Anaerobic, NADH-Dependent Haem Breakdown in a Family of Haemoproteins

Change log
Keith, Alasdair Donald 

Many pathogens function by internalising the haem molecules of their host organism and breaking down the porphyrin scaffold to sequester the Fe2+ ion. Typically, this breakdown mechanism is mediated by a haem oxygenase. However, a novel class of reaction has been discovered, which can be performed anaerobically using 'nature's reductant', NADH, and the Yersinia enterocolitica protein, HemS. To study the features of this reaction in more detail, conventional experimental methods were combined with Energy Landscape Theory. Deuterium labelling demonstrated that the reaction was initiated by hydride transfer and stopped-flow spectroscopy showed that the reaction proceeded via a short-lived intermediate. Since no structural information regarding NADH-binding to HemS was available, computational calculations were used to sample the conformational space around possible NADH-protein binding sites and to construct kinetic transition networks. From these networks, pathways showing the unfolding and approach of NADH to haem inside the pocket were determined. These pathways highlighted the roles of various residues, thus allowing for a targeted mutagenesis study. This study, carried out using both computation and laboratory-based experimentation, was especially focussed on a double phenylalanine gate located in the centre of the main cavity. Key insight concerning how this feature regulates the access of NADH to haem was gained.

Computational results suggested that the HemS homologues, HmuS, ChuS and ShuS, were also capable of promoting anaerobic haem breakdown, but that catalysis by ChuS and ShuS may be limited by competing functions. Bioinformatics was used to gauge what these possible alternative functions could be, and to place HemS within its wider phylogenetic context. The computational predictions were then tested in the laboratory. The three homologues were all shown to engage in the reductive haem breakdown process but to varying degrees of efficacy. These findings demonstrate that this novel haem breakdown reaction is not unique to HemS, but instead is a feature of a wider class of haemoproteins. A subset of these haemoproteins are known to bind certain DNA promoter regions, suggesting not only that they can catalytically degrade haem, but that they are also involved in transcriptional modulation responding to haem flux. Many of the bacterial species responsible for this class of protein (including those that produce HemS, ChuS and ShuS) are known to specifically target oxygen-depleted regions of the gastrointestinal tract. A deeper understanding of anaerobic haem breakdown processes engaged in by these pathogens could therefore prove useful in the development of future strategies for disease prevention.

Wales, David
Barker, Paul
Haem, Biophysical Chemistry, Molecular Biology, Energy Landscapes, Protein Structure
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
EPSRC (1944583)
Cambridge Trust Vice-Chancellor's Award; Departmental Funding