJ1 ( Figs 7 and 2A ). The final products of acetyl-CoA and propionyl-CoA could be assimilated efficiently via methylaspartate cycle or glyoxylate cycle 35 36 and the propionyl-CoA carboxylation pathway 37 in many haloarchaea. Integration of the PHA mobilization to either general β-oxidation cycle or the following specific pathways may reflect the evolutionary adaptation of haloarchaea to a high-salt environment.…”

Unusual Phosphoenolpyruvate (PEP) Synthetase-Like Protein Crucial to Enhancement of Polyhydroxyalkanoate Accumulation in Haloferax mediterranei Revealed by Dissection of PEP-Pyruvate Interconversion Mechanism

“…Cell growth measurement and residual glucose determination. The growth of H. mediterranei strains was monitored by the diphenylamine colorimetric method as described previously (36). The DNA content of cells was quantified by the absorbance at 595 nm using a Beckman Coulter DU800 spectrophotometer (Jersey City, NJ, USA).…”

“…These characteristics make H. mediterranei a potential industrial strain for PHA production. In previous years, our research group has reported PHBV synthesis and its regulation in H. mediterranei (28,(34)(35)(36). In addition, an improved PHBV-producing variant, ES1 with a deletion of genes for exopolysaccharide (EPS) production, was constructed (37).…”

Biosynthesis, property comparison, and hemocompatibility of bacterial and haloarchaeal poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

“…Currently, halophilic microbes are found to be a promising group as PHA-accumulating organisms [15]. They live in halophilic conditions and possess many advantages over non-halophilic microbes [15][16][17][18][19]. Most halophilic archaea can synthesize PHBV from structurally unrelated carbon sources such as starch, glucose, and glycerol [20][21][22].…”