Biochemistry: The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Organization of Metabolism: Catabolic Pathways
Tricarboxylic Acid Cycle
General Properties of Catalytic Cycles

The Tricarboxylic Acid Cycle, being one of the most critical Metabolic Pathways in aerobic organisms (Bacteria, Protozoa, Fungi, higher plants, and humans), is also a classic example of a catalytic cycle. All other cycles similarly involve one or more primary substrates alongside at least one regenerating substrate. Consequently, a catalytic cycle is invariably linked to a metabolic pathway responsible for synthesizing the regenerating substrate. Although this synthesis rarely needs to be rapid—since it typically only has to compensate for minor losses of the regenerating substrate due to Side Reactions—the underlying pathway provides a mechanism to biosynthesize any required quantity of any intermediate generated within the cycle. For instance, Cells draw substantial amounts of oxaloacetate, a-ketoglutarate, and succinyl-CoA from the tricarboxylic acid cycle to synthesize other cellular components. Specifically, aspartate and glutamate are produced directly from oxaloacetate and a-ketoglutarate via Transamination [Equation (8-16)]. It is often stated that the tricarboxylic acid cycle Functions in Biosynthesis; strictly speaking, however, when intermediates are drained from the cycle, it operates as an incomplete cycle. It is more accurate to view the pathway responsible for regenerating-substrate synthesis, along with certain cycle Enzymes, as serving to configure various biosynthetic routes.

Metabolic reaction sequences that function as part of a catabolic cycle while simultaneously contributing to biosynthetic (anabolic) processes are sometimes referred to as amphibolic. The pathways responsible for synthesizing regenerating substrates are termed anaplerotic, a designation introduced by Kornberg, which translates from Greek as "filling up."