Microbial Metabolism Energy Production

Energy production Nutrient molecules have energy associated with the electrons that form bonds between atoms Nutrient molecules have energy associated with the electrons that form bonds between atoms Catabolic reactions oxidize nutrients by removing electrons and concentrate their energy into the bonds of ATP Catabolic reactions oxidize nutrients by removing electrons and concentrate their energy into the bonds of ATP ATP has “high energy” or unstable bonds which allows the energy to be released quickly and easily. ATP has “high energy” or unstable bonds which allows the energy to be released quickly and easily.

ATP

ATP generation Cells use oxidation-reduction (redox) reactions in catabolism to extract energy from nutrient molecules Cells use oxidation-reduction (redox) reactions in catabolism to extract energy from nutrient molecules This energy is trapped by the generation of ATP by phosphorylation of ADP This energy is trapped by the generation of ATP by phosphorylation of ADP

Oxidation-reduction reactions Oxidation is the removal of electrons from a molecule Oxidation is the removal of electrons from a molecule Reduction is the gaining of electrons by a molecule Reduction is the gaining of electrons by a molecule Oxidation and reduction reactions are always coupled (redox reaction) Oxidation and reduction reactions are always coupled (redox reaction)

Many catabolic oxidation- reduction reactions are also dehydrogenation reactions The removal of electrons also means the removal of hydrogen atoms [i.e., not just an electron but also a proton (H + )] The removal of electrons also means the removal of hydrogen atoms [i.e., not just an electron but also a proton (H + )] These are transferred to an “electron carrier” These are transferred to an “electron carrier”

Electron carriers In catabolic reactions, energy is extracted from molecules in the form of electrons, which are transferred, along with H + ions, to electron carriers like NAD +. In catabolic reactions, energy is extracted from molecules in the form of electrons, which are transferred, along with H + ions, to electron carriers like NAD +. 2H + + 2e - NAD NADH + + H +

Mechanisms of ATP generation Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation Oxidative phosphorylation Photophosphorylation Photophosphorylation

Substrate-level phosphoryation ATP is generated when a high-energy phosphate is transferred directly to ADP from a phosphorylated substrate ATP is generated when a high-energy phosphate is transferred directly to ADP from a phosphorylated substrate

Oxidative phosphorylation Electrons are transferred from organic compounds through a series of electron carriers to O 2 or other oxidized inorganic or organic molecules Electrons are transferred from organic compounds through a series of electron carriers to O 2 or other oxidized inorganic or organic molecules The sequence of electron carriers is called the electron transport chain The sequence of electron carriers is called the electron transport chain The transfer of electrons from one carrier to the next generates energy which is used to make ATP from ADP by chemiosmosis The transfer of electrons from one carrier to the next generates energy which is used to make ATP from ADP by chemiosmosis

Photophosphorylation Only occurs in photosynthetic cells which contain light trapping pigment such as chlorophyll Only occurs in photosynthetic cells which contain light trapping pigment such as chlorophyll Light causes chlorophyll to give up electrons Light causes chlorophyll to give up electrons Energy released by the transfer of electrons from chlorophyll to carrier molecules is used to generate ATP Energy released by the transfer of electrons from chlorophyll to carrier molecules is used to generate ATP

How do chemoheterotrophs generate energy? Sources of energy: carbohydrate, fat, protein, minerals. Sources of energy: carbohydrate, fat, protein, minerals. Most microorganisms oxidize carbohydrates as the major source of cellular energy Most microorganisms oxidize carbohydrates as the major source of cellular energy Energy can also be derived from the oxidation of fats, proteins, and minerals. Energy can also be derived from the oxidation of fats, proteins, and minerals.

