Crystal structure of a biliverdin IXalpha reductase enzyme-cofactor complex

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Biliverdin reductase (BVR) is crucial for heme degradation, converting biliverdin IXalpha to bilirubin.
• Bilirubin, the end product, possesses antioxidant and anticompliment properties.
• Understanding BVR's mechanism is key to comprehending heme metabolism and its biological roles.

Purpose of the Study:

• To elucidate the structural basis of biliverdin reductase (BVR) activity.
• To investigate the interaction of BVR with its cofactor, nicotinamide adenine dinucleotide (NADH).
• To identify key residues involved in the catalytic mechanism of BVR.

Main Methods:

• X-ray crystallography was employed to determine the structures of apo rat BVR and its complex with NADH.
• High-resolution structures (1.2 Å and 1.5 Å) were obtained for the apo and holoenzyme, respectively.
• Site-directed mutagenesis (Tyr97Phe) was used to assess the role of specific residues in catalysis.

Main Results:

• The BVR structure reveals an N-terminal dinucleotide-binding domain and a C-terminal domain, with the active site at their interface.
• The NADH cofactor binds within the active site cleft, positioning its nicotinamide ring for hydride transfer to the si face.
• The Tyr97Phe mutant retained 50% activity, indicating Tyr97 is not essential and suggesting dominant hydride transfer catalysis.

Conclusions:

• The determined structures provide detailed insights into the BVR active site and cofactor binding.
• Catalysis is primarily driven by hydride transfer from NADH, potentially facilitated by conserved acidic residues (Glu96, Glu123, Glu126).
• The findings advance our understanding of heme catabolism and the enzymatic mechanisms involved.