X. CONTROL OF RESPIRATION
- Cells respond to changing metabolic needs by
controlling reaction rates.
- Anabolic pathways are switched off when their products are in
ample supply. The most common mechanism of control is feedback
inhibition.
- ATP production is controlled by regulating enzyme activity at
strategic points in catabolism.
- A key control point is the third step of
glycolysis which is catalyzed by an allosteric enzyme,
phosphofructokinase.
- The ratio of ATP to ADP or AMP reflects the energy status
of the cell, or energy charge. Phosphofructokinase is sensitive
to changes in this ratio.
- ATP and citric acid are allosteric inhibitors of
phosphofructokinase, so when their concentrations rise, the
enzyme slows glycolysis. As the rate of glycolysis slows, Krebs
cycle also slows since the supply of acetyl CoA is
reduced.
- ADP or AMP are allosteric activators for
phosphofructokinase, so when their concentrations relative to
ATP rise, the enzyme speeds up glycolysis which speeds up the
Krebs cycle.
- There are other allosteric enzymes at key locations which
also control the rates of glycolysis and the Krebs cycle.
