Structure and function of S-adenosylhomocysteine hydrolase

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• S-adenosylhomocysteine hydrolase (AdoHcyase) is crucial for regulating biological transmethylation reactions in mammals.
• S-adenosylhomocysteine (AdoHcy), a product of these reactions, acts as a feedback inhibitor, thus AdoHcyase controls its levels.
• Understanding AdoHcyase structure and function is key to comprehending metabolic regulation.

Purpose of the Study:

• To determine the three-dimensional structure of AdoHcyase in complex with NADH and an inhibitor.
• To elucidate the structural basis for AdoHcyase's role in regulating transmethylation.
• To compare the structural dynamics of human and rat AdoHcyase.

Main Methods:

• X-ray crystallography was employed to solve the enzyme's structure.
• Crystallographic direct methods (SnB program) were used to determine the selenium substructure.
• Multiwavelength anomalous diffraction data were analyzed as a special case of multiple isomorphous replacement.

Main Results:

• The three-dimensional structure of AdoHcyase complexed with NADH and DHCeA was determined.
• The enzyme exhibits an architecture resembling NAD-dependent dehydrogenases, featuring modified Rossmann folds in its catalytic and cofactor-binding domains.
• A significant rigid body movement (17 degrees) of the catalytic domain was observed upon inhibitor/substrate binding, differing between human and rat enzyme structures.

Conclusions:

• AdoHcyase possesses a conserved structural architecture with NAD-dependent dehydrogenases.
• The observed domain movement highlights a dynamic mechanism in enzyme regulation and substrate/inhibitor binding.
• The structural insights provide a foundation for understanding AdoHcyase's role in metabolic pathways and potential therapeutic targeting.