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

拼凑假说假设，在生物进化的早期阶段，只有少数酶存在，催化不同生物合成途径中许多底物的转化。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，其中每一种酶在单一代谢途径中仅转化一种底物。然而，现实更为复杂，因为许多现代酶也是混杂的。突出的例子是HisC（磷酸组氨醇转氨酶）和HisB（组氨酸磷酸酶）来自组氨酸生物合成，其还以可测量的效率催化同源酶SerC（磷酸丝氨酸转氨酶）和SerB直到现在还不清楚i）为什么现代His-酶是混杂的，ii）这些因素通常决定了向特定酶进化的极限。为了回答这些问题，我们计划将计算分析与分子生物学和生物化学实验相结合。关于问题i），将研究现代HisC和HisB酶的混杂是否已经存在于重构的前体酶中，以及混杂是否在缺乏SerC和SerC酶的物种中比在含有这些酶的物种中更明显。关于问题ii），我们想阐明是否可以通过蛋白质设计来增强混杂性，或者可以在不损害天然活性的情况下消除混杂性。此外，计划进行计算机模拟分析，以阐明基因重复和多样化对代谢途径稳健性和灵活性的贡献。基于该分析的结果，我们希望进一步鉴定具有高概率的混杂酶。

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.