Metabolism: Energy Release and Conservation Chapter 9 Metabolism: Energy Release and Conservation

Sources of energy most microorganisms use one of three energy sources the sun reduced organic compounds reduced inorganic compounds the chemical energy obtained can be used to do work Figure 9.1

Chemoorganotrophic fueling processess Figure 9.2

Chemoorganic fueling processes-respiration Most respiration involves use of an electron transport chain aerobic respiration: final electron acceptor is oxygen anaerobic respiration final electron acceptor is different exogenous NO3-, SO42-, CO2, Fe3+ or SeO42-. organic acceptors may also be used As electrons pass through the electron transport chain to the final electron acceptor, a proton motive force (PMF) is generated and used to synthesize ATP

Chemoorganic fueling processes - fermentation Uses an endogenous electron acceptor usually an intermediate of the pathway e.g., pyruvate Does not involve the use of an electron transport chain nor the generation of a proton motive force ATP synthesized only by substrate-level phosphorylation

Aerobic catabolism-An Overview Three-stage process large molecules (polymers)  small molecules (monomers) oxidation of monomers to pyruvate oxidation of pyruvate by the tricarboxylic acid cycle (TCA cycle)

many different substrtaes are funneled into the TCA cycle ATP made primarily by oxidative phosphory- lation Figure 9.3

Amphibolic Pathways Function both as catabolic and anabolic pathways Examples: Embden-Meyerhof pathway pentose phosphate pathway tricarboxylic acid (TCA) cycle Figure 9.4

The Breakdown of Glucose to Pyruvate Three common routes Embden-Meyerhof pathway pentose phosphate pathway Entner-Doudoroff pathway

The Embden-Meyerhof Pathway (glycolysis) Occurs in cytoplasmic matrix Oxidation of glucose to pyruvate can be divided in two stages -glucose to fructose 1,6 -bisphosphate (6 carbon) -fructose 1, 6-bisphosphate to pyruvate (two 3 carbon)

Glycolysis oxidation step – generates NADH ATP by substrate-level phosphorylation Figure 9.5

Summary of glycolysis glucose  2 pyruvate 2ATP 2NADH + 2H+

The Pentose Phosphate Pathway Can operate at same time as glycolytic pathway Operates aerobically or anaerobically an Amphibolic pathway

produce NADPH no ATP important intermediates Figure 9.6

sugar transformation reactions Figure 9.7

Summary of pentose phosphate pathway glucose-6-P  6CO2 12NADPH Glycolytic intermediates

The Entner-Doudoroff Pathway reactions of pentose phosphate pathway yield per glucose molecule: 1 ATP 1 NADPH 1 NADH reactions of glycolytic pathway Figure 9.8

The Tricarboxylic Acid Cycle Also called citric acid cycle and Kreb’s cycle Common in aerobic bacteria Anaerobes contain incomplete TCA cycle An Amphibolic pathway

Figure 9.9

Summary For each acetyl-CoA molecule oxidized, TCA cycle generates: 2 molecules of CO2 3 molecules of NADH one FADH2 one GTP

Electron Transport and Oxidative Phosphorylation Only 4 ATPs are synthesized directly from oxidation of glucose to CO2 (by substrate-level phosphorylation) Most ATP made when NADH and FADH2 are oxidized in electron transport chain (ETC)

The Electron Transport Chain Series of electron carriers transfer electrons from NADH and FADH2 to a terminal electron acceptor Electrons flow from carriers with more negative E0 to carriers with more positive E0

Electron transport chain… As electrons transferred, energy released In bacteria and archaea electron carriers are in located plasma membrane In eucaryotes the electron carriers are within the inner mitochrondrial membrane

large difference in E0 of NADH and E0 of O2 large amount of energy released Mitochondrial electron transport chain. Electron carriers are organized into 4 complexes that are linked by CoQ and Cyt c. Figure 9.10

Mitochondrial ETC electron transfer accompanied by Proton release into the intermembrane space occurs when electrons are transferred from carriers (e.g., FMN and A) that carry both electrons and protons to components (e.g., FES proteins and Cyt) that transport only electrons. electron transfer accompanied by proton movement across inner mitochondrial membrane Figure 9.11

Electron Transport Chain of E. coli branched pathway upper branch – stationary phase and low aeration lower branch – log phase and high aeration Figure 9.12

Oxidative Phosphorylation Process by which ATP is synthesized as the result of electron transport driven by the oxidation of a chemical energy source

Proton Motive Force Is the most widely accepted hypothesis to explain oxidative phosphorylation electron carriers are organized in the membrane such that protons move outside the membrane as electrons are transported down the chain proton expulsion results in the formation of a concentration gradient of protons and a charge gradient The combined chemical and electrical gradient (electro chemical ) across the membrane is the proton motive force (PMF)

Chemiosmosis Peter Mitchell in 1961 proposed that the electrochemical gradient (proton and pH) across a membrane is responsible for the ATP synthesis. He likened this process to osmosis, the diffusion of water across a membrane, which is why it is called chemiosmosis. Peter Mitchell received the Nobel Prize in 1978 for this concept.

PMF drives ATP synthesis(Chemiosmosis) Diffusion of protons back across membrane (down gradient) drives formation of ATP ATP synthase enzyme that uses PMF down gradient to catalyze ATP synthesis

ATP Synthase Figure 9.14 (a) The major structural features of ATP synthase deduced from X-ray crystallography and other studies. F1 is a spherical structure composed largely of alternating  and  subunits; the three active sites are on the  subunits. The  subunit extends upward through the center of the sphere and can rotate. The stalk ( and  subunits) connects the sphere to F0, the membrane embedded complex that serves as a proton channel. F0 contains one a subunit, two b subunits, and 9-12 c subunits. The stator arm is composed of subunit a, two b subunits, and the  subunit; it is embedded in the membrane and attached to F1. A ring of c subunits in F0 is connected to the stalk and may act as a rotor and move past the a subunit of the stator. As the c subunit ring turns, it rotates the shaft ( subunits). Figure 9.14 (a)

The binding change mechanism of ATP synthesis is depicted The binding change mechanism of ATP synthesis is depicted. The F1 sphere is viewed from the membrane side. The 3 active sites appear able to exist in 3 different conformations: an inactive open (O) conformation with low affinity for substrates, an inactive (L) conformation with fairly loose affinity for the substrates, and an active tight (T) conformation that has high affinity for substrates. In the first step, ADP and Pi bind to the O site. Then the  subunit rotates 120° with the input of energy, presumably from proton flow through F0. This rotation causes conformation changes in all three subunits resulting in a release of newly formed ATP and a conversion of the L site to an active T conformation. Finally, ATP is formed at the new T site while more ADP and Pi bind to the unoccupied O site and everything is ready for another energy-driven  subunit rotation. Figure 9.14 (b)

Inhibitors of ATP synthesis Blockers inhibit flow of electrons through ETC Uncouplers allow electron flow, but disconnect it from oxidative phosphorylation many allow movement of ions, including protons, across membrane without activating ATP synthase destroys pH and ion gradients some may bind ATP synthase and inhibit its activity directly

Maximum Theoretic ATP Yield from Aerobic Respiration Figure 9.15

Theoretical vs. Actual Yield of ATP Amount of ATP produced during aerobic and anaerobic respiration varies depending on growth conditions and nature of ETC Comparatively, anaerobic respiration yields fewer ATP that aerobic respiration In fermentation yileds very few ATP