Formation of Acetyl Coenzyme A
Summary: Pyruvate is degraded and combined with coenzyme A to form acetyl coenzyme A; hydrogens are released; and carbon dioxide is released.
The pyruvic molecules formed in glycolosis enter the mitochondria,
where they are converted to acetyl coenzyme A (acetyl CoA). In
this complex series of reactions, pyruvate undergoes oxidative
decarboxylation. First, a carboxyl group is removed as carbon
dioxide, which diffuses out of the cell. Then the two-carbon fragment
remaining is oxidized, and the hydrogens that were removed during
the oxidation are accepted by NAD+. Finally, the oxidized two-carbon
fragment, an acetyl group, is attached to coenzyme A, which is
manufactured in the cell from one of the B vitamins, pantothenic
acid. The reaction is catalyzed by a multienzyme complex that
contains several copies of each of three different enzymes. The
overall reaction for the formation of acetyl coenzyme A can be
stated as follows:
Note that the original glucose molecule has now been oxidized
to two acetyl groups and two carbon dioxide molecules. The hydrogens
removed have reduced NAD+ to NADH. At this point in aerobic respiration,
four NADH molecules have been formed from a single starting glucose
molecule, two during glycolosis and two during the formation of
acetyl CoA from pyruvate.
STEP 3: Citric Acid Cycle
STEP 1: Glycolosis
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