WATOC 2008 Poster

"HAEMOGLOBIN REVISITED", poster to be shown at WATOC2008, Sydney 14-19 August
A theoretical study of catalysis by soft materials, such as protein, has shown the relationship between the degree of catalysis and local transient rigidity [1]. Since the intrinsic rigidity of protein is quite low, a theoretical source of local rigidity has been found in the effects of surface tension on multi-subunit proteins [1]. This concept has already been used to generate new quaternary structures for the neuraminidases and galactose oxidase [2]. It was assumed that crystallisation of the proteins in preparation for x-ray crystallography had left their tertiary structures essentially the same as in solution, but their quaternary (multi-subunit) structures were possibly very different.
This poster will show a new model structure for haemoglobin based on the same concepts. The new structure is consistent with the complex chemical properties of haemoglobin and is supported by cross-linking studies.
Natural haemoglobin occurs as an α2β2 tetramer (α and β being different forms of haemoglobin) that shows cooperative binding of O2. There are also other interactions between the O2 binding, the pH and the concentrations of certain other chemicals. These interactions require that a binding site at one location in the tetramer does work on binding sites elsewhere in the tetramer. Some degree of rigidity would be required to allow specific transmission of this energy to the correct location.
The widely accepted structure of haemoglobin is based on x-ray crystallography, initially by Muirhead and Perutz in 1963. (Some recent NMR studies suggest that the solution structure is similar to the crystal, but they have problems with assumptions and verifiability.) In 1965 Monod, Wyman and Changeux proposed a new mechanism to explain cooperative O2 binding, as it was previously thought that the hemes interacted directly [3]. In Perutz’s structure the hemes could not directly interact and an intricate mechanical and chemical mechanism was proposed. As the complexity of haemoglobin chemistry has emerged since then, much ingenuity has been applied to modifying the models to fit the data. However, serious inconsistencies remain [4, 5]
The new quaternary structure to be shown in this poster differs markedly from Perutz’s structure. It provides an explanation for the apparent necessity that highly cooperative O2 binding involves α2β2 tetramers rather than homotetramers. The “switch” between high and low affinity forms involves isomeric quaternary structures only achievable with an α2β2 tetramer. The interactions between sites are mediated by delocalised π-electrons connected through the rigid central core of the tetramer. This mechanism is reminiscent of the mechanism proposed by Pauling before the x-ray crystal structure became known [3].
[1] Vanselow, D. G. Biophys. J. 2002, 82, 2293-2203.
[2] Vanselow, D. G. Native Proteins; http://nativeproteins.blogspot.com, 2008.
[3] Eaton, W.A.; Henry. E.R.; Hofrichter, J.; Mozzarelli, A. Nat. Struct. Biol. 1999, 6, 351-358.
[4] Yonetani, T.; Park, S.; Tsuneshige, A.; Imai, K.; Kanaori, K. J. Biol. Chem. 2002, 277, 34508-34520.
[5] Eaton, W. A..; Henry, E. R.; Hofrichter, J.; Bettati, S.; Viappiani, C.; Mozzarelli, A. IUBMB Life 2007, 59, 586-599.

A preprint of a full paper describing this work will be posted after the conference.