AMP to ATP ratio: switching between catabolism and anabolism
A rising AMP (adenosine monophosphate) to ATP ratio signals low energy and switches on catabolic pathways, the reactions that break fuels down to release energy and remake ATP.
Remember
Low ATP with high AMP and ADP switches catabolic pathways on and ATP-consuming anabolic pathways (which build larger molecules) off, while plentiful ATP does the opposite and favours storage.
ATP (adenosine triphosphate) is the cell's energy currency. It is made from ADP (adenosine diphosphate) and a phosphate using energy released from fuels, and spending it gives ADP and AMP (adenosine monophosphate).
When ATP is used faster than it is made, ATP falls while ADP and AMP rise. The cell reads a high AMP to ATP ratio as a low-energy signal. In many cells the sensor is AMPK (AMP-activated protein kinase), an energy-sensing enzyme that adds phosphate groups to other proteins to change their activity.
AMPK shifts the cell from building towards breaking down. Catabolic pathways break large molecules such as glucose into smaller ones and release energy, for example glycolysis and oxidative phosphorylation (ATP production in the mitochondria). Anabolic pathways build larger molecules from smaller ones using energy. AMPK switches off biosynthetic pathways (those that build molecules) to conserve ATP.
The reverse happens when ATP is plentiful. A high ATP level is a high-energy signal, along with NADH and NADPH (electron carriers, molecules that carry electrons and hydrogen between reactions). It activates anabolic pathways, so surplus fuel is stored for the long term as glycogen (the stored form of glucose) and fat. In a resting muscle cell with plenty of ATP, AMP stays low, so AMPK is not switched on.
When ATP is used faster than it is made, adenylate kinase (an enzyme that swaps phosphate groups between the adenine nucleotides, the family of molecules made from ATP, ADP and AMP) converts two ADP into one ATP and one AMP, so AMP rises more steeply than ADP.
Key facts
- Low-energy signal
- low ATP with high AMP and ADP, seen as a rising AMP to ATP ratio.
- Response to low energy
- catabolic pathways switch on to remake ATP, and ATP-consuming biosynthesis switches off.
- High-energy signals
- high ATP, NADH and NADPH favour storage and biosynthesis.
- Other low-energy signals
- NAD+ and FAD, the empty electron carriers.
- Sensor
- AMPK is activated by a rising AMP to ATP ratio.
- Amplifier
- adenylate kinase turns two ADP into ATP plus AMP, so AMP changes more than ADP.
Exam traps
- Storing fuel can sound sensible after exercise, but anabolic storage pathways need ATP, so low ATP with high AMP switches them off rather than turning them up.
- Low ATP does not slow glycolysis: glycolysis produces ATP overall and slowing it would deepen an energy deficit, whereas it is high ATP that slows phosphofructokinase, a key enzyme of glycolysis.
- A rising ATP to ADP ratio is not a low-energy signal: it means the cell has plenty of usable energy, which favours anabolic storage and slows ATP-producing catabolism.
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