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. Author manuscript; available in PMC: 2015 Dec 2.
Published in final edited form as: Adv Neurobiol. 2014;11:13–30. doi: 10.1007/978-3-319-08894-5_2

Figure 3.

Figure 3

Neurotransmitter glutamate (GLU) is mainly taken up by surrounding astrocytes subsequent to interaction with receptors in the synapse. In the astrocyte, glutamate is either converted to glutamine (GLN) catalyzed by glutamine synthetase (GS) as part of the glutamate-glutamine cycle or metabolized in the tricarboxylic acid (TCA) cycle. Glutamine is transferred to the glutamatergic neuron to be used for synthesis of glutamate catalyzed by phosphate activated glutaminase (PAG). Glutamate enters the TCA cycle by the activity of glutamate dehydrogenase (GDH) or an aminotransferase (AT), and the carbon skeleton may either be completely oxidatively metabolized via pyruvate recycling including malic enzyme (ME) activity or phosphoenolpyruvate carboxykinase and pyruvate kinase. Alternative, the carbon skeleton supports the pool of TCA cycle intermediates and in that way potentially increases the oxidation of acetyl CoA in the TCA cycle. De novo synthesis of glutamate and glutamine from glucose occurs via the concerted action of pyruvate dehydrogenase (PDH) and pyruvate carboxylase (PC) making a net synthesis of TCA cycle intermediates. CIT, citrate; OAA, oxaloacetate; PYR, pyruvate.