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

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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.