Histidine decarboxylase (HDC; EC 4.1.1.22), an enzyme that catalyzes histamine synthesis with high substrate specificity, is a member of the group II pyridoxal 5′-phosphate (PLP) -dependent decarboxylase family. Tyrosine is a conserved residue among group II PLP-dependent decarboxylases. Human HDC has a Y334 located on a catalytically important loop at the active site. In this study, we demonstrated that a HDC Y334F mutant is capable of catalyzing the decarboxylation-dependent oxidative deamination of histidine to yield imidazole acetaldehyde. Replacement of the active-site Tyr with Phe in group II PLP-dependent decarboxylases, including mammalian aromatic amino acid decarboxylase, plant tyrosine/DOPA decarboxylase, and plant tryptophan decarboxylase, is expected to result in the same functional change, given that a Y-to-F substitution at the corresponding residue (number 260) in the HDC of Morganella morganii, another group II PLP-dependent decarboxylase, yielded the same effect. Thus, it was suggested that the loss of the OH moiety from the active-site Tyr residue of decarboxylase uniquely converts the enzyme to an aldehyde synthase.

组氨酸脱羧酶（HDC； EC 4.1.1.22）是一种以高底物特异性催化组胺合成的酶，是II型吡pyr醛5'-磷酸酯（PLP）依赖性脱羧酶家族的成员。酪氨酸是II类PLP依赖性脱羧酶中的保守残基。人类HDC在活性位点的重要催化环上有一个Y334。在这项研究中，我们证明了HDC Y334F突变体能够催化组氨酸的脱羧依赖性氧化脱氨反应，生成咪唑乙醛。II类PLP依赖性脱羧酶中的活性位点Tyr替换为Phe，包括哺乳动物芳香族氨基酸脱羧酶，植物酪氨酸/ DOPA脱羧酶和植物色氨酸脱羧酶，预计会导致相同的功能变化，另一类依赖于PLP的II类脱羧酶摩根氏菌（Morganella morganii）产生了相同的效果。因此，建议脱羧酶的活性位点Tyr残基失去OH部分，将其独特地转化为醛合酶。