Quiz Me!

Catabolism: aerobic & anaerobic respiration

  Lecture Index
  Course Resources page
Last revised: Monday, February 21, 2000
Ch. 9 (p. 169-174; 176-179) in Prescott et al, Microbiology, 4th Ed.
Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!
Copyright 2000. Thomas M. Terry

Solution 2: Respiration

Electron transport system (ETS)

Specific carriers of ETS:

  1. mitochondria (in eucaryotes): NADH ---> (Flavoprotein ---> Iron sulfur proteins ---> Quinone ---> cytochrome b ---> cytochrome c ---> cytochrome a ---> cytochrome a3 ---> oxygen
  2. bacteria (prokaryotes) have different ETS carriers, shorter chains. In E. coli, can have two different terminal oxidases, one functions at high oxygen levels, one at lower oxygen levels. Cytochromes involved include: b558, b595, b562, d, and o

proton gradient and oxidative phosphorylation (oxphos)

Chemiosmotic hypothesis (Peter Mitchell, 1961)

differences between respiration in mitochondria (eucaryotes) and bacteria (procaryotes)

  1. In Eukaryotes:
    • ETS located in inner mitochondrial membrane. Proton gradient develops across inner mitochondrial membrane.
    • Mitochondria are very efficient at generating proton gradient. Can measure how many ~P bonds (in ATP) are made for each O2 consumed = P/O ratio.
    • With NADH as electron donor, P/O ratio can be 3 (means 3 ATP made per NADH).
    • But with FADH as electron donor, P/O ration only 2 (fewer protons are transported, less proton gradient).
    • Overall efficiency of respiration in mitochondria: ~ 40% (means that about 40% of energy in glucose actually gets converted to ATP).
  2. In Prokaryotes:
    • ETS located in cytoplasmic membrane. Proton gradient develops across this membrane.
    • Bacteria are not as efficient. ETS chains are shorter, P/O ratios are lower.
    • As a ballpark estimate, P/O ratios for NADH are only ~2. Overall efficiency of glucose oxidation is closer to 28%, not 40%.

Inhibitors of Oxidative Phosphorylation

Anaerobic respiration:

Examples of anaerobic respiration:

  1. Nitrate (NO3-).
    • Process called denitrification. Also called dissimilative nitrate reduction. Reduced waste products are excreted in significant amounts.
    • Redox potential is + 0.42 v (compared to + 0.82 v for oxygen). So organisms respiring anaerobically gain less energy than with oxygen.
    • Requires new terminal oxidase called nitrate reductase. Enzyme is repressed by oxygen, synthesis turned on in absence of oxygen.
    • Process can have several steps, proceed in two different directions:
      1. (A) nitrate (NO3-) ---> nitrite (NO2-) ---> ---> ---> ammonia (NH3)
      2. (B) nitrate (NO3-) ---> nitrite (NO2-) ---> nitrous oxide (N2O) ---> ---> dinitrogen gas (N2)
    • Second process is major pathway for loss of nitrogen compounds from soil, return of nitrogen to atmosphere.
    • Pseudomonas species are common denitrifiers, widespread in soils. When fertilized soils become flooded, oxygen is rapidly depleted, pseudomonads switch to anaerobic respiration and can use up soil nitrate, leaving field in unfertile state.
    • Note: Studied this in lab. Media must contain nitrate in addition to nutrients, otherwise won't work. Also, in scavenger hunt at end of course, one target microbe will be Pseudomonas, enrichment culture depends on its ability to grown anaerobically using nitrate reduction.
  2. 2. Sulfate (SO42-).
    • Process called sulfate reduction.
    • Sulfate (SO42-) ---> ---> ---> ---> Hydrogen Sulfide (H2S)
    • Small group of bacteria carry out this reaction; all obligate anaerobes.
    • Have unique cytochrome c3.
    • Sulfate is common in seawater. Often, H2S combines with iron, forms insoluble FeS ----> black sediments. Common in estuaries.
  3. 3. Carbon dioxide (CO2).
    • One of most common inorganic ions.
    • Methanogens: most important group of CO2 reducers. Obligate anaerobes, archaebacteria. Produce methane as waste product.
    • Reaction: CO2 + H2 + H+ ---> CH4 + H2O
    • Note: reaction also requires Hydrogen gas. Methanogens typically live alongside bacteria that produce hydrogen by fermentation, remove hydrogen as it is made.

TCA cycle: further catabolism of pyruvate

formation of acetyl-CoA

net effects of TCA cycle (see handout)

Catabolism of substances other than glucose

Take a Self-Quiz on this material
Return to Lecture Index
Return to MCB 229 Course Resources page