Shuttle reactions are supporting glycolysis/mitochondrial ATP production.
There are three shuttle pathways to transport reducing equivalents to mitochondria:
1. Glycerol Phosphate shuttle
2. Malate-Aspartate shuttle
3. Lactate shuttle
Glycerol Phosphate Shuttle
The glycerol-3-phosphate shuttle is a mechanism that regenerates NAD+ from NADH, as a side product during glycolysis (Figure 1).
NADH is oxidised in the mitochondria but is not transported through the inner membrane, so it is delivered through transport systems.
Glycerol phosphate dehydrogenase (GPD) is also found in the cytosol and on the mitochondrial membrane and catalyzes the conversion of dihydroxyacetone phosphate into glycerol