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Threonine is a Janus amino acid. Its side chain bears both methyl-group and a hydroxyl-group. In solution, this combination is hardly spectacular. However, in a context of a protein, turning of the side chain by its polar or nonpolar face can make a dramatic difference for packing or substrate accommodation.

One more thing is notable about threonine. This is the only D-structure in the canonical amino acid repertoire. Not a D-amino acid, though, itís a D-sugar. Due to the hydroxyl-group threonine can be classified both as an amino acid and as sugar, and if we name it as a sugar, then we should look at the stereocenter at the bottom of the structure, and this indicates a D-configuration, voila:

Interesting readings:

- Kiselev et al. Why Does Threonine, and Not Serine, Function as the Active Site Nucleophile in Proteasomes? J. Biol. Chem., 275, 2000, 14831-14837, doi: 10.1074/jbc.275.20.14831

The paper speculates about the choice of threonine in threonine protease, and shows that switch of Thr to Ser changes the kinetic mode of proteolytic degradation.

- Brister, M. A. et al. OGlcNAcylation and Phosphorylation Have Opposing Structural Effects in tau: Phosphothreonine Induces Particular Conformational Order. J. Am. Chem. Soc., 136, 2014, 3803-3816, doi: 10.1021/ja407156m

The paper illustrates some differences between post-translational modifications (glycosulation and phosphorylation) on threonine and serine. Basically, in the case of threonine the conformational impact is stronger.

- Burnett, J. C et al. A threonine turnstile defines a dynamic amphiphilic binding motif in the AAA ATPase p97 allosteric binding site. Org. Biomol. Chem., 15, 2017, 4096-4114, doi: 10.1039/C7OB00526A

This is a very notable paper, which shows that turning of two gating threonine side chains can help to accommodate both hydrophobic and hydrophilic groups in an inhibitor molecule.