Chapter 8 An Introduction to Microbial Metabolism
The metabolism of microbes Metabolism sum of all chemical reactions that help cells function Two types of chemical reactions: Catabolism -degradative; breaks the bonds of larger molecules forming smaller molecules; releases energy, end products: CO 2, H 2 O Anabolism biosynthesis; forms larger macromolecules from smaller molecules requires energy input end products: proteins etc.
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You are expected to know What are enzymes Enzyme Structure Enzyme examples and their cofactors Location of enzyme Action Regulation of enzyme Action Sensitivity of enzymes to their environment Competitive & non competitive inhibition of enzymes
Regulation of enzymatic activity and metabolic pathways Multi-enzyme systems Linear Cyclic Branched Divergent Convergent Constitutive enzymes always present, always produced in equal amounts or at equal rates, regardless of the amount of substrate Regulated enzymes not constantly present; production is turned on (induced) or turned off (repressed) in response to changes in the substrate concentration
Types of energy Energy: the capacity to do work or to cause change Forms of energy include Thermal Radiant/ Light Electrical Mechanical Atomic Chemical Only photosynthetic autotrophs can utilize light energy directly Photosynthesis converts light into chemical energy Provides nutritional and energy basis for all heterotrophs
Cell energetics Cells manage energy in the form of chemical reactions that make or break bonds and transfer electrons Reactions Exergonic Enzyme A + B C + Energy Energy + Endergonic Enzyme A + B C Release energy Energy released is temporarily stored in ATP Consume energy The energy in ATP is used in endergonic cell reactions
Cell energetics Reactions Synthesis/condensation reactions anabolic reactions to form covalent bonds between smaller substrate molecules require ATP release one molecule of water for each bond formed Hydrolysis reactions catabolic reactions that break down substrates into small molecules requires water to break bonds may releases ATP
Synthesis/condensation reactions Hydrolysis reactions
Electron and proton carriers Repeatedly accept and release electrons and hydrogen to facilitate the transfer of redox energy Most carriers are coenzymes: NAD, FAD, NADP, coenzyme A, and compounds of the respiratory chain
Adenosine Triphosphate (ATP) Metabolic currency Three part molecule consisting of: Adenine nitrogenous base Ribose 5-carbon sugar 3 phosphate groups adenosine = no phosphates adenosine monophosphate (AMP) = adenosine + 1 phosphate adenosine diphosphate (ADP) = adenosine + 2 phosphates adenosine triphosphate (ATP) = adenosine + 3 phosphates Removal of the terminal phosphate releases energy ATP utilization and replenishment is a constant cycle in active cells
Formation of ATP ATP can be formed by three different mechanisms 1) Substrate-level phosphorylation transfer of phosphate group from a phosphorylated compound (substrate) directly to ADP PO 4 Enzyme 1 ADP 1 ATP Phosphorylated substrate Substrate 2) Oxidative phosphorylation series of redox reactions occurring during respiratory pathway 3) Photophosphorylation ATP is formed utilizing the energy of sunlight
Bioenergetics pathways Bioenergetics studying mechanisms of cellular energy reserve management Includes catabolic and anabolic reactions Primary catabolism of fuels (glucose) proceeds through a series of three coupled pathways: 1. Glycolysis 2. Kreb s cycle 3. Respiratory chain, electron transport
Metabolic strategies Processing nutrients in many cases is based on three catabolic pathways that convert glucose to CO 2 and gives off energy Aerobic respiration glycolysis Kreb s cycle respiratory chain Anaerobic respiration glycolysis Kreb s cycle respiratory chain Fermentation glycolysis Final electron acceptor: Oxygen Not Oxygen but sulfate (SO 4 2- ), nitrate (NO 3- ) organic compounds
Overview of catabolic pathways
Aerobic respiration Series or enzyme-catalyzed reactions in which electrons are transferred from fuel molecules (glucose) to oxygen as a final electron acceptor Glycolysis glucose (6C) is oxidized and split into 2 molecules of pyruvic acid (3C), NADH is generated TCA processes pyruvic acid and generates 3 CO 2 molecules, NADH and FADH 2 are generated Electron transport chain accepts electrons from NADH and FADH; generates energy through sequential redox reactions called oxidative phosphorylation
Fate of pyruvate
The Nobel Prize in Physiology or Medicine 1953 Hans Krebs, Fritz Lipmann
Krebs cycle AEROBIC RESPIRATION Glycolysis Glucose (6 C) ATP NADH 2 pyruvate (3 C) CO 2 Acetyl CoA FADH 2 NADH Krebs CO 2 ATP Electrons Electron transport O 2 is final electron acceptor ATP produced = 38
Electron transport and Chemiosmosis
