Title
Identification of a Conserved Histidine as Critical for the Catalytic Mechanism and Functional Switching of the Multifunctional Proline Utilization A Protein
Author
Michael A Moxley, Lu Zhang, Shelbi Christgen, John J Tanner, Donald Frederick Becker
Year
2017
Journal
Biochemistry
Abstract
Proline utilization A from Escherichia coli (EcPutA) is a multifunctional flavoenzyme that oxidizes proline to glutamate through proline dehydrogenase (PRODH) and Δ1-pyrroline-5-carboxylate dehydrogenase (P5CDH) activities, while also switching roles as a DNA bound transcriptional repressor and a membrane bound catabolic enzyme. This phenomenon, referred to as functional switching, occurs through a redox-mediated mechanism in which flavin reduction triggers a conformational change that increases EcPutA membrane-binding affinity. Structural studies have shown that reduction of the FAD cofactor causes the ribityl moiety to undergo a crankshaft motion indicating that the orientation of the ribityl chain is a key element of PutA functional switching. Here, we test the role of a conserved histidine that bridges the FAD pyrophosphate to the backbone amide of a conserved leucine residue in the PRODH active site. An EcPutA mutant (H487A) was characterized by steady-state and rapid-reaction kinetics, and cell-based reporter gene experiments. The catalytic activity of H487A is severely diminished (> 50-fold) with membrane vesicles as the electron acceptor, and H487A exhibits impaired lipid binding and in vivo transcriptional repressor activity. Rapid-reaction kinetic experiments demonstrate that H487A is 3-fold slower than wild-type EcPutA in a conformational change step following reduction of the FAD cofactor. Furthermore, the reduction potential (Em) of H487A is about 40-mV more positive than wild-type EcPutA and H487A has an attenuated ability to catalyze the reverse PRODH chemical step of reoxidation by P5C. In this process, significant red semiquinone forms in contrast to the same reaction with wild-type EcPutA, in which facile 2-electron reoxidation occurs without forming measurable semiquinone. These results indicate that His487 is critically important for the proline/P5C chemical step, conformational change kinetics, and functional switching in EcPutA.
Instrument
J-815
Keywords
Circular dichroism, Secondary structure, Biochemistry