We also confirm by mass spectrometry that l-T4C oxidation by PYCRs leads to cysteine product formation. Inhibition kinetics show that l-Pro is a competitive inhibitor of PYCR1 \(\left( \right)\) with respect to l-T4C, consistent with these ligands occupying the same binding site. Interestingly, no activity was observed with either l-Pro or the analog dl-thiazolidine-2-carboxylate, indicating that the sulfur at the 4-position is critical for PYCRs to utilize l-T4C as a substrate. Both PYCRs exhibit significant l-T4C dehydrogenase activity however, PYCR2 displays nearly tenfold higher catalytic efficiency (136 M −1 s −1) than PYCR1 (13.7 M −1 s −1). Here, we characterize the dehydrogenase activity of human PYCR isozymes 1 and 2 with l-T4C using NAD(P) + as the hydride acceptor.
The conversion of l-T4C to l-Cys in bacterial cell lysates has been attributed to proline dehydrogenase and l-Δ 1-pyrroline-5-carboxylate (P5C) reductase (PYCR) enzymes but detailed kinetic studies have not been conducted.
The oxidation of l-T4C leads to l-cysteine formation in bacteria, plants, mammals, and protozoa. L-Thioproline ( l-thiazolidine-4-carboxylate, l-T4C) is a cyclic sulfur-containing analog of l-proline found in multiple kingdoms of life.
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