Chapter 10 Chemotrophic Energy Metabolism: Aerobic Respiration

cellular respiration (or respiration) uses an external electron acceptor to oxidize substrates completely to CO2

  • external electron acceptor: one that is not a by-product of glucose catabolism

  • respiration is the flow of electrons through or within a membrane, from reduced coenzymes to an external electron acceptor, usually accompanied by the generation of ATP

  • Coenzymes such as FAD (flavin adenine dinucleotide) and coenzyme Q (ubiquinone) are also involved

mitrochondria

  • Most aerobic ATP production in eukaryotic cells takes place in the mitochondrion - energy powerhouse of the eurkaryotic cell

    • mitrochondria ar though to have arisn from bacterial cells

    • been shown to carry out all the reactions of the citric acid cycle, electron transport, and oxidative phosphorylation ( Eugene Kennedy and Albert Lehninger 19848

  • aerobic respiration yields much more energy than fermentation does, generating up to 38 moles per glucose

  • mitochondria are found in almost all aerobic cells of eukaryotes in both chemotrophic and phototrophic cells

  • frequently clustered in regions of cells with the greatest need for ATP, like muscle cells

  • in electron micrographs, mitochondria usually appear as oval structures suggesting that they are large and numerous discrete entities

    • can take various shapes and sizes depending on the cell type

outer and inner membranes define 2 separate compartments and 3 regions

  • outer membrane contains porins that allow passage of solutes with molecular weights up to 5000

  • intermembrane space between the inner and outer membranes i thus continuous with the cytosol

  • The inner membrane is impermeable to most solutes, partitioning the mitochondrion into 2 separate compartments: intermembrane space and mitochondrial matrix

    • The inner membrane is 75% protein by weight, and the proteins include those involved in solute transport, electron transport, and ATP synthesis

    • The inner membrane of most mitochondria has many infoldings (tubular or various shapes) called cristae

      • structure of individual cristae with reconstruction of 3D view using EM tomography

        • 3 cristae with green, light blue, and dark blue, and innver mmebrane in gold

        • crista junctions connecting 2 of the cristae

        • the small size of the crista junctions limits diffusion of materials between the intracristal space and the intermembrane space

mitochondrial matrix

  • interior of the mitochondrion is filled wtih a semifluid matrix

  • contains many enzymes involved in mitochondrial function (ex citric acid cycle) as welll as DNA molecules and ribosomes

  • contain proteins encoded by their own DNA as well as some that are encoded by nuclear genes

outer membrane metabolic functions: phospholipid synthesis, fatty acid desaturation, fatty acid elongation    

inner membrane metabolic functions: electron transport, proton translocation for ATP syntheis, oxidative phsophorylation, pyruvate import, fatty acyl CoA import, metabolite transport

matric metabolic function: pyruvate oxidation, criitc acid cycle, ATP synethis, beta oxidation of fats, DNA replication, RNA synthesis (transcription), protein shnthesis (translation)

Respiration includes glycolysis, pyruvate oxidation, the citric acid cycle, electron transport, and ATP synthesis

bacteria do not have mitochondria but are capable fo aerobic respiration

  • their plase membrane and cytoplasm perform the same functions as the inner membrane and matrix of mitochondria

citric acid cycle (tricarboxylic acid cycle, TCA, Krebs cycle): oxidation in the round

  • in presence of oxygen, pyruvate is oxidized fully to CO2 with production of reduced xoenzymes and the released energy ued to drive ATP synethis

  • metabolizes acetyl CoA produced from pyruvate decarboxylation

  • acetyl CoA transfers it acetate group to 4-carbon acceptor called oxaloacetate generating citrate

  • after its formation, cirate undergoes 2 succuessive decarboxylations and also goes through several oxidation steps

  • eventually oxaloacetate is regenerated and can accept 2 more carbons from acetyl Co and cycle begins agian

pyruvate is converted to acetyle CoA by pyruvate derydrogenase complex (PDH)

  • conversion is a decarboxylation because one carbon is liberated as CO2

  • also oxidation bc 3 e- and one proton are trasnferred to NAD+ to form NADH

  • componeents work tg to catalyze oxidative decarboylation

citric acid cycle

  • 1st rxn, 2 carbon actate group is transferred from acetyl CoA to oxaloactate (4C) to form citrate (6C) then converted to isocitrate in the second step

