Maria-Eirini Pandelia, PhD
Department of Biochemistry
Brandeis University
(October 17, 2016)
Digging Up Nature’s Mechanisms: Surprises by Enzymes With Novel Metallocofactors
Metals are essential for life, from viruses to the cells of the human body. Metals such as iron and magnesium are needed for proper cell function. Because of the indispensable role they play, it is important to understand how they function in cells. Dr. Pandelia discussed her recent work examining the role of metals in different cellular processes. For example, her work has determined that iron and sulfur are essential for the formation of lipoyl cofactor, which is needed for metabolism. She also discussed the hepatitis B virus, which contains an unknown metal that is responsible for the growth of cancer in the liver associated with the virus. Her work highlights the multiple roles metals can play in cell biology.
My presentation was segregated into two parts, regarding the role of complex metallocofactors involved in enzyme catalysis and function.
The first part of the talk was dedicated to the enzyme Lipoyl Synthase (LipA). LipA catalyzes the last step in the formation of the lipoyl cofactor, which is critical to aerobic metabolism in various 2-oxoacid dehydrogenase complexes. LipA employs two [4Fe-4S] clusters to successively insert two sulfur atoms at the unactivated C6 and C8 positions of a protein-bound octanoyl group. In this reaction, LipA uses one of its [4Fe-4S] clusters and AdoMet to produce the 5′-deoxyadenosyl radical, which abstracts a hydrogen atom from substrate before each sulfur insertion. The role of the second [4Fe-4S] cluster was unknown. We have employed kinetic and spectroscopic analysis to investigate the structure and mechanism of LipA. Our results showed that LipA adopts a “cannibalistic” mechanism, whereby it sacrifices the second [4Fe-4S] cluster to provide sulfur for the formation of the lipoyl cofactor substrate, thus limiting the enzyme to one turnover in vitro.
The second part of the talk was about a small viral protein, designated protein X, which is an oncogenic factor in chronic infections affected by the Hepatitis B virus (HBV). Protein X is the smallest gene product of HBV and the main etiological agent of virus-mediated liver oncogenesis. Although there are innumerable reported functions and protein binding partners, the molecular mechanisms by which protein X promotes tumorigenesis are still unclear. Despite the multi-decade studies, hardly any biochemical or structural information is known, which has been the major obstacle for linking its structure and activity to the cascade of cellular processes it modulates. We have shown that HBx harbors a novel, previously unrecognized metallocofactor, thus opening the avenues for its characterization. Our findings are consistent with the metallocofactor being not solely a structural element, but having an active redox and structural role in the biological activity of protein X.