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) and completing the incomplete cuboidal cluster present in the [Fe₃S₄] WT Pf Fd (PDB: 1sj1) and its D14C (PDB: 3pni) variant. Structure alignment of backbone atoms from the heterometallic [AgFe₃S₄] protein and the homometallic [Fe₄S₄] D14C variant (2z8q) shows very minor differences, 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. More significant difference can be seen in the superimposed Fe – S clusters [Figure (4) is the enlarged 3D image of the superimposed [AgFe₃S₄] (atom colors corresponding to yellow: S, orange: Fe, gray: Ag) and [Fe₄S₄] (purple) clusters] 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, because to the longer Ag – S bond lengths compared to Fe – S bonds. However, the S – Ag – S bond angles are still close to the expected 90° for a primitive cubic system.

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Blue: indicates approx. which words should be highlighted in the text for the link with the 3D images Black: description of the figure, in Italic the one we don’t know if they can do it or not.