The mutations cause the leakage to the intracellular milieu of a highly reactive phosphorylated intermediate common to the biosynthetic pathways of glutamine and proline. Remarkably, however, cell growth is efficiently restored by many single mutations (12 at least) in the gene of glutamine synthetase. The deletion makes cells unable to grow in a culture medium lacking proline. Here, we report evolutionary repair experiments on Escherichia coli cells in which an enzyme crucial for the biosynthesis of proline has been deleted. It is not known, however, whether this evolutionary adaptation can be relaxed in response to challenges to organismal survival. Sequestration of reactive metabolic intermediates thus contributes to metabolic efficiency. Therefore, metabolic intermediates are often protected as protein-bound species and directly transferred between enzyme active sites in multi-functional enzymes, multi-enzyme complexes, and metabolons. Such metabolic intermediates are often reactive molecules which, if allowed to freely diffuse in the intracellular milieu, could lead to undesirable side reactions and even become toxic to the cell. Many metabolites are generated in one step of a biochemical pathway and consumed in a subsequent step.
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