Structural and redox plasticity in the heterodimeric periplasmic nitrate reductase

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• Denitrification is a crucial microbial process for nitrogen cycling.
• Respiratory nitrate reductase (NapAB) from Rhodobacter sphaeroides initiates denitrification by reducing nitrate.
• Understanding NapAB structure is vital for elucidating electron transfer mechanisms.

Purpose of the Study:

• To determine the high-resolution three-dimensional structure of Rhodobacter sphaeroides NapAB.
• To elucidate the structural basis of the interaction between the NapA and NapB subunits.
• To understand how the complex facilitates efficient electron transfer.

Main Methods:

• X-ray crystallography at 3.2 A resolution.
• Biophysical characterization of cofactor redox potentials.
• Structural analysis of subunit interfaces and electron transfer pathways.

Main Results:

• The structure reveals the heterodimeric NapAB complex with NapB binding NapA.
• Heme II of NapB is positioned near the [4Fe-4S] cluster of NapA.
• Extensive interactions (5,900 A(2)) involving NapB termini stabilize the complex.
• Redox potential shifts indicate structural plasticity enhancing electron transfer.

Conclusions:

• The determined NapAB structure provides atomic-level insights into denitrification.
• The interaction interface and cofactor positioning are optimized for efficient electron transfer.
• Structural plasticity plays a key role in the enzyme's function, transferring electrons from NapB's heme I to NapA's catalytic site.