M. Fahnenschmidt et al., Characterization of de novo synthesized four-helix bundle proteins with metalloporphyrin cofactors, PHYS CHEM P, 3(18), 2001, pp. 4082-4090
Two de novo synthesized proteins that form water-soluble four-helix bundle
structures were constructed based on the modular synthesis strategy of temp
late-assembled synthetic proteins. In the hydrophobic interior of these mod
ular proteins one bis-histidine binding site was introduced to incorporate
a single heme group or other metalloporphyrin cofactors. The exchange of tw
o tryptophan against two arginine residues near the heme-binding site leads
to variations in the stability of the modular proteins. The structural cha
racterization of the de nova synthesized heme proteins by EPR spectroscopy
revealed a heterogeneous binding situation of the cofactor in both systems.
The heme group is coordinated by two axial histidine ligands, whose planes
are oriented either parallel or twisted relative to each other. In one of
the two modular proteins this heterogeneity was reduced by addition of guan
idinium hydrochloride that aided in-the proper folding of the heme-binding
site. By using different central metals for the protoporphyrin IX cofactor
(Fe3+, Co3+, Zn2+) different functional properties were introduced into thi
s modular protein. While the Fe3+ porphyrin is easily reduced, the reductio
n of Co3+ porphyrin is very difficult and finally leads to the release of t
his metalloporphyrin cofactor into the aqueous surrounding. By incorporatin
g a porphyrin with a Zn2+ central metal a modular protein was obtained that
was capable of acting as electron donor in light induced triplet electron
transfer.