Figure 6. Activation of TOR via control of BCAA catabolism. While flies appear to use miR-277 for the control of branched-chain amino acid catabolism and signaling, a specific protein kinase can phosphorylate the branched-chain α keto acid dehydrogenase (BCKDH) enzyme in mammals. This kinase inhibits the enzyme and is itself inhibited by KIC, a substrate for the BCKDH reaction. This regulatory loop can work both as a feed-forward activation mechanism but also as a negative feed-back loop to prevent excessive accumulation of KIC. In addition, human miR-29b was reported to regulate the BCKDH complex.