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Microbial Metabolism
Microbiology and Virology; 3 Credit hours
Atta-ur-Rahman School of Applied Biosciences (ASAB)
National University of Sciences and Technology (NUST)
• Pyruvate dehydrogenase complex is the enzyme
which converts pyruvate to acetyl-CoA
• Acetyl-CoA is energy rich because a high energy thiol
links acetic acid to coenzyme A.
The Tri-carboxylic Acid Cycle
• TCA cycle enzymes are widely distributed among
• In procaryotes, they are located in the cytoplasmic
• In eucaryotes they are found in the mitochondrial
• The complete cycle appears to be functional in many
aerobic bacteria, free-living protists, and fungi.
The Electron Transport Chain
• Electron transport chain is composed of a series of
electron carriers that operate together to transfer
electrons from donors, like NADH and FADH2, to
acceptors, such as O
• The electrons flow from carriers with more negative
reduction potentials to those with more positive
potentials and eventually combine with O2 and H to
form water.
• The difference in reduction potentials between O2 and
NADH is large, about 1.14 volts, which makes possible
the release of a great deal of energy
The Electron Transport Chain
The Electron Transport Chain
ATP Synthase
ETC System of E. coli
• When oxygen is readily
available, the cytochrome
bo branch is used.
• When oxygen levels are
reduced, the cytochrome bd
branch is used because it
has a higher affinity for
• However, it is less efficient
than the bo branch because
the bd branch moves fewer
protons into the periplasmic
flavoprotein (FP),methanol dehydrogenase (MD)
Nitrate reductase (Nar), nitrite reductase (Nir), nitric oxide reductase (Nor), and nitrous
oxide reductase (Nos)
• Bacteria such as Desulfovibrio are another
example. They donate eight electrons to
sulfate, reducing it to sulfide (S2 or H2S).
• The anaerobic reduction of nitrate makes it unavailable
to the cell for assimilation or uptake. Therefore this
process is called dissimilatory nitrate reduction.
• P. denitrificans avoid the toxic effects of nitrite
by reducing it to nitrogen gas, a process
known as de-nitrification.