Download Chemoheterotrophs Chemoheterotrophs: Fat β (beta)

Chemoheterotrophs
Chapter 5 Metabolism:
Lipids, Anabolism, Autotrophs
Energy & carbon extraction from organic sources
Note how biochemically inferior humans are to many bacteria in this area:
Many microbial chemoheterotrophs can live on a single organic food source
(glucose only; citrate only; etc.)
What would happen to you if all you ate was sugar?
WHY?
Dr. Amy Rogers
Bio 139 Fall 2006
Office Hours: Mondays & Wednesdays,
~8:30-10:00 AM
Some figures taken from
Krogh Biology: A Guide to the Natural World
Chemoheterotrophs: Fat
•Catabolism of glucose for energy:
•Glycolysis
•Fermentation OR aerobic respiration (Krebs, ETC)
•Anaerobic respiration (ETC without oxygen as electron acceptor)
•Catabolism of proteins for energy:
•Deamination for aerobic respiration (products enter Krebs cycle)
•Decarboxylation in absence of oxygen
Fat catabolism: Triacylglycerols
Carbohydrates & Proteins are not the only
organic energy sources:
Lipids
(here, all saturated)
Lipids are frequently stored as fats in the form of
triacylglycerol (triglycerides)
1. Hydrolysis to glycerol & free fatty acids
Glycerol (3 carbons) enters glycolysis
2. Breakdown of fatty acids into “activated” 2 carbon units
β (beta)-oxidation
2 carbon units are oxidized,
broken off and bound to
Coenzyme A to make
Acetyl-CoA
Repeat β-oxidation, 2 carbons at a
time, until the fatty acid is gone.
(“activated” 2 carbon units: remember
pyruvate going from glycolysis to Krebs?)
All these oxidations, also
make reduced molecules:
NADH & FADH2
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Anabolism
• Bacteria are remarkable chemical factories
• Consider this week’s news article (bioplastics, biofuels)
• Various species of bacteria can make all kinds of weird stuff
(reduced electron carriers)
(NADH, FADH2)
• We have dissected a single catabolic
pathway (glycolysis & aerobic respiration)
in detail; many others exist
• What about anabolism (biosynthesis)?
DO NOT memorize any names/details from this figure
The point: Catabolic pathways provide both the energy
(ATP) and the initial substrates for anabolic pathways in
complex, interconnected webs of reactions.
Anabolism
• Every reaction in a biochemical pathway is
catalyzed by a specific enzyme
• For example, synthesis of the amino acid
tryptophan requires at least 13 different enzymes
Produced
during
glycolysis
• Lack of any one enzyme in a pathway
prevents synthesis of the final product
The product must be found in the environment,
or the organism dies
Essential amino acids, vitamins, etc.
Krebs cycle
intermediate
Auxotrophs & Prototrophs
A mutation that abolishes function of a single enzyme can make a
bacterium dependent on an outside source of a nutrient
(can’t synthesize it, gotta eat it)
Photoautotrophs
(as opposed to chemoheterotrophs)
Produce chemical energy from light;
carbon source: CO2
• Cyanobacteria, green algae, plants: Photosynthesis
We call such mutated strains of bacteria auxotrophs:
bacteria with specific unusual nutritional requirements as a
result of mutation (lost ability to make it for themselves)
• Use electrons from H2O to reduce CO2 to carbohydrate
• Produce oxygen (waste)
• Probably modified earth’s atmosphere by dumping O2
• Green & Purple Sulfur bacteria:
Prototroph: a wild type strain of the same species, with
all its genes intact, can grow without addition of that
particular nutrient to the media
• Use H2S, produce sulfur or sulfuric acid
• Strict anaerobes, and do not produce oxygen
• Came earlier in evolution, when atmosphere was hydrogen-rich
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Chemoautotrophs
• Inorganic energy sources (chemo-)
– Energy extracted by oxidizing inorganic chemicals
– Hydrogen, ferrous ions, ammonia, etc.
• CO2 as carbon source (-auto-)
– Can synthesize all organic substances they need
• Carbohydrates, fats, proteins, nucleic acids
(some bacteria can oxidize gold, copper, even uranium!)
Example: Nitrifying bacteria that use ammonia as an energy source
Archaea in deep ocean volcanic vents are in this group
Nitrifying Bacteria
The Nitrogen Cycle
(chemoautotrophs)
• Nitrifying bacteria use ammonia (NH3) &
nitrite (NO2) as energy sources
• Nitrogen is a necessary element for life
(amino acids, nucleic acids)
• The ultimate product of such oxidation is nitrate
(NO3)
• Nitrogen gas (N2) is abundant in the
atmosphere, but in this chemical form, it is
inaccessible to most living things
• These chemicals can be oxidized, therefore can be energy
sources!
• Nitrate is the form of nitrogen most usable by
plants
• Nitrifying bacterial play a crucial ecological role
• Atmospheric N2 must be fixed (converted
to a usable form)
Fixation: conversion of a gas into a solid or liquid form by chemical reaction
Nitrogen Cycle
Nitrogen Cycle
• Nitrification is an oxidation (NH3
NO3)
• autotrophic nitrifying bacteria get energy
• Nitrogen fixation is a reduction; energy is
consumed (N2
NH3)
(costs energy)
(terminal electron acceptor)
• Some nitrogen fixing bacteria are symbiotic, living inside
plants which provide the energy in exchange for the fertilizer
• Denitrification (returning organic nitrogen back to N2 gas), in
Oxidation by
nitrifying
bacteria
(yields energy)
turn, is a reduction. Under anaerobic conditions, some bacteria use
nitrate instead of oxygen as a terminal electron acceptor.
• See Nitrate Reduction test (lab #20)
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Relevant reading in
Black’s Microbiology:
(pages from 6th edition)
• Ch. 5
• Ch. 6, page 190 (auxotrophs & prototrophs)
• Ch. 25, pages 751-754 (nitrogen cycle)
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