Leucine is an amino acid with a branched hydrophobic side chain. In proteins, that side chain often participates in packing away from water, while the amino-acid backbone forms peptide bonds.
Leucine is also a metabolic signal. Direct leucine sensing by Sestrin2 connects amino-acid availability with the mTORC1 pathway; the original study used binding and signalling measurements to distinguish sensing from mere correlation.[ref]
Sestrin2 is a leucine sensor for the mTORC1 pathway by Rachel L. Wolfson and others. Leucine binding and regulation of the Sestrin2-GATOR2 interaction
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