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Synthetic complexes of the histidine brace

  • Full or part time
  • Application Deadline
    Applications accepted all year round
  • Self-Funded PhD Students Only
    Self-Funded PhD Students Only

Project Description

Cellulose is the world’s most abundant biopolymer. 1011 kg is generated every year, equivalent in energy to 670 billion barrels of oil, some 20 times the current annual global oil consumption. As a biofuel, therefore, it has the potential to displace petroleum-based sources, promoting energy security and reducing environmental impact. Yet, despite this burgeoning capacity the production of bioethanol from cellulosic sources is severely constrained by one major factor. The limitation is the remarkable chemical recalcitrance of cellulose, which—in its natural form—has an oxidative lifetime of 5 million years. This astonishing kinetic stability has thwarted all efforts to make effective use of cellulosic sources as biofuels.

AA9-LPMOs are enzymes secreted by fungi. An LPMO has the remarkable property of being able to degrade cellulose into soluble oligiosaccharides. It is seen as a key factor in the potential commercial enzymatic degradation of cellulose to bioethanol. In a key paper (PNAS 2011) , PHW and Gideon Davies published the full structure and active site details of AA9s, showing that it is a unique copper oxidase. Building on this important result this project aims to prepare and characterise small molecule model complexes of AA9s. The objective is to use these small molecules to understand more about AA9’s mechanism of action and, ultimately, to generate synthetic catalysts capable of degrading cellulose.

AA9 has an overall ‘anvil’ type structure, with an extended flat face measuring 40 by 30 Å. In the centre of this face is what is now known to be a copper binding site, made-up of two histidines and one tyrosine/ate (fig 1). One of the histidine groups is at the N-terminus of the protein and acts as an N-N chelating ligand coordinating through the nitrogen atoms of the amino terminus and the imidazole side chain. This histidine is methylated at the N; an unprecedented feature in bioinorganic chemistry. Three nitrogen atoms coordinate to the copper in a meridional configuration. A water molecule takes up the remaining equatorial coordination site with the tyrosine occupying a remote apical position. The overall coordination geometry is square pyramidal.

Active site of GH61. square pyramidal copper coordination sphere, note the remote Tyr (2.8 Å) and methylated histidine. Right, contour map from X-ray with density for Me group

As part of the project spectroscopic studies will be completed on state C and its mutants, over a range of pHs. It is anticipated that Resonance Raman studies can be completed on this state which appears to have a LM charge transfer band at ca 345 nm. The reactivity of State C will be explored particularly with respect to the oxidation of substrates which can be followed spectroscopically. This will allow the determination of oxidation kinetics.

Underpinned by the spectroscopic on AA9 we aim to prepare spectroscopic and functional models of the enzyme. Our preliminary calculations indicate that the following features are key structural features of any model complex: a Cu-meridional N3 coordination geometry with an NH or NH2 group in the central position; a hydrogen-bond to the coordinating atom of the substrate; a coordinated tyrosinate in the apical position.

We aim to prepare the following class of molecules, where X depicts the various possible oxygen species.

It is anticipated that Class A molecules (left) will be prepared in the first phase and Class B (right) in the later stages of the PhD. In each case the molecules will be characterised by UV vis spectroscopy and single crystal X-ray diffraction. Their kinetics of oxidation will be examined by reference to the similar studies carried out on AA9 itself. Moreover, the spectroscopic properties, particularly the low-temperature UV/vis and EPR properties of the complexes will be correlated with those of the enzyme. This is a key feature of this proposal as it seeks to make a definitive link between model complexes and enzyme.

All research students follow our innovative Doctoral Training in Chemistry (iDTC): cohort-based training to support the development of scientific, transferable and employability skills. All research students take the core training package which provides both a grounding in the skills required for their research, and transferable skills to enhance employability opportunities following graduation.

The Department of Chemistry holds an Athena SWAN Gold Award and is committed to supporting equality and diversity for all staff and students. Chemistry at York was the first academic department in the UK to receive the Athena SWAN Gold award. Paul Walton is internationally known for his work on gender equality.

Funding Notes

This project is open to students who can fund their own studies or who have been awarded a scholarship separate from this project. The Chemistry Department at York is pleased to offer Wild Fund Scholarships to those from countries outside the UK. Wild Fund Scholarships offer up to full tuition fees for those from countries from outside the European Union. EU students may also be offered £6,000 per year towards living costs. For further information see: View Website

References

It is further estimated that some 323 million tonnes of cellulose are thrown away by the US ever year, which—converted to bioethanol—could provide 30% of US fuel demand.
R. Jason Quinlan, Matt D. Sweeney, Leila Lo Leggio, Harm Otten, Jens-Christian N. Poulsen, Katja Salomon Johansen, Kristian B. R. M. Krogh, Christian Isak Jørgensen, Morten Tovborg, Annika Anthonsen, Theodora Tryfona, Clive P. Walter, Paul Dupree, Feng Xu, Gideon J. Davies, and Paul H. Walton. Proceedings of the National Academy of Sciences, 2011, 15079.
It is further estimated that some 323 million tonnes of cellulose are thrown away by the US ever year, which—converted to bioethanol—could provide 30% of US fuel demand.
R. Jason Quinlan, Matt D. Sweeney, Leila Lo Leggio, Harm Otten, Jens-Christian N. Poulsen, Katja Salomon Johansen, Kristian B. R. M. Krogh, Christian Isak Jørgensen, Morten Tovborg, Annika Anthonsen, Theodora Tryfona, Clive P. Walter, Paul Dupree, Feng Xu, Gideon J. Davies, and Paul H. Walton. Proceedings of the National Academy of Sciences, 2011, 15079.

Related Subjects

How good is research at University of York in Chemistry?

FTE Category A staff submitted: 47.06

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