Transcript Microbial Metabolism

Microbial Metabolism
Metabolic Reactions
Enzymology
Catabolism
Phototrophy
Anabolism
Metabolism Overview:
Reduction;
e- gain
from donor
Oxidation;
e- loss to
acceptor
Metabolic Pathways
• Although we can recognize substrate and product of
individual enzymatic reactions; metabolic functions are
often performed by several enzymatic reactions in a
series or “pathway”.
• Pathways can be linear, branched, cyclic or even spiral.
• Pathway activity is controlled in three ways:
– Metabolites and enzymes may be localized in different parts of
the cell; called metabolic channeling. (important in eukaryotes)
– The total amount of enzymes in a pathway can vary (gene
expression).
– Pathway activity is controlled by critical regulated enzymes.
These “pacemaker enzymes” are often the rate-limiting step in
the pathway.
“Pacemaker” Enzyme Activity
• Enzyme activity may change due to inhibitor or activator
molecules called effectors.
• Inhibitors can be competitive (bind at substrate active site)
• Noncompetitive inhibitors and activators bind to allosteric
(regulatory) sites; separate from the active site; These effectors
change the shape of the protein and its activity as a catalyst.
Metabolic Pathway Control Strategies
Feedback Inhibition:
(“end-product inhibition”; red)
• rate limiting enzyme is first in
pathway and is allosteric.
• end-product is a negative effector
(inhibitor) of first enzyme
Feed Forward Activation:
(“earlier-substrate activation”; blue)
+
• rate limiting enzyme of a branch
point is allosteric.
• earlier-substrate is a positive
effector (activator) of a forward
reaction enzyme.
Arrows =
enzymes
Reversible Metabolic Pathways
• Amphibolic Pathways:
– Catabolic direction
– Anabolic direction
• Separate regulatory enzymes each
way function as “check valves” for
flow control.
• Other pathway enzymes are
reversible; their equilibrium shifts
based on concentration of reactants
& products.
• Gycolysis / Gluconeogenesis is a
good example. Catabolic
breakdown of glucose for energy
versus the its anabolic formation,
respectively.
Glucose
Catabolism
(aerobic)
• ATP as the cellular energy
storage unit, can be formed
during respiration (R) or
fermentation (F).
• Both contain the Glycolysis
pathway; which produces ATP,
the electron carrier molecule
NADH, and pyruvate from
glucose.
• Aerobic Respiration will
proceed via Krebs Cycle and an
ETC if there is oxygen to react
as a terminal electron acceptor.
• Oxygen is not the only
possible terminal electron
acceptor in some bacteria (e.g.
NO3 or SO4 can be used);
called Anaerobic Respiration.
(anaerobic)
(ETC)
• Fermentation
proceeds when
there is no
terminal electron
acceptor for
respiration.
Products of Fermentation
Without any form of respiration, glycolysis products, pyruvate and NADH,
will accumulate. To keep making any more ATP by glycolysis, fermenting
cells must convert NADH (red.) back to NAD+ (ox.) by passing its
electrons to pyruvate. Reaction pathways that do this convert pyruvate
to many other compounds, depending on the organism.
Glycolysis:
6C glucose goes to 2x 3C pyruvate plus 2 ATP net, and 2 NADH. ATP must first be
invested to then yield energy from oxidation and substrate level phosphorylation of ATP.
Pyruvate Decarboxylation:
(Preparatory Step Before Kreb Cycle)
• Pyruvate loses a carbon in the form of
CO2 ; an electron is removed to convert
NAD+ to NADH, and coenzyme-A (CoA)
binds to the 2C acetyl group.
• Acetyl CoA enters the Kreb Cycle by
binding with 4C oxaloacetate to form 6C
citric acid.
Krebs Cycle:
• The cycle converts a citric acid back to
oxaloacetate; losing 2 CO2 ; releasing
electrons to yield 3 NADH plus 1 FADH,
and one ATP by substrate level
phosphorylation.
• For one glucose the cycle runs twice.
Energy Perspective on the Electron
Transport Chain (ETC) Function
The ETC is a series of membrane bound electron carriers
that transports electrons from high to low energy state,
ending with oxygen accepting electrons to water.
Energy release is first used to pump protons
(H+) across the membrane; a proton motive
force (PMF) then drives ATP synthesis.
Each NADH will make 3 ATP.
Each FADH will make 2 ATP
Energy
State
Each electron
transport step
releases energy
PMF= more protons on
this side of membrane.
FADH
Only 2
ATP per
FADH
Maximum yield
per glucose =
38 ATP
• Only achieved by
aerobic respiration of
mitochondria in
eukaryote cells.
• Aerobic respiration
by bacteria is less
efficient (< 24 ATP).
• Anaerobic
respiration is even
less efficient.
• Fermentation least
efficient (2 ATP)
Hydrolysis of Major Biomolecules
Enyzymes of Hydrolysis:
• Proteins by proteases.
• Polysaccharide and
other carbohydrates by
glycosidase.
•Nucleic acids (DNA or
RNA) by nucleases.
• Lipids by lipases.
Amphibolic Nature of Metabolism
Most catabolic pathways
have anabolic
counterparts, so not all
compounds are used to
generate ATP, but rather
shunted to make new
cell biomass.
Energy Source
Overview:
• In addition to organisms
feeding on organic carbon for
energy (chemoorganotrophs).
• There are chemolithotrophs,
which gain energy from
reduced inorganic compounds
(litho = rock).
• There are phototrophs that
yield energy from sunlight and
do not depend on any
chemical energy sources.
• Also note how the terminal
(final) electron acceptor
determines which respiration
type or fermentation.