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. Author manuscript; available in PMC: 2019 Jun 5.
Published in final edited form as: Biochemistry. 2018 Apr 13;57(22):3134–3145. doi: 10.1021/acs.biochem.8b00092

Figure 2.

Figure 2.

Proposed enzymatic mechanism for the yCBS reaction with absorption maxima.20 Possible quinonoid intermediates between E-serine (E-PLP-l-ser) and E-aminoacrylate (E-PLP-aa) or between E-aminoacrylate and E-cystathionine are not shown. The growing consensus among those working on fold-type-II PLP-dependent enzymes is that the ring nitrogen does not undergo protonation during the catalytic cycle in these enzymes.25 However, at pH 6.5 the expectation is that the ring nitrogen is protonated as shown.