No abstract

Moonlighting – the performance of more than one function by a single protein – is becoming recognized as a common phenomenon with important implications for systems biology and human health. The different functions of a moonlighting protein may use different regions of the protein structure, or alternative structures that occur due to post‐translational modifications and/or differences in binding partners. Often the different functions of moonlighting proteins are used at different times or in different places. The existence of moonlighting functions complicates efforts to understand metabolic and regulatory networks, as well as physiological and pathological processes in organisms. Because moonlighting functions can play important roles in disease processes, an improved understanding of moonlighting proteins will provide new opportunities for pharmacological manipulations that specifically target a function involved in pathology while sparing physiologically important functions.

Evolutionary biochemists define enzyme promiscuity as the ability to catalyze secondary reactions that are physiologically irrelevant, either because they are too inefficient to affect fitness or because the enzyme never encounters the substrate. Promiscuous activities are common because evolution of a perfectly specific active site is both difficult and unnecessary; natural selection ceases when the performance of a protein is “good enough” that it no longer affects fitness. Although promiscuous functions are accidental and physiologically irrelevant, they are of great importance because they provide opportunities for evolution of new functions in nature and in the laboratory, as well as targets for therapeutic drugs and tools for a wide range of technological applications.

The temperature variation of the NMR coupling constants of chorismic acid and of the bis(tetra-n-butylammonium) salts of chorismate and of 4-0-methylchorismate in water and in methanol has been studied. The results show that 10-40% of each of these species is present in the pseudo-diaxial form in aqueous solution at 25 °C. In methanol solution, chorismate exists as the pseudo-diequatorial conformer. The rate of the Claisen rearrangement of chorismate is 100 times slower in methanol than in water, while the rearrangements of chorismic acid and of 4-0-methylchorismate are slowed by 11-fold and 7-fold, respectively. These results together suggest that the non-enzymic rearrangement of chorismate involves a dipolar transition state having some of the character of a tight ion pair between the enol pyruvate anion and the cyclohexadienyl cation. The relatively small difference in the free energies of the two conformers of chorismate in aqueous solution further suggests that the enzyme chorismate mutase can directly select the pseudo-diaxial conformer (from which the Claisen rearrangement occurs) from solution.