Structure and reactivity of chlorite dismutase nitrosyls

Outer coordination sphere influences on cofactor maturation and substrate oxidation by cytochrome P460

Product selectivity of ammonia oxidation by ammonia-oxidizing bacteria (AOB) is tightly controlled by metalloenzymes. Hydroxylamine oxidoreductase (HAO) is responsible for the oxidation of hydroxylamine (NH2OH) to nitric oxide (NO). The non-metabolic enzyme cytochrome (cyt) P460 also oxidizes NH2OH, but instead produces nitrous oxide (N2O). While both enzymes use a heme P460 cofactor, they selectively oxidize NH2OH to different products. Previously reported structures of Nitrosomonas sp. AL212 cyt P460 show that a capping phenylalanine residue rotates upon ligand binding, suggesting that this Phe may influence substrate and/or product binding. Here, we show via substitutions of the capping Phe in Nitrosomonas europaea cyt P460 that the bulky phenyl side-chain promotes the heme-lysine cross-link forming reaction operative in maturing the cofactor. Additionally, the Phe side-chain plays an important role in modulating product selectivity between N2O and NO during NH2OH oxidation under aerobic conditions. A picture emerges where the sterics and electrostatics of the side-chain in this capping position control the kinetics of N2O formation and NO binding affinity. This demonstrates how the outer coordination sphere of cyt P460 is tuned not only for selective NH2OH oxidation, but also for the autocatalytic cross-link forming reaction that imbues activity to an otherwise inactive protein.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.