Specificity of Small c-Type Cytochromes in Anaerobic Ammonium Oxidation

Mohd Akram,Thomas R.M Barends,Josephine Bock,Andreas Dietl

doi:10.1021/acsomega.1c02275

Mohd Akram, Thomas R.M Barends + Show 2 more

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https://doi.org/10.1021/acsomega.1c02275

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Journal: ACS omega	Publication Date: Aug 9, 2021
Citations: 8	License type: CC BY 4.0

Affiliation: Max Planck Institute for Medical Research

Abstract

Anaerobic ammonium oxidation (anammox) is a bacterial process in which ammonium and nitrite are combined into dinitrogen gas and water, yielding energy for the cell. This process relies on a series of redox reactions catalyzed by a set of enzymes, with electrons being shuttled to and from these enzymes, likely by small cytochrome c proteins. For this system to work productively, these electron carriers require a degree of specificity toward the various possible redox partners they encounter in the cell. Here, we compare two cytochrome c proteins from the anammox model organism Kuenenia stuttgartiensis. We show that they are highly homologous, are expressed at comparable levels, share the same fold, and display highly similar redox potentials, yet one of them accepts electrons from the metabolic enzyme hydroxylamine oxidase (HAO) efficiently, whereas the other does not. An analysis of the crystal structures supplemented by Monte Carlo simulations of the transient redox interactions suggests that this difference is at least partly due to the electrostatic field surrounding the proteins, illustrating one way in which the electron carriers in anammox could attain the required specificity. Moreover, the simulations suggest a different “outlet” for electrons on HAO than has traditionally been assumed.

Highlights

Anaerobic ammonium oxidation is a bacterial process in which ammonium (NH4+) and nitrite (NO2−) are converted into dinitrogen gas (N2) and water, yielding energy for the cell.[1]
When 100 mM imidazole was added to oxidized Kustc0562, the UV−vis spectrum showed a shift of the Soret band from 411 to 406 nm, which was accompanied by the disappearance of a weak charge-transfer band at 690 nm, indicative of methionine coordination to the heme iron[14] (Figure S2a)
Several central anammox metabolic enzymes have been purified and characterized biochemically and/or structurally,[3−10] little is known about the ways in which these proteins exchange electrons with other parts of the anammox metabolism

Summary

Introduction

Anaerobic ammonium oxidation (anammox) is a bacterial process in which ammonium (NH4+) and nitrite (NO2−) are converted into dinitrogen gas (N2) and water, yielding energy for the cell.[1]. The first step is the one-electron reduction (NiR).[3−6] of NO2− to nitric oxide (NO) by a nitrite The resulting NO is combined with reductase NH4+ to yield the extremely reactive and unusual intermediate hydrazine (N2H4) by the unique hydrazine synthase (HZS)[7] in a process that takes up a further three electrons This reaction likely proceeds via hydroxylamine (NH2OH) as an intermediate. The hydrazine is oxidized to N2 by hydrazine dehydrogenase (HDH),[9,10] releasing four electrons at an extremely low redox potential of −750 mV Together, these redox reactions fuel an electron transport chain that generates a proton gradient across the anammoxosomal membrane. This proton gradient, in turn, is used to drive ATP synthesis.[1]

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