Nanding Chen

Mentor: Dr. Yan Kung

Chemistry Department

 

Understanding the Structural Basis of HMG-CoA Reductase Cofactor Specificities

 

The mevalonate pathway is responsible for the production of isoprenoid natural products, which are good candidates for biofuel and drug compounds. The rate-limiting and the first committed step in this pathway is catalyzed by the enzyme 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (HMGR), which reduces HMG-CoA to mevalonate. In NADH-utilizing class II HMGR, a C-terminal “flap” domain appears to close upon NADH binding. However, in NADPH-utilizing class I HMGR, the structure of this domain has not yet been obtained, as its inherent flexibility poses a challenge for crystallography.

In addition, structures of NADH-bound class II HMGR reveal an α-helix connecting to β-strands, whereas the corresponding region of NADPH-bound class I HMGR consists of only a short loop. This structural difference between HMGRs could further contribute to different binding mechanisms of NADPH and NADH, thereby explaining their distinct cofactor specificities.

NADPH-utilizing class I HMGR was mutated to construct a disulfide bond that will facilitate X-ray crystallographic studies of the mobile C-terminal domain; in addition, the cofactor-binding regions of class I and class II HMGRs were exchanged by mutagenesis. Crystal structure of the mutant proteins, together with kinetic studies on cofactor binding, will provide us with insights into the structural basis of cofactor preferences of HMGR. Understanding the cofactor specificities can help us engineer the enzyme more effectively, and better utilize the mevalonate pathway for biosynthesis.