The ups and downs of ectoine: structural enzymology of a major microbial stress protectant and versatile nutrient

Abstract

Ectoine and its derivative 5-hydroxyectoine are compatible solutes and chemical chaperones widely synthesized by Bacteria and some Archaea as cytoprotectants during osmotic stress and high- or low-growth temperature extremes. The function-preserving attributes of ectoines led to numerous biotechnological and biomedical applications and fostered the development of an industrial scale production process. Synthesis of ectoines requires the expenditure of considerable energetic and biosynthetic resources. Hence, microorganisms have developed ways to exploit ectoines as nutrients when they are no longer needed as stress protectants. Here, we summarize our current knowledge on the phylogenomic distribution of ectoine producing and consuming microorganisms. We emphasize the structural enzymology of the pathways underlying ectoine biosynthesis and consumption, an understanding that has been achieved only recently. The synthesis and degradation pathways critically differ in the isomeric form of the key metabolite N-acetyldiaminobutyric acid (ADABA). γ-ADABA serves as preferred substrate for the ectoine synthase, while the α-ADABA isomer is produced by the ectoine hydrolase as an intermediate in catabolism. It can serve as internal inducer for the genetic control of ectoine catabolic genes via the GabR/MocR-type regulator EnuR. Our review highlights the importance of structural enzymology to inspire the mechanistic understanding of metabolic networks at the biological scale.

The MocR/GabR Ectoine and Hydroxyectoine Catabolism Regulator EnuR: Inducer and DNA Binding

“…The isomer of α-ADABA, γ-ADABA, is the substrate for the ectoine synthase EctC (Czech et al, 2019), the key enzyme for the production of ectoine (Peters et al, 1990;Ono et al, 1999;Hermann et al, 2020). We wondered if interactions of γ-ADABA with EnuR could be found in docking experiments but no stable binding was observed.…”

“…Ectoine nutrient utilization regulator (EnuR) [also referred to as EhuR (Yu et al, 2017), or EutR (Suvorova and Rodionov, 2016)] is a member of the MocR/GabR family (Suvorova and Rodionov, 2016;Pascarella, 2019) and serves as a repressor protein involved in the transcriptional control of ectoine/5hydroxyectoine catabolic gene clusters (Jebbar et al, 2005;Schulz et al, 2017a,b; Figure 1A). The tetrahydropyrimidines ectoine and 5-hydroxyectoine (Galinski et al, 1985;Inbar and Lapidot, 1988) are among the most widely synthesized compatible solutes by members of the Bacteria (da Costa et al, 1998;Pastor et al, 2010;Czech et al, 2018;Hermann et al, 2020;Imhoff et al, 2020). Their accumulation is used by microorganisms to fend off the detrimental consequences of high osmolarity on cellular hydration and extremes in temperatures on growth (Pastor et al, 2010;Czech et al, 2018;Kunte et al, 2020).…”

“…In this enzyme bimodule, EutE presumably supports the release of α-ADABA from the EutD active site through transient interactions (Mais et al, 2020). Eventually, the metabolites generated through the joint EutD/EutE enzyme activities are converted by additional enzymes encoded in ectoine/5-hydroxyectoine catabolic gene clusters, or elsewhere in the genome sequence, to L-aspartate to fuel the TCA-cycle (Schwibbert et al, 2011;Hermann et al, 2020;Mais et al, 2020). It should be noted in this context that the gene content of ectoine/5-hydroxyectoine catabolic clusters are variable (Schwibbert et al, 2011;Schulz et al, 2017b;Hermann et al, 2020;Reshetnikov et al, 2020), and consequently, the complete degradation route(s) of ectoines are not fully understood.…”

“…Instead, the ectoine-derived metabolites α-ADABA and DABA serve this function by reacting with the PLP molecule covalently attached to the ATD of EnuR (Schulz et al, 2017a,b;Yu et al, 2017). Ectoine/5-hydroxyectoine importer and catabolic gene clusters are often juxtapositioned to a gene (enuR) encoding EnuR-type proteins (Schulz et al, 2017a;Hermann et al, 2020). This genetic arrangement implies a wider role for the EnuR repressor in controlling the use of ectoines as nutrients.…”

“…We use the marine α-proteobacterium Ruegeria pomeroyi DSS-3, a member of the widely distributed and ecophysiologically important Roseobacter clade (Moran et al, 2004), as our model system for the analysis of the catabolism of ectoines ( Schulz et al, 2017a,b;Hermann et al, 2020;Mais et al, 2020). In contrast to other members of the Roseobacter clade (Simon et al, 2017), R. pomeroyi DSS-3 cannot synthesize ectoines (Schulz et al, 2017b).…”