Heme and non-Heme Iron Proteins
We determine structural properties of biological macromolecules using crystallographic techniques. We concentrate on bio-chemically interesting reactions such as CO (carbon monoxide) or NO (nitric oxide) binding to heme or non-heme iron proteins such as superoxide dismutase, catalase, myoglobin and nitrophorin. In cooperation with the Pacheco group (chemistry department of the UW-Milwaukee) we prepare photolabile compounds that can be used to initiate reactions in these molecules. The static structures of these compounds bound to the protein will be a prerequisite for further investigations by time-resolved methods including time-resolved crystallography. In a typical experiment, the temperature is lowered to cryogenic temperatures (100 K and lower) and the reaction is initiated at that temperature. An increase of temperature allows the molecules to surmount certain energy barriers. The structure is probed using conventional crystallography by lowering the temperature back to the initial temperature.
click here to see an enlarged figure of our static crystallographic experiment with the L29W myoglobin mutant
Cis/Trans-Isomerizations in Biomolecules
Cis/Trans-isomerizations are at the base of light perception in the living world. The most famous reaction is probably the isomerization of retinal located in the rhodopsin (seeing purple) of our eye retina. Other organisms use chemically completely different molecules, which, however, also show cis/trans isomerizations, to drive their reactions towards light. Plants, for example, use the chromophor phytochromobilin contained in the protein phytochrome to perceive red light to regulate germination and greening. The phytochromobilin is chemically related to substances made by the bile in mamals as a result of heme degradation. This is the reason for the the expression “bilin” in the name of these compounds. These bile salts consists of the 4 pyrrole rings of the heme that are, however, not linked to a closed ring but rather resemble a linear structure. Cis/Trans isomerization takes place usually at the double bond that is involved in connecting rings C and D. In the literature the cis/trans isomerization that occurs here is called Z/E isomerization referring to the German words Zusammen (together) and Entfernt (distant), respectively. We were the first who determined a true E-isoform of a bilin chromophore in a biological macromolecule; that of the phycoviolobilin (PVB) of a cyanobacterial phycobilin called phycoerythrocyanin. Please note the peculiar difference in the naming convention for the phyto (plant) bilins and phyco (seaweed, plant-related) biliproteins and their chromophores.
On the left the chemical structure of the bilin chromophore phycoviolobilin (PVB) is depicted, which is an open tetrapyrrol in its 15E configuration. On the right the physical (3D) structure of the same chromophore is shown as determined by X-ray crystallography to 2.2 A resolution.