Discussion
The tetracoordinated copper center in the oxidized and reduced forms of plastocyanin exhibits longest bond with
Answer: D
In both forms the longest bond is to methionine-S.
The type 1 copper centre of plastocyanin is bound by two histidine nitrogens, the thiolate sulfur of a cysteine and the thioether sulfur of a methionine, in a distorted tetrahedral arrangement. The Cu-S(Cys) bond is short (about 2.1 Å) and strongly covalent; it gives the intense blue colour of the oxidised protein through S→Cu(II) charge transfer. The Cu-S(Met) bond is long, roughly 2.8 to 2.9 Å, and stays long whether copper is Cu(II) or Cu(I).
The protein holds the site in nearly the same geometry in both states (the 'entatic' or rack-induced state), which keeps the reorganisation energy small and electron transfer fast. So options naming cysteine for either form are wrong.
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