Structural Probes Of Reactive Intermediates Of Dehaloperoxidase From Amphitrite ornata

Abstract

The enzyme dehaloperoxidase (DHP) from the marine worm Amphitrite ornata is a unique hemoglobin that functions as a peroxidase, capable of converting 2,4,6-trihalo- phenols (TBP, TCP, and TFP) into the corresponding 2,6-dihalogenated quinones as well as other products. In this overview talk we discuss the structure and function of DHP using X-ray crystallography and nuclear magnetic resonance (NMR) to discuss the large differences between DHP function and hemoglobin function despite the strong structural similarities. The position of halogenated phenols inside the distal pocket is one anomalous feature of DHP that is not observed any other hemoglobin or myoglobin. The X-ray crystal structure of DHP reveals that the distal histidine is flexible and has two major conformations. The closed conformation (named by analogy with Sperm Whale myoglobin) is enforced by the binding of a sixth ligand to the heme iron. In the open conformation, observed in the deoxy DHP X-ray crystal structure, the distal histidine is in a solvent exposed conformation. The role of the histidine in coupling the binding of substrate and inhibitors will be discussed in the context of the mechanism for formation of compound I, compound II and a novel intermediate called compound RH that appears to be crucial to the cycling of DHP between hemoglobin and peroxidase function. The NMR data reveal that there are both interior and exterior binding sites for the substrate. This aspect will be discussed along with evidence from optical and EPR spectroscopy to understand the electron transfer kinetics of DHP.