Anaplerotic reactions

Anaplerotic reactions (from the Greek ἀνά= 'up' and πληρόω= 'to fill') are chemical reactions that form intermediates of a metabolic pathway. Examples of such are found in the citric acid cycle (TCA cycle). In normal function of this cycle for respiration, concentrations of TCA intermediates remain constant; however, many biosynthetic reactions also use these molecules as a substrate. Anaplerosis is the that have been extracted for biosynthesis (in what are called cataplerotic reactions).

The TCA cycle is a hub of metabolism, with central importance in both energy production and biosynthesis. Therefore, it is crucial for the cell to regulate concentrations of TCA cycle metabolites in the mitochondria. Anaplerotic flux must balance cataplerotic flux in order to retain homeostasis of cellular metabolism.

There are 4 major reactions classed as anaplerotic, and it is estimated that the production of oxaloacetate from pyruvate has the most physiologic importance.

The malate is created by PEP carboxylase and malate dehydrogenase in the cytosol. Malate, in the , can be used to make pyruvate (catalyzed by malic enzyme) or oxaloacetic acid, both of which can enter the citric acid cycle.

Glutamine can also be used to produce oxaloacetate during anaplerotic reactions in various cell types through "glutaminolysis", which is also seen in many c-Myc transformed cells.

Pyruvate carboxylase deficiency is an inherited metabolic disorder where anaplerosis is greatly reduced. Other anaplerotic substrates such as the odd-carbon-containing triglyceride triheptanoin can be used to treat this disorder.

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