Environmental Engineering Reference
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electrons that are transferred between compounds by microorganisms also find home
in intermediate compounds. Enzymes responsible for the redox transformations cap-
ture electrons from electron-rich compounds ( dehydrogenases ) and store them in
intermediate co-enzymes such as nicotine adenine dinucleotide (NAD). A typical
reaction involving the NAD/NADH pair is NAD + +
H + .
The synthesis of pyruvic acid by the partial oxidation of glucose is called gly-
colysis . This reaction is a convenient one to show how ATP mediates in a metabolic
process. Figure 6.64 shows the partial oxidation process. The process starts with the
conversion of glucose to glucose-6-phosphate in which one ATP molecule is lost.
Subsequent rearrangement to fructose-6-phosphate and loss of another ATP gives
fructose-1,6-diphosphate. At this point there is a net energy loss. However, during
further transformations, two molecules ofATP are gained and hence overall energy is
stored during the process. The conversion of glyceraldehyde-3-phosphate to pyruvic
acid is the most energy-yielding reaction for anaerobic organisms. This ATP/ADP
cycle is called the fermentative mode of ATP generation and does not involve any
electron transport.
The complete oxidation of glucose should liberate 38 molecules ofATP, equivalent
to the free energy available in glucose. If that much has to be accomplished, the
electrons generated during the process should be stored in other compounds that
then undergo reduction. Electronacceptors generally used by microbes in our natural
environment include oxygen, nitrate, Fe(III), SO 2 4 , and CO 2 . It is to mediate the
transfer of electrons from substrate to electron acceptors that the microorganisms
need intermediate electron transport agents. These agents can also store some of
the energy released during ATP synthesis. Some examples of these intermediates
are cytochromes and iron-sulfur proteins. The same function can also be performed
by compounds that act as H + -carriers (e.g., flavoproteins). The redox potentials of
some of the electron transport agents commonly encountered in nature are given in
Table 6.17. Chappelle (1993) cites the example of E. coli that uses the NADH/NAD
cycle to initiate redox reactions that eventually releases H + ions out of its cell. The
NADH oxidation to NAD in the cytoplasm is accompanied by a reduction of the
flavoprotein that releases H + from the cell to give an FeS protein which further
converts to flavoprotein via acquisition of 2H + from the cytoplasm, and in concert
withcoenzymeQreleases2H + outofthecell.Theresultingcytochrome b transfersthe
electrontomolecularoxygenformingwater.Thenetresultistheuseoftheenergyfrom
redox reactions to transport hydrogen ions out of the cell. The energy accumulated
in the process is utilized to convert ADP to ATP. The entire sequence of events is
pictorially summarized in Figure 6.65 and is sometimes called chemiosmosis , the
process of harnessing energy from electron transport. More details of these schemes
are given in advanced textbooks such as by Schelegel (1992). The main point of this
discussion is the part played by electron transport intermediates and ATP synthesis
in the metabolic activities of a living cell. Thus, microorganisms provide an efficient
route by which complex molecules can be broken down. The entire process is driven
by the energy storage and release capabilities of microorganisms that are integral
parts of their metabolism.
Thermodynamic considerations of the energy of reactions mediated by micro-
organisms can provide us information on the feasibility of the transformations. The
2H + +
2 e
NADH
+
 
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