Investigation into the Mechanism of Human Carbonic Anhydrase II

Human carbonic anhydrase II (HCAII) acts as a carbon dioxide sequestration catalyst, removing carbon dioxide from the blood stream and converting it into bicarbonate and a proton. The active site contains a tetrahedrally coordinated zinc atom coordinated by three facially-capping histidine residues. The fourth coordination site is occupied by a water molecule. The mechanism of its active site has been studied since the 1960s.1 The majority of the mechanism is well understood, as it is comprised of two distinct stages. Firstly a zinc hydroxide is formed via deprotonation of the zinc-aqua species via a proton shuttle. This allows for an initial nucleophilic attack on the carbon dioxide from the hydroxide moiety, forming a coordinated bicarbonate species on the zinc centre. The bicarbonate species produced is then exchanged by a water molecule and a second reaction involving the transfer of a proton from the bound water molecule occurs to the surrounding solvent and regeneration of the zinc hydroxide takes place, ready for another cycle.

Disagreement arises as Lipscomb et al. believe that a proton is internally transferred from the original hydroxide group to an oxygen atom on the incoming carbon dioxide, whereas Lindskog et al. believe that there is an internal rotation of the oxygen atom bound directly to the zinc.2 The two mechanisms also differentiate between each other by how the bicarbonate binds to the zinc centre. In the mechanism suggested by Lindskog et al. a bidentate bicarbonate is proposed, whereas Lipscomb et al. have suggested this is a unidentate bicarbonate. Several attempts have been made to understand in more detail which of the two proposed mechanisms is correct.3, 4 HCAII has been the focus of significant attention by the small molecule modelling community.3-24 This is due to the further understanding of the mechanistic details, as well as finding a synthetic catalyst for the sequestration of carbon dioxide for a range of commercial opportunities of a complex. The aims of this project involved synthesising a suitable zinc hydroxide mimic complex and studying the reaction with carbon dioxide.

Initial challenges included design and synthesis of a suitable ligand system. Chapter 2 discusses the previous problems with ligands synthesised in the Walton group and how cis,cis-1,3,5-tris(3,5-ditertbutylphenylpropenylideneimino) cyclohexane, (3,5 tBu TCT) looks to overcome them. Chapter 3 describes successful complexation of this ligand system to a zinc ion, with supporting NMR data. This complex can be reacted with carbon dioxide to give 13C NMR and IR data. In addition, in several synthesise of these species, it was observed that a chloride-coordinated zinc complex was preferentially stable over the hydroxide as this was repeatedly observed in ESI and LIFDI MS. A crystal structure was successfully obtained for this species, along with that of a zinc-coordinated nitrate species. This is isoelectronic to the desired zinc bicarbonate species, allowing useful analysis of specific bond lengths in chapter 5.