Abstract
The invariance principle of enzyme enantioselectivity must be absolute because it is absolutely essential to the homochiral biological world. Most enzymes are strictly enantioselective, and tryptophanase is one of the enzymes with extreme absolute enantioselectivity for L-tryptophan. Contrary to conventional knowledge about the principle, tryptophanase becomes flexible to catalyze D-tryptophan in the presence of diammonium hydrogenphosphate. Since D-amino acids are ordinarily inert or function as inhibitors even though they are bound to the active site, the inhibition behavior of D-tryptophan and several inhibitors involved in this process was examined in terms of kinetics to explain the reason for this flexible enantioselectivity in the presence of diammonium hydrogenphosphate. Diammonium hydrogenphosphate gave tryptophanase a small conformational change so that D-tryptophan could work as a substrate. As opposed to other D-amino acids, D-tryptophan is a very bulky amino acid with a benzene ring in its heterocyclic moiety, and so we suggest that this structural feature makes the catalysis of D-tryptophan degradation possible, consequently leading to the flexible enantioselectivity. The present results not only help to understand the mechanism of enzyme enantioselectivity, but also shed light on the origin of homochirality.
Highlights
Chiral homogeneity, especially the origin of L-dominant amino acids in the biological world, has intrigued scientists for many years, though there has been no general consensus as yet to the origins of homochirality
We suggest that the side group of D-tryptophan is more bulky than D-histidine due to the benzene ring, and the bulkiness makes it possible to interact with the catalytic site of tryptophanase
We indicated that a benzene ring of D-tryptophan in addition to DAP was important for the flexible enantioselectivity of tryptophanase
Summary
Especially the origin of L-dominant amino acids in the biological world, has intrigued scientists for many years, though there has been no general consensus as yet to the origins of homochirality. If early polypeptides were synthesized by spontaneous abiotic processes in a primitive racemic environment, the mechanism of homochirality may have already been incorporated into them, and their descendants may be traced to present enzymes Such a mechanism is involved in enzyme enantioselectivity in extant life. This indicates that the forming of the aldimine bond is essential to react with D-enantiomers, it is not enough for the flexible tryptophanase enantioselectivity to arise in the presence of DAP Perhaps other factors such as conformational change are required as well. Previous reports showed that tryptophanase underwent a small reversible conformational change in the presence of DAP [14] We think this small conformational change possibly modifies the enantioselectivity of tryptophanase so that D-enantiomers can be activated via the external aldimine bond formation. This report studies the reason why the flexible enantioselectivity emerges in the presence of DAP
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