Bond or Cage Effect: How Nitrophorins Transport and Release Nitric Oxide

MARCELO A. MARTÍ; MARIANO C. GONZÁLEZ LEBRERO; ADRIÁN E. ROITBERG; DARIO A. ESTRIN

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY

AMER CHEMICAL SOC

Año: 2008 p. 1611 - 1618

0002-7863

Abstract: Most blood-sucking insects possess salivary proteins which, upon injection into the victim’s tissue,help them improve their feeding. One group of these salivary proteins takes advantage of the vasodilatorproperties of NO to perform this task. These proteins are the so-called nitrophorins (NPs). NPs are hemeproteins that store and transport NO, which, when released in the victim’s tissue, produces vasodilationand inhibition of blood coagulation. It has been proposed that NO binds tightly to NP at a low pH of around5.6 and that once NPs are injected in the victims tissue, at a pH of approximately 7.4, a conformationalchange occurs which lowers NO affinity, allowing it to be released. In this work we have studied the NOrelease mechanism of NP4 at a molecular level using state of the art computer simulation techniques. Wehave used molecular dynamics (MD) simulations to study NP4 conformational dynamics at both pH values5.6 and 7.4 and computed the corresponding free energy profile for NO release using a multiple steeringmolecular dynamics scheme. We also have used hybrid quantum mechanical/molecular mechanics (QM/MM) techniques to analyze the heme-NO structure and the Fe-NO bond strength in the different NP4conformations. Our results provide the molecular basis to explain that NO escape from NP4 is determinedby differential NO migration rates and not by a difference in the Fe-NO bond strength. In contrast to mostheme proteins that control ligand affinity by modulating the bond strength to the iron, NP4 has evolved acage mechanism that traps the NO at low pH and releases it upon cage opening when the pH rises.