Supply-demand analysis of energy metabolism in Lactococcus lactis under anaerobic conditions

Jordaan, Sandra (2011-12)

Thesis (MSc)--Stellenbosch University, 2011.

Thesis

ENGLISH ABSTRACT: The interests in understanding the metabolic processes of microbial systems are numerous. The interest in the species Lactococcus lactis (L. lactis) lies in applications to the food industry and in studies comparing the metabolism of related organisms. The aim of this study was to perform in vivo supply-demand analysis on anaerobically fermenting L. lactis. This was done by perturbing both the supply and demand pathways, then measuring glycolytic flux (by means of 13C NMR spectroscopy) and intracellular ATP/ADP (by means of 31P NMR spectroscopy) at steady state – where the central metabolite, ATP, is produced at the same rate as it is consumed and its concentration thereby remains constant. The L. lactis reference strain MG1363 was supplemented with glucose and analysed “online” by 13C-NMR under anaerobically-fermentative conditions. The rates of glucose consumption and lactate production were determined from this 13C flux. Due to experimental difficulties with the online detection of 31P (possibly due to the low biomass yield and the choice of growth medium), ATP/ADP levels had to be determined offline: from the same batch cultures as the 13C samples, fermentations were performed and halted at time points when the cells had attained a steady state. These fermentation cultures were then subjected to cell lysis and centrifugation in order to extract intracellular metabolites. These cell extracts were analysed offline by 31P NMR in order to determined levels of phosphate metabolites, specifically ATP and ADP. Perturbation of the supply pathway was achieved by utilising a genetically modified strain (the CS8 strain with over-expressed las operon) and comparing it to the reference strain MG1363. This resulted in a slight increase in ATP/ADP, but also yielded a slightly reduced flux, which is contrary to expectations from a mutant with over expressed glycolytic enzymes. The demand pathway was perturbed by two methods: 1) utilisating a genetically modified strain (the BK1506 strain with over-expressed F1-ATPase) and comparing it to the reference strain MG1363, and 2) by treating wild-type MG1363 with sodium acetate and comparing flux and ATP/ADP values to the untreated wild-type. Sodium acetate dissociates in the cytoplasm and causes dissipation of the transmembrane proton motive force, which is re-established by upregulation of membrane-bound H+-translocating ATPases. While the use of genetically modified strains provided only one flux-vs-ATP/ADP data point to compare to the wild-type (not sufficient for complete supply-demand analysis), the treatment of the wild type with uncoupler yielded several data points where flux and ATP/ADP values differed according to the concentration of uncoupler added. The CS8 strain demonstrated a 19 % reduced glucose flux (24 % reduced lactate flux) with respect to the wild type MG1363. The BK1506 strain demonstrated a 72 % increase in glucose flux (33 % increase in lactate flux) with respect to the wild type. The treatment with 2 mM acetate resulted in a 72 % increase in glucose flux (123 % increase in lactate flux), whereas treatment with 4 mM acetate resulted in a 107 % and a 126 % increase in glucose and lactate fluxes, respectively. The treatment with different concentrations of acetate provided several data points with corresponding flux and ATP/ADP, enabling the calculation of the elasticity coefficient of the supply pathway to changes in ATP/ADP (εsupply ) which was found to be -5.6 and -6.3 for glucose and lactate, respectively. ATP/ADP The elasticity coefficient was high compared with values obtained in similar studies on other organisms. Considering that at steady state the supply and demand fluxes are equal, the high supply elasticity (which is easier to measure), when incorporated into control coefficient summation theorems, gives the indication that: 1) a greater amount of control may reside in the ATP demand pathway (the elasticity of which is more difficult to determine experimentally, but which may well be lower than the supply elasticity), and 2) ATP/ADP homeostasis is good, as indicated by a high elasticity of the supply pathway to ATP/ADP. This study represents a basis for further supply-demand analysis with non-growing batch cultures of L. lactis.

