Pyridoxal 5́-phosphate (PLP) is an important cofactor for amino acid decarboxylases with many biological functions, including the synthesis of signalling molecules, such as serotonin, dopamine, histamine, γ-aminobutyric acid, and taurine. Taurine is an abundant amino acid with multiple physiological functions, including osmoregulation, pH regulation, antioxidative protection, and neuromodulation. In mammalian tissues, taurine is mainly produced by decarboxylation of cysteine sulphinic acid to hypotaurine, catalysed by the PLP-dependent cysteine sulphinic acid decarboxylase (CSAD), followed by oxidation of the product to taurine. We determined the crystal structure of mouse CSAD and compared it to other PLP-dependent decarboxylases in order to identify determinants of substrate specificity and catalytic activity. Recognition of the substrate involves distinct side chains forming the substrate-binding cavity. In addition, the backbone conformation of a buried active-site loop appears to be a critical determinant for substrate side chain binding in PLP-dependent decarboxylases. Phe94 was predicted to affect substrate specificity, and its mutation to serine altered both the catalytic properties of CSAD and its stability. Using small-angle X-ray scattering, we further showed that CSAD presents open/close motions in solution. The structure of apo-CSAD indicates that the active site gets more ordered upon internal aldimine formation. Taken together, the results highlight details of substrate recognition in PLP-dependent decarboxylases and provide starting points for structure-based inhibitor design with the aim of affecting the biosynthesis of taurine and other abundant amino acid metabolites.

5-磷酸吡哆醛 (PLP) 是氨基酸脱羧酶的重要辅助因子，具有许多生物学功能，包括合成信号分子，如血清素、多巴胺、组胺、γ-氨基丁酸和牛磺酸。牛磺酸是一种丰富的氨基酸，具有多种生理功能，包括渗透调节、pH 调节、抗氧化保护和神经调节。在哺乳动物组织中，牛磺酸主要通过半胱氨酸亚磺酸脱羧为亚牛磺酸，在 PLP 依赖性半胱氨酸亚磺酸脱羧酶 (CSAD) 的催化下产生，然后产物氧化为牛磺酸。我们确定了小鼠 CSAD 的晶体结构，并将其与其他依赖 PLP 的脱羧酶进行比较，以确定底物特异性和催化活性的决定因素。对底物的识别涉及形成底物结合腔的不同侧链。此外，掩埋的活性位点环的骨架构象似乎是 PLP 依赖性脱羧酶中底物侧链结合的关键决定因素。预计 Phe94 会影响底物特异性，其突变为丝氨酸会改变 CSAD 的催化特性及其稳定性。使用小角度 X 射线散射，我们进一步表明 CSAD 在溶液中呈现开/关运动。apo-CSAD 的结构表明活性位点在内部醛亚胺形成时变得更加有序。综合起来，