Less electronegative than H2: negative redox potential
More electronegative than H2: positive redox potential
Fluorine as an electron acceptor
Although fluorine is the most electronegative atom, it has properties that prevent it from being an electron acceptor
Nicotine Adenine Dinucleotide
NAD+
Strong reduction potential: -.32V
2 electrons & 2 protons donated to destroy double bond
NADP+ has a phosphate bond
Electron Carriers
(prosthetic and coenzymes)
transfer of electrons from donor to acceptor through intermediary (also source of protons)
1) freely diffusible
2) attached to enzymes in cytoplasmic membranes
Energy released in redox reactions is highly conserved in high energy phosphate bonds
ATP, ADP, AMP
ATP is a great source of energy
ADP is a good source of energy
AMP is a poor source of energy
3 Classes of catabolism
1. Fermentation
2. Respiration
3. Photoheterotrophy
Fermentation
-partial breakdown of organic food without net transfer of electrons to a terminal electron acceptor
Respiration
-breakdown of organic molecules with electron transfer to a terminal electron acceptor
Photoheterotrophy
-energy from light absorption is used to breakdown an organic molecule
Catabolism substrates
Lipids
Glycerol (glycolysis)
Fatty acids (oxidative breakdown to acetyl group)
Amino acids
Decarboxylation (glycolysis)
Deamination (excretion)
Aromatic compounds (acetyl groups- TCA cycle)
Substrate level phosphorylation
ATP is synthesized during the catabolism of an organic compound
Transfers high energy bond from one group to another, converting loe energy bond to high energy
Glycolysis
Products
NAD+ reduced to NADH
Consume ATP, produce 3 ATP
Pyruvate
(all happens twice)
Citric Acid Cycle
Broad overview
There are 10 variatios in archaea & bacteria
Pyruvate is completely oxidized to CO2
Can sometimes go backward for making carbon skeletons
Intermediate molecules can be used for other processes
Alpha ketogluterate
Citric Acid Cycle Precursor Molecules
-glutamate -> other AAs
-oxaloacetate-> asparate -> other AAs
Pyruvate as Citric Acid Cycle
Precursor Molecule
Can be used to make alanine
3-phosphoglycerate
Citric Acid Cycle Precursor Molecules
Can be used to make serine
Citric Acid Cycle overall reaction
Pyruvate + 4NAD+ + FAD --> 3CO2 + 4 NADH + FADH2
Electron Transport Phosphorylation
Donors and ATP yield
8 NADH = 24 ATP
2 FADH2 = 4 ATP
2 GTP = 2 ATP
NADH dehydrogenase
Each level of chain is reduced by the product of the previous form
ATP synthase uses proton gradient to make ATP
If asked to sterilize a heat sensitive material, which would you use:
B) ethylene oxide
Which statement is incorrect?
B) biofilms are more sensitive to antibiotics
Which is not a chemical agent used to control microbes?
C) filtration
Aerobic Respiration
(3 parts and overall reactions of each)
1. Glycolysis
Glucose -> 2 Pyruvate
2. Tricarboxylic Acid Cycle
2 Pyruvate -> 6 CO2
3. Electron Transport Chain
12 H+ + 3 O2 -> 6H20
NADH dehydrogenase & ETC
-transfer hydrogen atoms from NADH
-inner surface of cell membrane
-in respiration, hydrogen atoms passed to flavoproteins
FMN or FAD containing proteins (flavoproteins)
-riboflavin (vitamin B2)
-flavoprotein accept H atoms (2 electrons and 2 protons are transferred)
Cytochromes
heme containing proteins (iron-porphyrin ring)
-different classes of cytochromes with different reduction potentials, but all have an iron section to be reduced and oxidized
Porphyrin
Cytochrome example: heme group in blood
Porphyrin Degrading Enzymes get blood out of clothes
Iron-sulfur proteins
-Fe2S2 and Fe4S4-have a range of reduction potentials
Quinones
-hydrophobic-lipid soluble-diffuse through membrane
-similar to vitamin K
Proton motive force
-electon transport carriers-oriented in the membrane in such a way that a separation of proton from electrons occurs across the membrane during electron transport
-protons pumped to the outside
-electrons move down the electron transport chain until they reduce oxygen
-oxygen is reduced to water-hydrogen for the water come from inside the cytoplasm
Gradients used in Proton Motive Force
Electrical and pH gradients
Protons are pumped out of cell and energy is stored in these gradients