AFRIKAANSE OPSOMMING: Daar is groot belangstelling daarin om die metaboliese prosesse van mikrobiese sisteme beter te verstaan. Die belang van die spesie Lactococcus lactis (L. lactis) lê beide in die toepassing in die voedselbedryf en in studies wat die metabolisme van verskeie organismes vergelyk. Die doel van hierdie studie was om in vivo vraag-aanbod analise uit te voer op anaerobies-fermenterende L. lactis. Dit was gedoen deur beide die aanbod en vraag reaksie-blokke te moduleer en dan die glikolitiese fluksie (d.m.v. 13C KMR spektroskopie) en die intrasellulêre ATP/ADP (d.m.v. 31P KMR spektroskopie) in ’n bestendige toestand te meet (wanneer die sentrale metaboliet, ATP, teen dieselfde tempo geproduseer en verbruik word en sy konsentrasie daardeur konstant bly). Die L. lactis verwysing-stam MG1363 is met glukose aangevul en 13C fluksie is aanlyn onder anaerobies-fermenterende kondisies gemeet. Die tempo van glukose verbruik en laktaat produksie is vanaf die 13C fluksie bereken. Eksperimentele probleme met die aanlyn bepaling van 31P (dalk as gevolg van lae biomassa en/of die keuse van groeimedium) moes ATP/ADP vlakke af-lyn indirek bepaal word: fermentasies van dieselfde lot-kulture as die 13C monsters is opgestel en by sekere tydpunte gestop wanneer ‘n bestendige toestand bereik was (waar ATP/ADP konstant bly). Hierdie fermenterende kulture is blootgestel aan sel-lise en sentrifugasie om intrasellulêre metaboliete te onttrek. Dié sel-ekstrakte is deur 31P KMR geanaliseer om die vlakke van fosfaat metaboliete, spesifiek ATP en ADP, te bepaal. Die aanbod blok is gemoduleer deur gebruik te maak van ‘n geneties-gemodifiseerde stam (die CS8 stam met ‘n ooruitgedrukte las operon) en met die verwysing stam MG1363 te vergelyk. Dié gemuteerde stam het ’n effense toename in ATP/ADP getoon, maar het gelyktydig ook ’n afname in glikolitiese fluksie getoon, wat onverwags is vir ’n stam met ooruitgedrukte glikolitiese ensieme. Die vraag blok is met twee metodes gemoduleer: 1) deur gebruik te maak van ‘n geneties-gemodifiseerde stam (die BK1506 stam met ‘n ooruitgedrukte F1-ATPase), en 2) deur die wildetipe MG1363 met natrium asetaat te behandel en daardeur ATP verbruik van biomassa produksie te ontkoppel en die vraag na ATP te vermeerder. Daarna word die fluksie en ATP/ADP waardes met die onbehandelde wildetipe vergelyk. Natrium asetaat dissosieer in die sitoplasma en verswak die transmembraan elektriese potensiaal, wat dan weer versterk word deur membraan-gekoppelde H+-ATPase ensieme wat protone uit die sitoplasma uit pomp. Terwyl die gebruik van geneties-gemodifiseerde stamme net een fluksie-tot-ATP/ADP datapunt voorsien om met die wildetipe te vergelyk (wat nie voldoende is vir totale vraag-aanbod analise nie), het die behandeling van die wildetipe met ontkoppelaar meerdere datapunte voorsien waar fluksie en ATP/ADP waardes verskil volgens die konsentrasie van ontkoppelaar wat bygevoeg is. Die CS8 stam het ’n 19 % verminderde glukose fluskie getoon, asook ’n 23 % verminderde laktaat fluksie, in vergelyking met die wilde tipe MG1363. Die BK1506 stam het ’n 73 % toename in glukose fluskie getoon, asook ’n 34 % toename in laktaat fluksie, in vergelyking met die wilde tipe. Behandeling met 2 mM natrium asetaat het ’n 64 % toename in glukose fluksie veroorsaak, sowel as ’n 124 % toename in laktaat fluksie, en behandeling met 4 mM natrium asetaat het 108 % toename in glukose fluksie en 127 % toename in laktaat fluskie veroorsaak. Behandeling met verskillende konsentrasies natrium asetaat het genoeg data punte (fluksies met toepaslike ATP/ADP waardes) verskaf om die berekening van elastisiteits-koëffisiënt van die aanbod reaksie-blok tot veranderinge in ATP/ADP (εsupply ) te bereken. Die waardes was -5.6 vir glukose fluksie en -6.3 vir laktaat fluksie. ATP/ADP Die elastisiteits koëffisiënt was relatief hoog in vergelyking met waardes wat in soorteglyke studies op ander organismes bepaal is. Aangesien die fluksies van die aanbod en vraag reaksie blokke by ’n bestendige toestand dieselfde tempo het, kan die hoë waarde van die aanbod elastisiteits-koëffisiënt (wat die makliker een is om te meet) na die volgende afleidings lei: 1) meer kontrole mag in die ATP verbruikende reaksie-blok geleë wees (die elastisiteits-koëffisiënt is moeiliker om eksperimenteel te bepaal maar mag wel laer as die aanbod-elastisiteit wees), en 2) ATP/ADP homeostase word goed gehandhaaf, soos aangetoon deur die hoë aanbod-elastisiteit. Hierdie studie dien as ’n basis vir verdere vraag-aanbod analise in nie-groeiende L. lactis lot-kulture.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/17779
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