Carbohydrate catabolism Microbes use two general processes to generate energy from glucose Microbes use two general processes to generate energy from glucose Aerobic respiration Aerobic respiration Fermentation Fermentation Both start with glycolysis Both start with glycolysis (= Emden Meyerhoff pathway) (= Emden Meyerhoff pathway)

Aerobic Respiration Glycolysis (Embden-Meyerhof) Glycolysis (Embden-Meyerhof) Glucose is oxidized to pyruvic acid Glucose is oxidized to pyruvic acid Pyruvic acid is oxidized to acetyl CoA Pyruvic acid is oxidized to acetyl CoA TCA cycle (Kreb’s cycle) TCA cycle (Kreb’s cycle) Acetyl CoA is oxidized to CO 2 Acetyl CoA is oxidized to CO 2 Electron transport chain Electron transport chain Reduced NADH and FADH 2 from the above are oxidized through a series of redox reactions through an electron transport chain. Reduced NADH and FADH 2 from the above are oxidized through a series of redox reactions through an electron transport chain.

Glycolysis Starting point for cellular respiration and fermentation. Starting point for cellular respiration and fermentation. 10 step catabolic pathway 10 step catabolic pathway Two stages Two stages Preparatory stage Preparatory stage Energy conserving stage Energy conserving stage

ADP Glucose 6-phosphate Fructose 1,6-diphosphate Glyceraldehyde 3-phosphate Dihydroxyacetone phosphate Fructose 6-phosphate ATP P ADP ATP P P Glycolysis: preparatory stage 2 ATPs are used 2 ATPs are used Glucose is split to form 2 molecules of Glyceraldehyde- 3-phosphate Glucose is split to form 2 molecules of Glyceraldehyde- 3-phosphate P P P Hexokinase Phosphoglucoisomerase Phosphofructokinase aldolase Triose phosphate isomerase

ADP Glyceraldehyde 3-phosphate Diphosphoglyceric acid ATP Glycolysis: energy conserving stage For each initial glucose molecule; For each initial glucose molecule; 2 Glyceraldehyde- 3-phosphate oxidized to 2 Pyruvic acid 2 Glyceraldehyde- 3-phosphate oxidized to 2 Pyruvic acid 4 ATP produced 4 ATP produced 2 NADH produced 2 NADH produced NAD + NADH P P P P PPPP P P 3-phosphoglyceric acid ADP ATP 2-phosphoglyceric acid PP PP H2OH2O H2OH2O Phosphoenolpyruvic acid ADP ATP Pyruvic acid ADP ATP Triose phosphate dehydrogenase Phosphoglycerokinase Phosphoglyceromutase Enolase Pyruvate kinase

Summary of glycolysis Glucose (C 6 H 12 O 6 ) is split and oxidized through a ten step pathway to two molecules of pyruvic acid (C 3 H 4 O 3 ) Glucose (C 6 H 12 O 6 ) is split and oxidized through a ten step pathway to two molecules of pyruvic acid (C 3 H 4 O 3 ) Net gain of 2 ATP molecules, 4 from energy conserving phase (by substrate level phosphorylation) minus 2 from preparatory phase Net gain of 2 ATP molecules, 4 from energy conserving phase (by substrate level phosphorylation) minus 2 from preparatory phase 2 NADH molecules produced 2 NADH molecules produced Pyruvic acid can now undergo either fermentation or respiration Pyruvic acid can now undergo either fermentation or respiration

Alternatives to glycolysis Many bacteria have an alternative pathway to glycolysis for the oxidation of glucose Many bacteria have an alternative pathway to glycolysis for the oxidation of glucose Entner-doudoroff reaction Entner-doudoroff reaction Phosphogluconate pathway Phosphogluconate pathway Some bacteria oxidize inorganic compounds instead of glucose to get energy. (the Lithotrophs) Some bacteria oxidize inorganic compounds instead of glucose to get energy. (the Lithotrophs)

Use of Inorganic ions as electron “SOURCES” (Lithotrophs)

Bacteria, by Energy sources Phototrophs Phototrophs Chemotrophs Chemotrophs Oxidize organic compounds for Energy: Chemoorganotrophs Oxidize inorganic compounds for Energy: Oxidize inorganic compounds for Energy:Chemolithotrophs