- Some intermediates of the TCA cycle serve as precursors for biosynthesis.
- For efficient operation of this cyclic metabolism, the intermediates used for biosynthesis should be replenished otherwise the concentration of oxaloacetate would be too low to start the TCA cycle.
- Oxaloacetate is replenished through a process called the anaplerotic sequence.
Anaplerotic sequence
- Bacteria growing on carbohydrates synthesize oxaloacetate from pyruvate or phosphoenolpyruvate (PEP) .
- Many organisms, from bacteria to mammals, carboxylate pyruvate to oxaloacetate and this is catalyzed by pyruvate carboxylase consuming ATP:
pyruvate carboxylase
pyruvate + HCO3- + ATP <-------------------------- > oxaloacetate + ADP + Pi
- This enzyme needs biotin. Acetyl-CoA activates this enzyme in many bacteria, as in animals, but some bacteria such as Pseudomonas aeruginosa have a pyruvate carboxylase that is not activated by acetyl-CoA.
- A PEP carboxylase mutant of Escherichia coli is unable to grow in a glucose–mineral salts medium, but can grow when supplemented with TCA cycle intermediates. This bacterium has PEP carboxylase as the anaplerotic sequence, not pyruvate carboxylase.
- This property is shared by many other bacteria including Bacillus anthracis, Thiobacillus novellus, Acetobacter xylinum and Azotobacter vinelandii.
PEP carboxylase
PEP + HCO3- <-------------------------- > oxaloacetate + Pi