Haloarchaea (class Halobacteria) live in extremely halophilic conditions and evolved many unique metabolic features, which help them to adapt to their environment. The methylaspartate cycle, an anaplerotic acetate assimilation pathway recently proposed for Haloarcula marismortui, is one of these special adaptations. In this cycle, acetyl-CoA is oxidized to glyoxylate via methylaspartate as a characteristic intermediate. The following glyoxylate condensation with another molecule of acetylCoA yields malate, a starting substrate for anabolism. The proposal of the functioning of the cycle was based mainly on in vitro data, leaving several open questions concerning the enzymology involved and the occurrence of the cycle in halophilic archaea. Using gene deletion mutants of H. hispanica, enzyme assays and metabolite analysis, we now close these gaps by unambiguous identification of the genes encoding all characteristic enzymes of the cycle. Based on these results, we were able to perform a solid study of the distribution of the methylaspartate cycle and the alternative acetate assimilation strategy, the glyoxylate cycle, among haloarchaea. We found that both of these cycles are evenly distributed in haloarchaea. Interestingly, 83% of the species using the methylaspartate cycle possess also the genes for polyhydroxyalkanoate biosynthesis, whereas only 34% of the species with the glyoxylate cycle are capable to synthesize this storage compound. This finding suggests that the methylaspartate cycle is shaped for polyhydroxyalkanoate utilization during carbon starvation, whereas the glyoxylate cycle is probably adapted for growth on substrates metabolized via acetyl-CoA.

Developing new carbon dioxide (CO 2 ) fixing enzymes is a prerequisite to create new biocatalysts for diverse applications in chemistry, biotechnology and synthetic biology. Here we used bioinformatics to identify a “sleeping carboxylase function” in the superfamily of medium-chain dehydrogenases/reductases (MDR), i.e. enzymes that possess a low carboxylation side activity next to their original enzyme reaction. We show that propionyl-CoA synthase from Erythrobacter sp. NAP1, as well as an acrylyl-CoA reductase from Nitrosopumilus maritimus possess carboxylation yields of 3 ± 1 and 4.5 ± 0.9%. We use rational design to engineer these enzymes further into carboxylases by increasing interactions of the proteins with CO 2 and suppressing diffusion of water to the active site. The engineered carboxylases show improved CO 2 -binding and kinetic parameters comparable to naturally existing CO 2 -fixing enzymes. Our results provide a strategy to develop novel CO 2 -fixing enzymes and shed light on the emergence of natural carboxylases during evolution.

Growth on acetate or other acetyl-CoA-generating substrates as a sole source of carbon requires an anaplerotic pathway for the conversion of acetyl-CoA into cellular building blocks. Haloarchaea (class Halobacteria) possess two different anaplerotic pathways, the classical glyoxylate cycle and the novel methylaspartate cycle. The methylaspartate cycle was discovered in Haloarcula spp. and operates in ∼40% of sequenced haloarchaea. In this cycle, condensation of one molecule of acetyl-CoA with oxaloacetate gives rise to citrate, which is further converted to 2-oxoglutarate and then to glutamate. The following glutamate rearrangement and deamination lead to mesaconate (methylfumarate) that needs to be activated to mesaconyl-C1-CoA and hydrated to β-methylmalyl-CoA. The cleavage of β-methylmalyl-CoA results in the formation of propionyl-CoA and glyoxylate. The carboxylation of propionyl-CoA and the condensation of glyoxylate with another acetyl-CoA molecule give rise to two C4-dicarboxylic acids, thus regenerating the initial acetyl-CoA acceptor and forming malate, its final product. Here we studied two enzymes of the methylaspartate cycle from Haloarcula hispanica, succinyl-CoA:mesaconate CoA-transferase (mesaconate CoA-transferase, Hah_1336) and mesaconyl-CoA hydratase (Hah_1340). Their genes were heterologously expressed in Haloferax volcanii, and the corresponding enzymes were purified and characterized. Mesaconate CoA-transferase was specific for its physiological substrates, mesaconate and succinyl-CoA, and produced only mesaconyl-C1-CoA and no mesaconyl-C4-CoA. Mesaconyl-CoA hydratase had a 3.5-fold bias for the physiological substrate, mesaconyl-C1-CoA, compared to mesaconyl-C4-CoA, and virtually no activity with other tested enoyl-CoA/3-hydroxyacyl-CoA compounds. Our results further prove the functioning of the methylaspartate cycle in haloarchaea and suggest that mesaconate CoA-transferase and mesaconyl-CoA hydratase can be regarded as characteristic enzymes of this cycle.

Haloarchaea are extremely halophilic heterotrophic microorganisms belonging to the class Halobacteria (Euryarchaeota). Almost half of the haloarchaea possesses the genes coding for enzymes of the methylaspartate cycle, a recently discovered anaplerotic acetate assimilation pathway. In this cycle, the enzymes of the tricarboxylic acid cycle together with the dedicated enzymes of the methylaspartate cycle convert two acetyl coenzyme A (acetyl-CoA) molecules to malate. The methylaspartate cycle involves two reactions catalyzed by homologous enzymes belonging to the CitE-like enzyme superfamily, malyl-CoA lyase/thioesterase (haloarchaeal malate synthase [hMS]; Hah_2476 in Haloarcula hispanica) and ␤-methylmalyl-CoA lyase (haloarchaeal ␤-methylmalyl-CoA lyase [hMCL]; Hah_1341). Although both enzymes catalyze the same reactions, hMS was previously proposed to preferentially catalyze the formation of malate from acetyl-CoA and glyoxylate (malate synthase activity) and hMCL was proposed to primarily cleave ␤-methylmalyl-CoA to propionyl-CoA and glyoxylate. Here we studied the physiological functions of these enzymes during acetate assimilation in H. hispanica by using biochemical assays of the wild type and deletion mutants. Our results reveal that the main physiological function of hMS is malyl-CoA (not malate) formation and that hMCL catalyzes a ␤-methylmalyl-CoA lyase reaction in vivo. The malyl-CoA thioesterase activities of both enzymes appear to be not essential for growth on acetate. Interestingly, despite the different physiological functions of hMS and hMCL, structural comparisons predict that these two proteins have virtually identical active sites, thus highlighting the need for experimental validation of their catalytic functions. Our results provide further proof of the operation of the methylaspartate cycle and indicate the existence of a distinct, yetto-be-discovered malyl-CoA thioesterase in haloarchaea.IMPORTANCE Acetate is one of the most important substances in natural environments. The activated form of acetate, acetyl coenzyme A (acetyl-CoA), is the highenergy intermediate at the crossroads of central metabolism: its oxidation generates energy for the cell, and about a third of all biosynthetic fluxes start directly from acetyl-CoA. Many organic compounds enter the central carbon metabolism via this key molecule. To sustain growth on acetyl-CoA-generating compounds, a dedicated assimilation (anaplerotic) pathway is required. The presence of an anaplerotic pathway is a prerequisite for growth in many environments, being important for environmentally, industrially, and clinically important microorganisms. Here we studied specific reactions of a recently discovered acetate assimilation pathway, the methylaspartate cycle, functioning in extremely halophilic archaea.KEYWORDS CitE proteins, acetate assimilation, haloarchaea, thioesterase