KINETIC PROPERTIES OF MUTANT HOLOCARBOXYLASE SYNTHETASES

全新羧化酶合成酶（HCS）缺乏症是一种引起生物素反应性多种羧化酶缺乏症的代谢性疾病。我们分析了几种突变HCS蛋白的动力学特性。与野生型相比，位于HCS假定的生物素结合位点的两个突变体显示生物素的Km值升高。另一方面，其余5个突变位于生物素结合位点之外。含有这些突变的酶显示生物素的正常或低Km值（非Km突变）。具有非Km突变体的患者以及具有Km突变体的患者的症状对生物素治疗有反应。在携带这些突变的培养细胞中，对生物素补充丙酰辅酶a羧化酶活性的响应性与相应的Vmax降低密切相关。具有突变HCS蛋白且Vmax较低的患者对生物素治疗表现出较差的临床和生化反应。HCS基因型的测定对表征HCS缺陷患者的临床表型具有重要意义。

Holocarboxylase synthetase (HCS) deficiency is a metabolic disorder causing a biotin-responsive multiple carboxylase deficiency. We analyzed the kinetic property of several mutant HCS proteins. Two mutants located within the putative biotin-binding site of HCS showed elevated Km values for biotin compared to those of the wild-type form. On the other hand, the remaining five mutations were outside of the biotin-binding site. The enzymes containing these mutations showed normal or low Km values for biotin (non-Km mutant). Symptoms of patients who have the non-Km mutants, as well as those of patients who have the Km mutants, responded to biotin therapy. The responsiveness to biotin supplement of propionyl-CoA carboxylase activity in cultured cells bearing some of these mutations correlated well to the corresponding reduction in the Vmax. Patients who have mutant HCS proteins with lower Vmax showed poor clinical and biochemical responses to biotin therapy. The determination of HCS genotype can be valuable for characterizing the clinical phenotype in HCS deficient patients.