Download M220 Lecture 11 - Napa Valley College

M220 Lecture 11
Microbial Metabolism (continued)
In living systems oxidation and reduction reactions are coupled. When highly reduced substrates such
as glucose are oxidized, the hydrogens (with their electrons) that are lost, must be gained by other
chemicals. These other chemicals are therefore reduced as they gain hydrogens. Coenzymes can
receive these hydrogens (become reduced) to make enzymatic reactions work. These same coenzymes
will later become oxidized when they donate their hydrogens to allow other enzymatic reactions to
occur.
Glucose is a highly reduced substrate (many hydrogens). It has the potential to go through many
oxidation reactions yielding much energy. (Remember, oxidations are coupled with reductions). Energy
is captured in ATP by the process of phosphorylation. Substrate level phosphorylations occur when high
energy phosphates are directly transferred from phosphorylated compounds to ADP. Phosphorylations
that occur as part of the electron transport chain (system) are known as oxidative phosphorylations.
Hydrogens that deliver their electrons to the electron transport chain (system) for oxidative
phosphorylation are transported on reduced coenzymes (NAD and FAD). Here the reduced coenzymes
give up their hydrogens (with their electrons) to ETC compounds and become oxidized in the process.
Coenzymes are now free to undergo reduction and pick up hydrogens once again.
Respiration is any energy yielding oxidation in which the terminal hydrogen acceptor is an inorganic
compound. Gaseous oxygen is not necessarily involved.
1. Aerobic Respiration-any energy yielding oxidation in which the terminal hydrogen acceptor is
the inorganic compound oxygen. (Highest energy yield)
2. Anaerobic Respiration-any energy yielding oxidation in which the terminal hydrogen acceptor is
an inorganic compound other than oxygen. Terminal hydrogen acceptors include compounds
such as nitrate (NO3-1), sulfate (SO4-2) or carbonate (CO3-2).
Fermentation is any energy yielding oxidation in which the terminal hydrogen acceptor is an organic
compound. This does not require oxygen. In fact, this is anaerobic. The Electron Transport Chain is not
used. (Lowest energy yield)
There are three major reaction pathways used in the complete oxidation of glucose. They are Glycolysis,
Krebs Cycle (Citric Acid Cycle or Tricarboxylic Acid Cycle) and the Electron Transport System or Chain
(Cytochrome System). Note, that there is also a step between Glycolysis and the Krebs Cycle known as
the Preparatory or Intermediate step.
Note: Enzymatic reactions for ATP production occur on the plasma (cell) membranes of prokaryotes.
Now we examine the complete catabolism of glucose using aerobic respiration.
I.
Features of Glycolysis
A. Net gain of 2 ATPs from substrate level phosphorylation. Actually 2 ATPs are used to
prime reactions, but 4 ATPs are actually produced.
B. 2 hydrogen pairs (2H will be used to represent a hydrogen pair) are produced. Each 2H
is attached to the coenzyme NAD. Therefore, 2 NAD each with a hydrogen pair is
produced. The reduced NAD has the potential to carry the 2H to the ETC (later) for ATP
production by oxidative phosphorylation.
C. Glycolysis is anaerobic it does not require or use oxygen whether oxygen is present or
not.
D. Glycolysis is a series of 10 separate enzymatic reactions. The 6 carbon glucose is
eventually split into two 3 carbon pyruvic acids.
Intermediate or Preparatory Step-follows glycolysis
Before entering the Krebs Cycle, each pyruvic acid must lose one molecule of CO2 and become a
2 carbon unit (acetyl group). Since 2 pyruvic acids are produced from each glucose that entered
glycolysis, 2CO2’s are produced in this step per glucose. Furthermore, for each pyruvic acid that
is converted to an acetyl, a 2H (hydrogen pair) is removed (oxidation reaction) and attached to
NAD (becomes reduced). Again, there are two pyruvic acids per glucose so 2 NAD’s each with a
2H is created per glucose in this preparatory step. Each acetyl group is now attached to the
coenzyme called CoA to become acetyl CoA. The acetyl CoA delivers the 2 carbon acetyl group
to the Kreb’s Cycle reactions.
II.
Feature of Krebs Cycle
A. Series of biochemical reactions in which the large amount of potential chemical energy
stored in acetyl CoA is released in steps. Intermediate compounds in the Krebs Cycle
are oxidized and the hydrogens are added to the coenzymes NAD and FAD (they
become reduced) along the way. Remember that reduced NAD and FAD will carry the
hydrogens along with their electrons to the ETC for ATP production.
B. Acetyl CoA delivers its acetyl group (2C) to a 4C compound in the Krebs Cycle called
oxaloacetic acid. Citric Acid a 6C compound is formed.
C. In step 3, a hydrogen pair (2H) is added to NAD. Also a CO 2 is removed
(decarboxylation). The compound now has 5 carbons.
D. In step 4, another 2H is added to NAD and another CO2 is removed. We now have a 4C
compound.
E. In step 5, ATP is produced through substrate level phosphorylation.
F. In step 6, succinic acid (succinate) is converted to fumaric acid (fumarate). This
dehydrogenation reaction adds 2H to FAD (thus reducing FAD). FAD like NAD will carry
the hydrogen pair to the ETC for ATP production.
G. In step 8, another 2H is added to NAD. The compound oxaloacetic acid is formed and
the cycle can be repeated.
Note, for each glucose, 2 acetyl groups are formed. The above features (A-G) are repeated for
each acetyl group. The entire Krebs Cycle turns twice per glucose.