NAD+ is the best electon donor
O2 is the best electron acceptor
Complex 1
1. NADH donates 2H+ and 2e- to FAD
2. FADH donates 2e- to iron-sulfur protein
but the 2H+ are released into the environment
3. 2H+ from the cytoplasm and 2e- from Fe-S protein reduce coenzymeQ
Complex III
4. Coenzyme Q passes e- to cytochrome bc1
5. Fe-S protein (Rieske protein) passes the electrons to cytochrome c1 and eventually to cytochrome c (periplasmic protein)
Complex IV
6. Cytochrome c diffuses to cytochrome aa3 and donates electrons
7. O2 is reduced to water
ATP Synthase
Oxidative Phosphorylation
FO portion crosses membrane
F1 portion is inside cell and catalyzes ATP synthesis
Protons enter FO, cause conformation change of F1 and create ATP
ATP Synthase Inhibitors
block electron flow and thus there is no proton motive force established
eg. CO, CN- -bind to cytochromes
ATP Synthase Uncouplers
prevent ATP synthesis without affecting electron transport
eg. Dinitrophenol, dicumarol-make membranes leaky destroying the proton motif force
ATP produced by glucose catabolism
Efficiency
–Efficiency of energy capture= 40%
–Efficiency of an Internal combustion engine= 20%
Aerobic respiration: oxygen as the terminal acceptor
What happens without oxygen?
Found: Wetland soils and water, human digestive tract
Environmental modulation of ETS
1. Food is limiting- less NADH entering the system (alternative NADH dehydrogenase)
2. Low oxygen conditions (different terminal electron acceptor)
Chemoheterotrophs: Anaerobic respiration
-alternative electron acceptors (6 of them)
Fe 3+
NO3-
NO2-
S
CO2
SO42-
Chemoheterotrophs: Anaerobic respiration
Proton gradient & energy yield
Still have proton gradient and ATP synthase but energy yield is lower
Assimilative metabolism
-compounds that serve as terminal electron acceptors can be a nutrient source (ex. Nitrate, Nitrite are terminal electron acceptors also used for amino acid synthesis)
Dissimilative metabolism
-electron acceptors
-large amounts of compounds reduced
-reduced electron acceptor is excreted into the environment
Nitrate (NO3-) reduction
-obligate and facultative anaerobes
-reduced waste products (nitrite) are excreted in significant amounts
Redox potential=+ 0.42 v (+ 0.82 v for oxygen).
-Nitrate reduction is repressed by the presence of oxygen
Nitrate Reduction
Reduction of nitrate to nitrogen gas
(Examples of microbes)
Examples:
Paracoccus dinitrificans
Pseudomonas stutzeri
Rhodobacter sphaeroides
Sulfate (SO42-) sulfate reduction
-sulfate to sulfite to hydrogen sulfide
-marine environments, hydrothermal vents
-obligate anaerobes
**Not many protons are pumped through ATP Synthase
Sulfate Reduction
Carbonate reduction (CO2)
Hydrogen gas is a strong electron donor, can be used to reduce CO2 to methane
Hydrogenotrophy
Carbonate reduction
use of molecular hydrogen as an electron donor
Methanogens
Carbonate reduction
most important group of CO2 reducers (Archaea)
Produce methane as waste product
Found in landfills and arctic poles, also found in digestive tract of cows
Methanogens & Homoacetogens
Methanogens have electron acceptors (coenzymes) that differ from other bacteria and archaea.
-still have proton motive force but very low energy yield
Homoacetogens- Not seen in eukaryotes. Use methane but produce acetic acid (acetogenesis)
Metal Electron Acceptors
Ferric Ion (Fe3+), Manganese, chlorate, uranium
Fe2+ was likely the dominant electron acceptor in early Earth
Geobacter metallireducens- can reduce metals by grabbing with pilli
Can be used to clean environment
Organic Electron Acceptors (example)
Ex. Fumerate is reduced by succinate
How do organisms decide if they should do fermentation or anaerobic respiration?
Usually do fermentation when no final electron acceptor is present
Fermentation Products
H2, butyrate, formate
Excreted from cell when produced
*Glycolysis is not the only way to get fermentation products
What happens when there is not enough energy to synthesize ATP?
Ion pumps can create a gradient (ex. Na+) that stores energy
API-20E test
•System of 20 individual, miniaturized tests used to determine the metabolic capabilities of the organism.