Centro de Investigaciones en Química Biológica de Córdoba (CIQUIBIC, Universidad Nacional de Córdoba-Consejo Nacional de Investigaciones Científicas y Técnicas (UNC-CONICET)), Departamento de Química Biológica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, X5000HUA Córdoba, Argentina.
J Biol Chem. 2012 Jan 13;287(3):1955-61. doi: 10.1074/jbc.M111.287813. Epub 2011 Nov 29.
Initiation of glucose polymerization by glycogenin autoglucosylation at Tyr-194 is required to prime de novo biosynthesis of glycogen. It has been proposed that the synthesis of the primer proceeds by intersubunit glucosylation of dimeric glycogenin, even though it has not been demonstrated that this mechanism is responsible for the described polymerization extent of 12 glucoses produced by the dimer. We reported previously the intramonomer glucosylation capability of glycogenin without determining the extent of autoglucopolymerization. Here, we show that the maximum specific autoglucosylation extent (MSAE) produced by the non-glucosylated glycogenin monomer is 13.3 ± 1.9 glucose units, similar to the 12.5 ± 1.4 glucose units measured for the dimer. The mechanism and capacity of the dimeric enzyme to carry out full glucopolymerization were also evaluated by construction of heterodimers able to glucosylate exclusively by intrasubunit or intersubunit reaction mechanisms. The MSAE of non-glucosylated glycogenin produced by dimer intrasubunit glucosylation was 16% of that produced by the monomer. However, partially glucosylated glycogenin was able to almost complete its autoglucosylation by the dimer intrasubunit mechanism. The MSAE produced by heterodimer intersubunit glucosylation was 60% of that produced by the wild-type dimer. We conclude that both intrasubunit and intersubunit reaction mechanisms are necessary for the dimeric enzyme to acquire maximum autoglucosylation. The full glucopolymerization capacity of monomeric glycogenin indicates that the enzyme is able to synthesize the glycogen primer without the need for prior dimerization.
糖原核心蛋白通过 Tyr-194 残基的自身葡糖基化作用引发葡萄糖聚合，从而启动糖原的从头生物合成。据推测，引物的合成是通过二聚体糖原核心蛋白的亚基间葡糖基化进行的，尽管尚未证明该机制负责描述的二聚体产生的 12 个葡萄糖聚合程度。我们之前报道了糖原核心蛋白的单体内葡糖基化能力，但没有确定自身葡糖基聚合的程度。在这里，我们表明，未经葡糖基化的糖原核心蛋白单体的最大特定自身葡糖基化程度（MSAE）为 13.3 ± 1.9 个葡萄糖单位，与二聚体测量的 12.5 ± 1.4 个葡萄糖单位相似。还通过构建仅通过亚基内或亚基间反应机制进行葡糖基化的异二聚体来评估二聚体酶的机制和完全葡糖基聚合能力。通过二聚体亚基内葡糖基化产生的非葡糖基化糖原核心蛋白的 MSAE 是单体的 16%。然而，部分葡糖基化的糖原核心蛋白几乎可以通过二聚体亚基内机制完成其自身葡糖基化。异二聚体亚基间葡糖基化产生的 MSAE 是野生型二聚体的 60%。我们得出结论，亚基内和亚基间反应机制对于二聚体酶获得最大自身葡糖基化都是必要的。单体糖原核心蛋白的完全葡糖基聚合能力表明，该酶能够在无需预先二聚化的情况下合成糖原引物。
