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. 2017 Sep 15;7:11704. doi: 10.1038/s41598-017-11948-w

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© The Author(s) 2017

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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General mechanism for the first half reaction of aminotransferases. (1) A conserved Asp residue protonates the pyridine nitrogen of PLP (N1). (2) The transaldimination reaction with the amino acid substrate occurs only when the internal aldimine is protonated at Nε. (3) The 1,3-prototropic shift represents the rate limiting step of the half reaction. The amino group of the Lys residue acts as a base catalyst extracting the Cα hydrogen. According to the Dunathan hypothesis, co-planarity of the Schiff base bond of the external aldimine with the pyridine plane forms an extended π-system that, together with protonation of N1, allows the negative charge on Cα carbanion to be stabilized by a combination of Coulombic and resonance effects³. The same Lys residue serves as an acid catalyst in the protonation of C4′ completing the 1,3-proton transfer. (4) Hydrolysis of the resulting ketimine yields pyridoxamine phosphate (PMP) and the leaving α-keto acid.