Sources and biological significance for the promiscuity of modern enzymes from histidine biosynthesis

拼凑假说认为，在生物进化的早期阶段，只有很少的酶存在，这些酶在不同的生物合成途径中催化许多底物的转化。从这些混杂的前体酶开始，基因复制和多样化事件将产生专门的酶，每种酶只在单一代谢途径中转化一种底物。然而，现实更为复杂，因为许多现代酶也是杂乱无章的。突出的例子是组氨酸生物合成的HISC(组氨酸磷酸转氨酶)和HISB(组氨酸磷酸转氨酶)，它们也以相当高的效率催化丝氨酸生物合成的同源酶SERC(磷酸丝氨酸转氨酶)和SERB(磷酸丝氨酸磷酸)的反应。至今仍不清楚：1)为什么现代组氨酸转氨酶是混杂的；2)哪些因素总体上决定了向专门酶进化的极限。为了回答这些问题，我们计划结合分子生物学和生物化学实验进行计算分析。关于问题I)将调查现代HISC和HISB酶的混杂是否已经存在于重组的前体酶中，以及在缺乏SERC和SERC酶的物种中混杂是否比在含有这些酶的物种中更明显。关于问题II)我们想要阐明混杂是否可以通过蛋白质设计来增加，或者是否可以在不损害本地活动的情况下被消除。此外，还计划进行电子计算机分析，以阐明基因复制和多样化对代谢途径的稳健性和灵活性的贡献。根据这项分析的结果，我们想要识别更多的酶，这些酶很可能是混杂的。

The patchwork hypothesis postulates that in an early phase of biological evolution only few enzymes existed, which catalyzed the conversion of many substrates in different biosynthetic pathways. Starting from these promiscuous precursor enzymes, gene duplication and diversification events would have yielded specialized enzymes, each of which converted only one substrate within a single metabolic pathway. Reality is, however, more complex insofar as also many modern enzymes are promiscuous. Prominent examples are HisC (histidinole phosphate aminotransferase) and HisB (histidinole phosphatase) from histidine biosynthesis, which catalyze with measurable efficiency also the reactions of the homologous enzymes SerC (phosphoserine transaminase) and SerB (phosphoserine phosphate) from serine biosynthesis.It has remained unclear until now i) why modern His-enzymes are promiscuous and ii) which factors determine in general the limits of the evolvability towards specialized enzymes. In order to answer these questions, we are planning to perform a combination of computational analysis with molecular biology and biochemical experiments. Regarding problem i) it will be investigated whether the promiscuity of modern HisC and HisB enzymes was already present in reconstructed precursor enzymes and whether promiscuity is more pronounced in species lacking SerC and SerC enzymes than in species containing these enzymes. Regarding problem ii) we want to elucidate whether promiscuity can be augmented via protein design or can be eliminated without compromising the native activities. In addition, an in silico analysis is planned in order to clarify the contribution of gene duplication and diversification for the robustness and flexibility of metabolic pathways. Based on the results of this analysis, we want to identify further enzymes that are promiscuous with high probability.