Journal:JBIC:19

Heterometallic [AgFe₃S₄] ferredoxin variants – synthesis, characterization and the first crystal structure of an engineered heterometallic iron-sulfur protein

Maja Martic, Ida Noémi Jakab-Simon, Lærke Tvedebrink Haahr, Wilfred Raymond Hagen and Hans Erik Mølager Christensen^[1]

Molecular Tour
The crystal structure of the Pyrococcus furiosus (Pf) ferredoxin (Fd) D14C variant with the novel [AgFe₃S₄] heterometallic cluster was determined to 1.95 Å resolution (PBD entry 4dhv), being the first reported structure of an engineered heterometallic iron-sulfur protein. The crystal structure of the shows that the (clearly seen on the electron density map), as predicted from previous spectroscopic and electrochemical studies. The heterometal is coordinated to the and to the (residues are coordinated with Fe ions of heterometal), [Fe₄S₄] D14C variant (2z8q, heterometallic [AgFe₃S₄] protein is in cyan and homometallic [Fe₄S₄] is in green) and completing the incomplete cuboidal cluster present in the [Fe₃S₄] WT Pf Fd (PDB: 1sj1) and its D14C (PDB: 3pni) variant (for more details see also "Crystal structures of the all cysteinyl coordinated D14C variant of Pyrococcus furiosus ferredoxin: (4Fe-4S) <-> (3Fe-4S) cluster conversion"). Structure alignment of backbone atoms from the heterometallic [AgFe₃S₄] protein and the homometallic [Fe₄S₄] D14C variant (2z8q) shows , i.e. the root mean square deviation (RMSD) is 0.4 – 0.7 Å, observed due to the alternate conformation of the main chain atoms, flexible loops and small changes at the N- and C-termini (heterometallic [AgFe₃S₄] protein is in cyan and homometallic [Fe₄S₄] is in green). can be seen in the superimposed Fe–S clusters (atom colors corresponding to yellow: S, orange: Fe, gray: Ag) from these two variants, which is due to the presence of the second row transition metal ion (Ag) coordinated to the four S-ligands, i.e. the presence of Ag results in a distorted geometry of the cluster compared to the all-iron arrangement, Ag – S bond lengths compared to Fe – S bonds. However, the S – Ag – S are still close to the expected 90° for a primitive cubic system.