Fig. 4.

Three examples for covalently bound POMs in the PDB (A: 4F6T, B: 2OGX, C: 4BVP). (A) Covalently bound [Mo₈O₂₆O(Glu)N(His)H_n]ⁿ⁻⁵: one molybdenum atom is covalently bound to a N_ɛ2 histidine nitrogen atom, whereas the other molybdenum is bound to the O_ɛ2 oxygen atom of a glutamic acid. Protein scaffold is illustrated as green cartoon (with 30% transparency), whereas the binding amino acids are represented as sticks (color code: carbon atoms = green, nitrogen atoms = blue and oxygen atoms = red). The POM is depicted in ball and stick representation (color code: molybdenum atoms = deep teal, oxygen atoms = red). (B) Covalently bound [W₃O₁₀H_xN₃]^(6−x)− lying on a crystallographic threefold axis. Each tungsten atom is covalently bound to the N_ɛ2 histidine nitrogen atom of three symmetry related monomers (indicated by different coloring: green, yellow and red, respectively). Protein scaffold is illustrated as cartoon (with 30% transparency), whereas the binding histidines are represented as sticks (color code: carbon atoms = green/yellow/red, nitrogen atoms = blue). The POM is depicted in ball and stick representation (color code: tungsten atoms = marine, oxygen = red). (C) Covalently bound [Mo₈O₂₈]⁸⁻: one molybdenum atom is covalently bound to the N_ɛ2 histidine nitrogen of a His₆-tag and another molybdenum atom to the hydroxyl group of a serine. Both side chains are originating from different protein monomers (indicated by different coloring: green and cyan, respectively). Protein scaffold is illustrated as cartoon (with 30% transparency), whereas the binding side chains are represented as sticks (color code: carbon atoms = green/cyan, nitrogen atoms = blue, oxygen atoms = red). The POM is depicted in ball and stick representation (color code: molybdenum atoms = deep teal, oxygen atom = red).