Go
to the main content page
The
oxidative pathways
A
large amount of ATP is produced by the complete oxidation
of carbon-containing compounds (e.g., glucose) to carbon
dioxide and water in reactions of the Krebs cycle and oxidative
phosphorylation. These reactions take place inside the membrane-bound
components of cells called mitochondria. Carbohydrate can
enter the oxidative pathways at a branch point in the glycolytic
pathway. At the branch point, two kinds of reaction can
occur: pyruvate, a three-carbon compound, can either react
to form lactate or lose a carbon dioxide molecule to form
a two-carbon unit, which then enters the Krebs cycle. If
the muscle is working hard and using ATP rapidly, lactate
is formed. If it is working slowly, a larger fraction of
the carbon enters the oxidative pathways.
During
reactions of the Krebs cycle, carbon dioxide and hydrogen
atoms are removed from the compounds that enter the cycle.
The carbon dioxide diffuses out of the muscle and is carried
bythe blood to the lungs, where it is exhaled. The electrons
from the hydrogen atoms are passed through an electron-transport
chain consisting of the series of reactions involving cytochrome
molecules. These events occur in the mitochondria, in which
ADP is also combining with phosphate (Pi ) to form ATP.
In the last step of this reaction sequence, the hydrogen
atoms combine with oxygen to form water. The net reaction
for each glucose molecule that enters the glycolytic pathway
and proceeds through the Krebs cycle and oxidative phosphorylation
is summarized below.
C6H12O6+36ADP+36Pi=6CO2+36ATP+6H2O
Although the oxidative pathways result in the formation
of more ATP from each glucose molecule than does the glycolytic
pathway, the complete process is much slower than glycolysis.
The slowest steps are those involving the passage of the
carbon-containing compounds from the sarcoplasm into the
mitochondria and the delivery of oxygen, which ultimately
comes from the air that is breathed. In red blood cells
oxygen is combined with hemoglobin, a protein containing
four identical subunits, during its transfer from the lungs
to the muscles. Muscle contains myoglobin, which has a structure
similar to a single subunit of hemoglobin. Myoglobin, which
combines with and can store oxygen, is responsible for transporting
much of the oxygen through the sarcoplasm to the mitochondria.
Myoglobin is especially important in the heart and “red”
muscles, which rely heavily on oxidative metabolism for
the production of ATP.
During
intense exercise of skeletal muscle, ATP is supplied almost
entirely by carbohydrate metabolism. During rest or very
light exercise, however, skeletal muscle depends largely
on the oxidation of stored fats—actually of their breakdown
products, fatty acids—for the production of ATP. Two-carbon
units and hydrogen atoms are removed from fats in a stepwise
fashion. The two-carbon units enter the Krebs cycle, as
do the identical units derived from carbohydrates, as the
compound acetyl co-enzyme A. The hydrogen atoms from the
breakdown of the fat and from the reactions of the Krebs
cycle proceed through oxidative phosphorylation, and ATP
is produced.