    • isocritrate has hydroyl group that is easily oxidized / dehydrogenated in next step

citric acid-3 and citric acid-4

  • isocitrate is oxidized to oxalosuccinate with NAD+ as the electron acceptor

  • oxalosuccinate immediately undergoes decarboxylation to form a-ketoglutarate (5C) (CAC-3)

  • a-ketoglutarate is oxidized to succinyl CoA (CAC-4)

  • succinyl CoA has been generated like acetyl CoA it has a high-energy thioester bond

citric acid-5 & -6

  • energy from hydrolysis of thioester bond in succinyl CoA is used to generate one ATP (bacteria and plants) or GTP (aniamsl) (CAC-5)

  • succinate is oxidized to furmate in the next step using FAD as electron acceptor

  • furmate is hydrated to produce malate (CAC-7)

  • in reaction CAC-8, malate is converted to oxaloacetate as electrons are accepted by NAD+ to produce NADH

citric acid cycle summary:

  1. 2 carbons enter cycle as acetyl CoA, which joins oxaloscetate to form the 6-carbon citrate

  2. decarboxylation occurs at 2 steps to balance the input of 2 carbons by releaseing 2 CO2

  3. oxidation occurs at 4 steps with NAD+ the electron acceptor in 3 steps and FAD in 1 step

  4. ATP is generated at 1 point with GTp as an intermeidate in the case of animal cells

  5. 1 turn of the cycle is completed as oxaloacetate, the orginal 4C acceptor, is regenerated

acetyl CoA + 3NAD+ + FAD + ADP + Pi → 2 CO2 + 3NADH + FADH2 + CoA—SH + ATP

inclduing glycolysis, pyruvate decarboxylation, and citric acid cycle:

glucose + 10NAD+ + 2FAD + 4ADP + 4Pi → 6 CO2 + 10NADH + 2FADH2 + 4ATP

citric acid cycle represents the main conduit of arobic energy metabolism for a variety of substrates besides sugar (fats and proteins)

  • fats are highly reduced compounts that liberate mroe energy per gram upon oxidation than do carbohydrates- long term enrgy storage form for mnay organism

  • most fat is stored as deposits of triacylglycerols, natural triesters of glycerol and long chain fatty acids

catabolism of triacylglycerols - glycerol is channeled into the glcolytic pathway by oxidative conversion to dihydroacetone phosphate

  • fatty acts are linked to coenzyme A to form fatty acyl CoA then degraded by beta oxidation

  • triglceride + 3 H2O → glycerol + fatty acids

  • glycerol (glycerol kinase) → glycerol phosphate (glycerol phosphate dehydrognease) → dihydroacetone phsophate

fatty acid degradation - most fatty acids are oxidatively converted to acetyal CoA in the mitochondrion

  • the fatty acids are degraded in series of repetitive cycles which removes 2 C at a time until completely degraded

  • each cycle involves

  1. oxidation

  2. hydration

  3. reoxidation

  4. thiolysis

  • this results in production of 1 FADH, 1 NADH, 1 acetyl CoA per cycle

fatty acyl CoA degradation

  • fatty acyl CoA is oxidized forming a double bond b/w alpha and beta carbons (2)

  • 2 electrons and protons removed are transferred to FAD forming FADH2

  • water is added across the double bond by a hydratase (3)

  • beta carbon is oxidized converting the hydroxyl group to a keto group (4)

  • electrons and proton are transferred to NAD+ to form NADH

  • bond b/w alpha and beta carbons is broken and 2-carbon fragment is transferred to a second acetyl CoA (5)

  • steps 2-5 are repeated until the orignial fatty acid is completely degraded

  • most fatty acids have an even number of carbons and are completely degradedl those with an uneven number of carbons require extra steps before feeding into the citiric acid cycle

  • unsaturated fatty acids equire 1 or 2 additional enzymes

  • excessive catabolism of fat- exhcause CoA- ketosis (fats are not oxidied completely lowering blood pH- keto-acidosis, experienced by patienets with uncontrollled diabetes (severe dehydration, falling blood pressure, coma and swelling of the brain)

ordering of electron carriers

  • position of each is determined by its standard reduction potential

  • electron transfer from NAADH or FADH2 at the top O2 at the bottom is spontaneous and exergonic

  • complex 1: nadh-coenzyme q oxidoreductase

  • complex