Electron transport and oxidative phosphorylation Final processing of electrons and hydrogen and the major generator of ATP Chain of redox carriers that receive electrons from reduced carriers (NADH and FADH 2 ) ETS shuttles electrons down the chain, energy is released and subsequently captured and used by ATP synthase complexes to produce ATP Oxidative phosphorylation
The Formation of ATP and Chemiosmosis Chemiosmosis as the electron transport carriers shuttle electrons, they actively pump hydrogen ions (protons) across the membrane setting up a gradient of hydrogen ions proton motive force Hydrogen ions diffuse back through the ATP synthase complex causing it to rotate, causing a 3-D change resulting in the production of ATP British biochemist Peter Mitchell carried out his research in a renovated country house in rural Cornwall. Won Nobel prize in 1978 in chemistry http://www.nature.com/scitable/
Electron transport and ATP synthesis in bacterial cell envelope
The final step Oxygen accepts 2 electrons from the ETS and then picks up 2 hydrogen ions from the solution to form a molecule of water. Oxygen is the final electron acceptor 2H + + 2e - + ½O 2 H 2 O
Theoretic ATP yield for aerobic respiration
Anaerobic respiration Functions like aerobic respiration except it utilizes oxygen containing ions, rather than free oxygen, as the final electron acceptor Nitrate (NO 3 - ) and nitrite (NO 2 - ) Most obligate anaerobes use the H + generated during glycolysis and the Kreb s cycle to reduce some compound other than O 2
Fermentation Incomplete oxidation of glucose or other carbohydrates in the absence of oxygen Uses organic compounds as terminal electron acceptors Yields a small amount of ATP Production of ethyl alcohol by yeasts acting on glucose Formation of acid, gas, and other products by the action of various bacteria on pyruvic acid
Alcoholic & Acidic fermentation Most fermentations do not generate ATP beyond that produced by substrate-level phosphorylation. Microbes compensate for low efficiency of fermentation by consuming large quantities of substrate and excreting large quantities of products.
Fermentation pathways Homolactic fermentation: Produces 2 molecules of lactic acid Ethanolic fermentation: Produces 2 molecules of ethanol and two CO 2 Heterolactic fermentation: Produces 1 molecule of lactic acid, 1 ethanol, and 1 CO 2 Mixed-acid fermentation: Produces acetate, formate, lactate, and succinate, as well as ethanol, H 2, and CO 2
Products of pyruvate fermentation Clostridium Butyric acid Ethanol Yeasts CO 2 + H 2 Proteus Acetoacetic acid Formic acid Succinic acid Oxaloacetic acid Pyruvate Acetylmethylcarbinol NAD H Lactic acid Acetyl CoA Streptococcus, Lactobacillus Acetic acid Acetobacterium Mixed acids Escherichia, Shigella Propionic acid Propionibacterium 2,3 butanediol Enterobacter
Application of fermentation in diagnostic microbiology To quickly identify the microbe causing a disease and prescribe an effective antibiotic, hospitals use rapid and inexpensive biochemical tests. Carbohydrate fermentation test MacConkey agar
Comparing aerobic respiration, fermentation and anaerobic respiration
Catabolism Anabolism Glycolysis Biosynthesis and the crossing pathways of metabolism Many pathways of metabolism are bi-directional or amphibolic Chromosomes Enzymes Membranes Cell wall Storage Membranes Storage Cell structure Nucleic acids Proteins Starch Cellulose Lipids Fats Macromolecule Nucleotides Amino acids Deamination Carbohydrates GLUCOSE Fatty acids Beta oxidation Building block Metabolic pathways Pyruvic acid Acetyl coenzyme A Krebs cycle NH 3 CO 2 H 2 O Simple products
Biosynthesis and the crossing pathways of metabolism Catabolism Anabolism Glycolysis Catabolic pathways contain molecular intermediates (metabolites) that can be diverted into anabolic pathways Pyruvic acid converted into amino acids through amination Chromosomes Nucleic acids Nucleotides Enzymes Membranes Proteins Amino acids Deamination Cell wall Storage Starch Cellulose Carbohydrates Membranes Storage Lipids Fats Fatty acids Beta oxidation Cell structure Macromolecule Building block Amino acids can be converted into energy sources through deamination GLUCOSE Pyruvic acid Acetyl coenzyme A Metabolic pathways Krebs cycle NH 3 CO 2 H 2 O Simple products
Photosynthesis: Earth s lifeline The ultimate source of all the chemical energy in cells comes from the sun light 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2
Occurs in 2 stages: Photosynthesis Light-dependent photons are absorbed by chlorophyll, carotenoid, and phycobilin pigments Water split by photolysis, releasing O 2 gas and provides electrons to drive photophosphorylation H 2 O 2H + e 2 ATP NADPH Glucose Released light energy used to synthesize ATP and NADPH Chloroplast O 2 CO 2
Photosynthesis reactions
Photosynthesis Light-independent reaction dark reactions Calvin cycle uses ATP to fix CO 2 to ribulose-1,5-bisphosphate and convert it to glucose