Two Other Interesting Proteins

Two other very interesting structures were found while conducting this study. When iron is in protein molecules, the most common amino acid to which it is bound is histidine, so these structures are interesting because they have amino acids bonded to the iron which are not the typical histidine molecules.

Transferase

The first of the two molecules is galactose-1-phosphate uridylyltransferase. This is an important enzyme used in the conversion of the carbohydrate galactose into a more usable carbohydrate glucose-6-phosphate. Specifically, this enzyme catalyzes the transfer of uridine 5'-phosphoryl from UDP-glucose to galactose-1-phosphate during the conversion, giving glucose 1-phosphate and UDP-galactose. The glucose 1-phosphate is made into glucose 6-phosphate, and the UDP-galactose is made into UDP-glucose.¹

As can be seen from this view of the protein, the iron is bonded to three histidine groups (in the usual manner through a nitrogen in the ring) and one glutamic acid carboxylate.

This iron center has a fairly distorted square pyramidal geometry. The average N-Fe bond is 219 pm and the two oxygen atoms are at 185 and 214 pm.2

¹ Thoden, J.B.; Ruzicka, P.A.; Frey, I.R.; and Holden, H.M. Biochemistry 1997, 36, 1212-1222.

² Using the default bond distance of 250 pm, there was a bond drawn
between the carbon and iron atoms, which are 226 pm apart. Since there is
not actually a bond to the carbon in the glutamic acid, the bond was removed
by setting the maximum bond distance to 220 pm. When this is done we get the
more correct bonding picture of the molecule where the only atoms from the glutamic
acid in the foldable Molecular Origami model are the two oxygens.

Photosynthetic Reaction Center

This interesting view is the iron in the photosynthetic reaction center of a bacterium. The reaction center controls the primary processes of photosynthesis, the changing of energy from electromagnetic radiation into chemical energy.

As can be seen in the view, the iron is surrounded by four histidine molecules and one glutamic acid. The geometry of this iron is different from the iron in transferase because of the larger number of histidines, and could best be described as a distorted octahedron.3 Again, valuable information could be obtained by comparing the angles and distances of the bonds from the iron to the two atoms in the glutamic acid.

The axial nitrogens are at an average of 232 pm, while the equatorial nitrogens have an average distance of 214 pm. The oxygens are 212 and 215 pm from the iron atom. The iron-ligand bond distances are slightly longer than the above transferase. This can be explained, as the extra histidine in 1aig should lead to a higher electron density around the iron, and an overall more shielded iron center.

³The carbon atom is only 244 pm from the iron atom, so unless
this default distance is lowered to below 244 pm, the program will draw a
bond between those two atoms. For this view, the value was set to 242 pm.