Page 1 of 9 BIOL 3702 Lecture Outline Chapter 9: Metabolism: Energy, Release, and Conservation Fueling Processes ◆ Chemoorganotrophs differ in the types of electron acceptors they use in their various energy-yielding processes ◆ The process involving exogenous (external) acceptors in the electron transport chain is termed respiration ✳ Final electron acceptor is oxygen, the process is termed aerobic respiration ✳ Final electron acceptor is not oxygen, the process is termed anaerobic respiration ◆ The electron transport system uses the donated electron to make ATP via oxidative phosphorylation ◆ When the electron transport system is not used and endogenous (internal) acceptors are employed, the process is termed fermentation and ATP is formed via substrate-level phosphorylation Aerobic Respiration ◆ Aerobic catabolism can be divided into three different stages: ✳ Stage I - larger molecules are broken down into their constituent parts with little energy released ✳ Stage II - either an aerobic or anaerobic process whereby even simpler molecules are produced as is ATP as well as NADH and/or FADH2 ✳ Stage III - complete oxidation of molecules under aerobic conditions to form CO2 as well ATP, NADH, and FADH2 (the latter two molecules generate even more ATP through the electron transport system ◆ Some of the pathways in metabolism are amphibolic, i.e., they function both catabolically and anabolically Breakdown of Glucose ◆ Microbes use a number of pathways to catabolize glucose to pyruvate, among which include the following: ✳ Glycolysis ✳ Pentose phosphate pathway ✳ Entner-Doudoroff PathwayBIOL 3702 Lecture Outline Chapter 9 Page 2 of 9 ◆ Glycolysis (Embden-Meyerhof or Glycolytic Pathway) ✳ Most common pathway found in all major groups of microbes ✳ Functions in the absence or presence of oxygen ✳ Located in the cytoplasmic matrix of both procaryotes and eucaryotes ✳ Divided into two parts ● Six carbon stage - glucose (6 carbon [C6] molecule) is phosphorylated twice and converted to fructose 1,6-bisphosphate [C6] – Input of 2 ATP molecules – No energy produced ● Three carbon stage - catabolism of fructose 1,6-bisphosphate [C6] to two molecules of pyruvate [C3] and generate per pyruvate – One molecule of NADH – Two molecules of ATP by substrate-level phosphorylation ✳ Substrate-level phosphorylation is the synthesis of ATP by coupling ADP phosphorylation with an exergonic reaction ✳ Overview of process ● Six-carbon stage – ATP + Glucose [C6] = glucose-6-phosphate [C6] – Glucose-6-phosphate [C6] + ATP = fructose-1,6-bisphosphate [C6] ● Three-carbon stage – Fructose-1,6-bisphosphate [C6] cleaved into glyceraldehyde-3-phosphate [C3] – Glyceraldehyde-3-phosphate [C3] converted to pyruvate [C3] + 2 ATP + NADH (this process occurs twice - balance the carbon atoms!!! 2 x C3 = C6) ✳ Summary of glycolysis ● 2 pyruvate molecules ● 2 ATP molecules - sum total: 2 used to begin glycolysis, 4 produced by substrate-level phosphorylation ● 2 NADH molecules ◆ Pentose Phosphate Pathway (Hexose monophosphate pathway) ✳ May be used simultaneously with glycolysis and Enter-Doudoroff pathway ✳ Operates under either aerobic or anaerobic conditions ✳ Important in both catabolism and biosynthesisBIOL 3702 Lecture Outline Chapter 9 Page 3 of 9 ✳ Summary of pentose phosphate pathway ● NADPH formed serves as a source of electrons for reduction of molecules during biosynthesis ● Four- and five-carbon sugars are synthesized that will be used for a variety of purposes ● Intermediates are used to produce ATP ● Serves as a catabolic pathway for five-carbon sugars ● More important as an anabolic pathway than as a catabolic pathway to produce energy ◆ Entner-Doudoroff Pathway ✳ Begins the same way as the pentose phosphate pathway to produce glyceraldehyde-3-phosphate ✳ Glyceraldehyde-3-phosphate catabolized to pyruvate via the 3-carbon stage of glycolysis ✳ Total yield per glucose molecule ● One ATP molecule ● One NADH molecule ● One NADPH molecule Tricarboxylic Acid Cycle ◆ Tricarboxylic Acid Cycle (Krebs Cycle, TCA Cycle, Citric Acid Cycle) is widely distributed among microbes ◆ Comprised of five basic steps: ✳ Oxidation of pyruvate [C3] to acetyl-coenzyme A (AcCoA) [C2] with the release of CO2 and the formation of NADH ✳ AcCoA [C2] condenses with oxaloacetate (OAA) [C4] to form citric acid (citrate) [C6], the first compound of the 6-carbon stage ✳ Through a series of reactions in the six-carbon stage,citric acid [C6] loses a carbon as CO2 to form α-ketoglutarate [C5] while generating one NADH molecule ✳ In the five-carbon stage, α-ketoglutarate [C5] loses a carbon as CO2 to form succinyl coenzyme A (succinyl-CoA) [C4] while generating one NADH molecule ✳ Through a series of reactions in the four-carbon stage, succinyl-CoA [C4] is converted to OAA [C4] while generating one NADH molecule, one FADH2 molecule, and one GTP molecule (equivalent to ATP; produced via substrate-level phosphorylation)BIOL 3702 Lecture Outline Chapter 9 Page 4 of 9 ◆ Summary of TCA cycle per molecule of pyruvate ✳ 4 NADH molecules ✳ 1 FADH2 molecule ✳ 1 ATP (as GTP) molecule ✳ 3 CO2 molecules Electron Transport Chain ◆ In eucaryotes, the electron transport chain is located in the mitochondrial inner membrane ✳ Comprised of a series of electron carriers arranged into four complexes connected by coenzyme Q and cytochrome c ✳ Electrons flow from electron carrier to another ✳ Final electron acceptor is to O2 ◆ Energy that is released during these transfers is captured as ATP via the formation of proton and electrical gradients ◆ The ATP is made via the process of oxidative phosphorylation - ATP formation driven by electron transfer between carriers ✳ NADH will drive the synthesis of 3 ATPs ✳ FADH2 will drive the synthesis of 2 ATPs ◆ The same principles apply to the electron transport system in procaryotes, but the structural details differ as do physiological responses ✳ Vary in cytochrome composition ✳ Electrons can enter at several different points in the chain ✳ Chains may be branched ✳ Different branches may operate under different environmental conditions Oxidative Phosphorylation ◆ Several theories have been postulated for this mechanism of ATP synthesis ◆ Most widely accepted theory is the chemiosmotic theory ✳ During electron transport in eucaryotes,