Preferred sites and pathways for electron transfer in blue copper proteins

Area of Science:

• Biochemistry
• Biophysical Chemistry
• Metalloprotein Chemistry

Background:

• Blue single copper proteins are vital electron carriers with restricted redox center accessibility.
• Understanding electron transfer (E.T.) mechanisms in these proteins is crucial for elucidating biological electron transport.
• The specific pathways for electron entry and exit at the copper ion remain an area of investigation.

Purpose of the Study:

• To investigate the mechanisms and locations of long-range electron transfer in blue single copper proteins.
• To explore the role of chromium ion labeling in identifying E.T. pathways.
• To determine if a common E.T. mechanism exists across different blue single copper proteins.

Main Methods:

• Reductive labeling of azurin, plastocyanin, and stellacyanin with chromium (Cr) ions.
• Formation of substitution-inert Cr(III)-adducts after reduction with Cr(II) ions.
• Characterization of Cr(III) binding sites relative to the copper center using structural information.

Main Results:

• Cr(III) labels were successfully formed in azurin, plastocyanin, and stellacyanin.
• In azurin, Cr(III) binds to a carboxylate group approximately 10 Å from the copper center.
• In plastocyanin and stellacyanin, Cr(III) labels bind to carboxylate groups 12 Å and 6 Å, respectively, from the copper center.
• A pi-facilitated E.T. pathway is proposed, involving an extended imidazole system in azurin and a weakly coupled pi-system in plastocyanin and stellacyanin.

Conclusions:

• A common feature of long-distance intramolecular E.T. in blue copper proteins may involve pi-facilitated pathways.
• These pathways represent a potential regulatory mechanism distinct from direct E.T. at the copper-coordinating imidazole.
• The findings suggest a conserved strategy for electron transfer regulation in this metalloprotein class.