MGY377 Lecture 10 Review

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University of Toronto St. George
Molecular Genetics and Microbiology
John Brumell

MGY377 Lecture 10 – Metabolism III - Chemolithotrophs o Use alternate electron donors in the absence of organic compounds or light for energy generation o Use inorganic compounds (H , H2S, 2 , Fe , NH , NO3) to 2enerate energy o Obtain carbon from CO2 o Obtain energy from oxidation of inorganic compounds  Electron transport chain, essentially same as chemoorganotrophs o Reducing power for biosynthesis comes from  Directly from inorganic compound  Reverse electron flow o H2 oxidizing bacteria  H2 is a common product of bacterial metabolism  Includes aerobic and anaerobic bacteria and archaea  Hydrogenases  Mediates oxidation of H2  Membrane-bound enzyme that passes electrons directly to ETC  Reductant (NADH) generated directly from H2 by soluble form in cytoplasm o Generate reductant that can be used for Calvin cycle o Lower on the electron tower o In some bacteria only (e.g., Ralstonia eutropha)  Two proton pumping steps generate a PMF for ATP production o Sulphur oxidizing bacteria  Oxidize many sulphur (sulphide) compounds  Final product is H+ and SO 2-(sulphuric acid) 4  Oxidation of sulphur compounds is step wise  Electrons transferred to ETC  Enter at different components depending on reduction potential  Sulphite (SO )3is an important intermediate in oxidation steps 2-  Oxidized to sulphate (SO 4 by two pathways o Sulphite oxidase pathway o APS reductase pathway  Reductant (NADH) generated by reverse electron flow o Iron oxidizing bacteria  Fe2+ (ferrous)  Fe3+ (ferric)  Ferrous iron  Insoluble at neutral pH  Spontaneously oxidizes  Precipitates as ferric hydroxide o Growth is evident from the production of Fe3+, which readily forms ferric hydroxide, leading to the yellow-orange colour  Found in acidic, iron-rich environments  Coal-mining dumps  Acid polluted environments  Bogs  Very short route of electron transport to oxygen  Fe2+ is oxidized by rusticyanin  Copper-containing protein in periplasm  Mediates oxidation of Fe2+  Passes electrons to cyt c o Reduces O2 and pumps H+ out  Reductant (NADH) generated by reverse electron flow o Nitrifying bacteria  Widely distributed in soil and water  Oxidation of ammonia (NH3) is step wise  NH3 oxidizers (nitrosifyers): NH4+  NO2- o NH4+ is oxidized to hydroxylamine by AMO o Hydroxylamine is oxidized by HAO  Generates 4 electrons  2 electrons go to cyt c to generate PMG  2 electrons go to AMO  NO2 oxidizers (nitrite oxidizers): NO2-  NO3-  High reduction potentials for both redox couples  Reductant (NADH) generated by reverse electron flow  Slow growth - Anaerobic bacteria o Use alternate electron acceptors in the absence of oxygen for energy generation o Common in prokaryotes o Anaerobic respiration  Terminal electron acceptor other than oxygen  Involves ETC  Can compete with aer
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