Two Solutions to a Single Issue: Detoxification of Nitro Xenobiotics by Flavin Oxidoreductases
Kevin Francis *
Department of Chemistry, Texas A&M University-Kingsville, Kingsville, Texas, United States of America.
Victoria Patterson
Department of Chemistry, Texas A&M University-Kingsville, Kingsville, Texas, United States of America.
Madelyn Wallace
Department of Chemistry, Texas A&M University-Kingsville, Kingsville, Texas, United States of America.
*Author to whom correspondence should be addressed.
Abstract
This review article details the current knowledge of nitronate monooxygenase (NMO) and nitroalkane oxidase (NAO) both of which detoxify organic nitro compounds. The chemical versatility of these molecules are first described by explaining why they are unusually strong carbon acids. This reactivity led to their widespread use in industry and inevitable release into the environment as pollution. Oxidation of nitro compounds is likely an adaptation of existing enzymatic activities to convert them into carbonyl compounds that can be broken down by normal metabolism. NMO has a marked preference for anionic nitronates while NAO exclusively utilizes neutral substrates. Kinetic studies demonstrating this fact are presented along with a detailed structural analysis to rationalize this observation. NMO uses a conserved histidined to electrostatically stabilize the carbanion formed from nitro compounds where as NAO has a negatively charged aspartate in the active site that repels anionic substrates. Details of the chemical mechanisms of each enzyme are presented showing that NMO uses a flavin radical during catalysis whereas NAO forms a covalent adduct between the cofactor and substrate. Information presented in this article is intended to shed light on two platforms that can be exploited for bioremediation to alleviate nitroalkane pollution.
Keywords: Nitroalkanes, alkyl nitronates, nitronate monooxygenase, nitroalkane oxidase, flavin oxidoreductases, enzyme mechanism, pH and kinetic isotope effects, bioremediation