Structural and Functional Study of the Type III Pantothenate Kinase from Thermotoga Maritima
Coenzyme A (CoA) is one of the most ubiquitous and essential cofactors in all living organisms. Pantothenate kinase (PanK) catalyzes the first step in the five-step universal pathway of CoA biosynthesis. Three types of PanK have been characterized so far. Prokaryotic PanK (PanK-I) and eukaryotic PanK (PanK-II) were identified previously. A third type of PanK (encoded by coaX gene) was identified by genetic complementation in 2005. PanK-III has a wider phylogenetic distribution than the long known PanK-I, and is nearly universally present in most of the major bacteria divisions, including many pathogenic bacteria. Different from the type I and type II PanKs, PanK-III is not feedback inhibited by CoA, and can not use pantothenamide antibiotics as substrate. In addition, PanK-III has a high Km for ATP (in the mM range) and requires a monovalent cation to have activity. The focus of my research is to unravel the underlying molecular basis for the unique enzymatic properties of PanK-III through crystallographic and other biochemical methods. I have solved the first crystal structure of PanK-III from Thermotoga maritima (TmCoaX). As the structure reveals, PanK-III belong to the acetate and sugar kinase/heat shocks protein 70/actin (ASKHA) protein superfamily, same as PanK-II, whereas PanK-I belongs to P-loop kinase superfamily. Recently, I also solved the crystal structures of two binary complexes of PanK-III with substrate pantothenate and product phospho-pantothenate, respectively, as well as a ternary complex of PanK-III with pantothenate and ADP. Combined with isothermal titration calorimetry, we present a detailed structural and thermodynamic characterization of the interactions between PanK-III and its substrates ATP and pantothenate. Comparison of substrate binding and catalytic sites of PanK-III with that of eukaryotic PanK-II revealed drastic differences in the binding modes of both ATP and pantothenate, even though both PanK-II and PanK-III belong to the same ASKHA superfamily and may share a common catalytic mechanism. In conclusion, our studies not only are important for understanding the fundamental metabolic pathways in PanK-III-harboring pathogenic bacteria, but also provide a structural basis for designing specific inhibitors.