Pitt Study Could Advance Fight Against Diabetes, Obesity

Researchers crack enzyme code that plays role in glycerol metabolism

Researchers at the University of Pittsburgh School of Medicine have become the first to decipher the three-dimensional structure of a membrane-bound enzyme that plays a crucial role in glycerol metabolism —a discovery that could lead to important advances against obesity, diabetes, and a potential host of other diseases.

Their findings were reported in the March 4 issue of the Proceedings of the National Academy of Sciences.

The sugar-alcohol glycerol is an essential source of energy that is required to help drive cellular respiration. In addition to powering some of the most central reactions of the body, glycerol also provides key precursors needed to regulate fatty acid and sugar metabolism. Figuring out the complex ways that cells break down or produce glycerol and use this vital chemical could be critical to combating obesity, diabetes, and other chronic disorders. Recent findings also have linked glycerol metabolism to cellular processes related to aging, infectivity in certain organisms such as Mycobacterium tuberculosis, and other energy-related illnesses.

“Everybody wants a golden bullet for obesity, and certainly we need better ways of controlling diabetes,” said Joanne I. Yeh, the study’s senior author and a professor of structural biology at Pitt. “I think that glycerol metabolism will be on the forefront of developing treatments for these diseases and so many others, since it is a pivotal yet underappreciated link among some very important metabolic pathways.”

The protein structure that Yeh’s team solved is a large enzyme called Sn-glycerol-3-phosphate dehydrogenase—known simply as GlpD—found in abundance in the cell membranes of almost all organisms, including humans. GlpD is a monotopic membrane protein, which means that although it is embedded partially in the cell membrane, the protein does not span the entire membrane to the interior of the cell. As a result, it is technically challenging to produce enough highly purified and active protein to obtain clear, relevant information about the enzyme’s atomic structure. This study marks the highest resolution structure of a monotopic membrane protein that scientists have solved to date, one of only a handful of structures of this important class of membrane proteins that have been determined.

“These findings and data help to fill an important scientific and technical gap in the structural field. They also present new information and ideas about how the enzyme works and the importance of the cell membrane in stabilizing the enzyme and in processes related to energy production,” said Yeh, who published the paper with postdoctoral research associate Unmesh N. Chinte and research assistant professor Shoucheng Du, both in Pitt’s Department of Structural Biology.

Studying the proteins and enzymes involved in oxidative and glycerol metabolism as well as characterizing their structures, functions, and regulatory relationships has been a major research focus of Yeh’s lab. It took Yeh and her colleagues only three months—an unusually short time—to decipher the set of 3-D structures of GlpD isolated from E. coli bacteria, thanks to other methodologies they developed in earlier studies.

With the GlpD structure in hand, Yeh’s team is already examining how mutating, or changing, certain amino acids in the enzyme affects its function and fold. These studies target the roles that these specific amino acids play in enzymatic function and regulation of activity. These questions are important because glycerol metabolism is a key link between sugar and fatty acid metabolism. The Pitt group also has determined the atomic resolution structures of other enzymes involved in mediating glycerol and oxidative metabolism. In all, these structural results provide some of the first three-dimensional views of these highly important proteins and enzymes.

The study was funded by the National Institutes of Health. A link to the online paper is available at www.pnas.org/cgi/reprint/0712331105v1.