Amphibolic pathways can serve either in energy-yielding
catabolic or in energy requiring biosynthetic processes, depending
on the cellular circumstances. For example, the citric acid cycle
generates NADH and FADH2 when functioning catabolically. But it can
also provide precursors for the synthesis of such products as
glutamate and aspartate (from a-ketoglutarate and oxaloacetate,
respectively), which in turn serve as precursors for other
products, such as glutamine, proline, and asparagine