An oxygen transport maquette:

Design and engineering of an O2 transport protein

R. L. Koder, J. L. R. Anderson, L. A. Solomon, K. S. Reddy, C. C. Moser & P. L. Dutton.

Nature, 458, 305-309, 19 March 2009

Abstract: The principles of natural protein engineering are obscured by overlapping functions and complexity accumulated through natural selection and evolution. Completely artificial proteins offer a clean slate on which to define and test these protein engineering principles, while recreating and extending natural functions. Here we introduce this method with the design of an oxygen transport protein, akin to human neuroglobin. Beginning with a simple and unnatural helix-forming sequence with just three different amino acids, we assembled a four-helix bundle, positioned histidines to bis-histidine ligate haems, and exploited helical rotation and glutamate burial on haem binding to introduce distal histidine strain and facilitate O2 binding. For stable oxygen binding without haem oxidation, water is excluded by simple packing of the protein interior and loops that reduce helical-interface mobility. O2 affinities and exchange timescales match natural globins with distal histidines, with the remarkable exception that O2 binds tighter than CO.

The movie below is a schematic view of the functional action of the oxygen transport maquette. The basic four alpha-helix bundle design (seen end-on, left, or in a side-view, right) has histidines (green pentagons) pointed towards the bundle interior to bind to either side of the iron atom in the middle of the planar heme (brown rectangle). One of these helices is under strain from the normally charged glutamate residues which prefer to be in the watery exterior of the bundle. This preference encourages dissociation of one histidine and frees the iron for ligation to a nearby oxygen molecule (pair of red dots).

The plug-in to view this movie can be found at http://www.apple.com/quicktime